M Neus

Alain Dufresne
email: alain.dufresne@cermav.cnrs.fr

Plasticized Starch/ Tunicin Whiskers Nanocomposites : 1. Structural Analysis

Nanocomposite materials were obtained using glycerol plasticized starch as the matrix and a colloidal suspension of cellulose whiskers as the reinforcing phase. The cellulose whiskers, prepared from tunicin, consisted of slender parallelepiped rods with a high aspect ratio. After mixing the raw materials and gelatinization of starch, the resulting suspension was cast and evaporated under vacuum. The composites were conditioned at various moisture contents in order to evaluate the effect of this parameter on the composite structure. The resulting films were characterized using scanning electron microscopy, differential scanning calorimetry, water absorption experiments and wide angle X-ray scattering. An accumulation of plasticizer in the cellulose/amylopectin interfacial zones was evidenced. The specific behavior of amylopectin chains located near the interface in the presence of cellulose probably led to a transcrystallization phenomenon of amylopectin on cellulose whiskers surface.

Introduction

There is a growing interest in the non-food usage of starch-based products for applications in which synthetic polymers have traditionally been the materials of choice.

Especially, the incorporation of granular starch as filler [1][2][3] or disrupted starch granules [4][5][6][7][8] into commodity plastics has generated worldwide a considerable attention in using starch to enhance biodegradability of plastic materials. Starch is the cheapest biopolymer and it is totally biodegradable. It is also available in large quantities from several renewable plant sources produced in abundance beyond available markets. But, in starch filled plastics, bacteria and fungi digest the starch fraction, and the remainder one is not degraded by any biological activity, representing up to 95% of the whole material.

A second generation of starch-based materials have been studied in which granular starch must be mixed with enough non-aqueous plasticizer (generally polyols, such as glycerol) to enable melting below the decomposition temperature of starch. This procedure yields a product in which starch forms a continuous polymeric entangled phase or a completely disordered molecular structure of the granular starch. This type of starch is known as thermoplastic starch (TPS) 9 or destructured starch (DS) 10 , which can be manufactured using technology already developed for the production of synthetic plastics, thus representing a minor investment. The potential advantages of such materials, apart of their environmental gains, are the abundant availability of the raw materials from renewable resources, not depending on fossil sources, and also their low cost, which represents both economic and social benefits.

By itself, starch is a poor choice as a replacement for any plastic. It is mostly water soluble, difficult to process, and brittle when used without plasticizer addition. Furthermore, its mechanical properties are very sensitive to the moisture content, which is difficult to control.

In previous works 11,12 composite materials were obtained from a potato pulp cellulose microfibrils suspension and an aqueous suspension of gelatinized potato starch as the matrix.

Improved thermomechanical properties and a decrease of the water sensitivity of these systems were reported. However, the understanding of the phenomena involved in these improvements requires the processing and the characterization of model systems. Such model systems can be obtained using cellulose whiskers as a model cellulosic filler.

Physical incorporation of cellulose whiskers, and especially tunicin whiskers, as cellulosic model filler into polymeric matrix for the processing of model composites has been largely used, since the first announcement of using cellulose whiskers as a reinforcing phase. 13 This extensive use [14][15][16][17][18] can be explained by the regular shape, high aspect ratio and monocrystalline nature of cellulose whiskers. The main problem associated to the modeling of the starch-based materials is the presence of four components (starch, cellulose, main plasticizer and water). These components can be found in different phases (amorphous, crystalline, liquid). In addition, competitive interactions should occur between these components. All these factors make the system largely more complex than in the case of unplasticized amorphous or semicrystalline matrix filled with cellulose whiskers.

In the present study, the structure of the complex system obtained from plasticized starch reinforced with tunicin whiskers is analyzed as a function of cellulose and relative humidity content. From this knowledge, the mechanical behavior of these materials will be analyzed in the second part of the paper. 19

Experimental

Starch matrix. The starch gels were prepared by gelatinization of waxy maize starch (almost pure amylopectin, amylose content is lower than 1%) kindly supplied by Roquette S.A. (Lestrem, France), dispersed in a mixture of water and glycerol (Prolabo, 98% purity). Gels contained 10 wt% of waxy maize starch, 5 wt% glycerol and 85 wt% water. These ratios were the average values found in the literature for the processing of TPS. [20][21][22][23][24][25][26][27][28] The gelatinization was performed in a stirred autoclave reactor operating at 160°C for 5 min. These conditions were optimized by varying systematically the processing temperature and duration in the ranges 150-190°C and 5-60 min, respectively. The criteria for the optimization of the process were the complete disappearance of ghosts within the starch gel and the avoiding of starch degradation. The determination of the disappearance of ghosts was carried out by optical microscopy and the observation of starch degradation was checked by visual inspection of films appearance, degradation leading to a tanning of resulting films. After mixing, the suspension was degassed under vacuum in order to remove the remaining air and cast in a Teflon mold stored at 70°C under vacuum to allow water evaporation.

Cellulose Whiskers. Cellulose microcrystals, or whiskers, were extracted from tunicate (a see animal). The mantle of tunicate is formed of cellulosic microfibrils (tunicin) particularly well organized and therefore highly crystalline. Colloidal suspensions of whiskers in water were prepared as described elsewhere. 13,14,29,30 Mantels were first cut into small fragments that were deproteinized by three successive bleaching treatments, following the method of Wise et al. 29 The bleached mantle (the tunicin) was then disintegrated in water with a Waring blender (at a concentration of 5 wt%). The resulting aqueous tunicin suspension was mixed with H 2 SO 4 to reach a final acid/water concentration of 55 wt%. Hydrolysis conditions were 60°C for 20 min under strong stirring. The suspension was neutralized and washed with water. After sonication a dispersion of well individualized cellulose whiskers resulted, which did not sediment or flocculate as a consequence of surface sulfate groups created during the sulfuric acid treatment. 30 Film Processing. The starting products (starch + glycerol + water + cellulose whiskers suspension) were mixed in order to obtain composite films with a homogeneous dispersion and with different compositions. The glycerol content was fixed at 33 % (dry basis of starch matrix). The cellulose whiskers content was varied from 0 to 25 wt% (cellulose/starch + glycerol). Similar processing conditions as those described for the unfilled starch matrix were used to gelatinize starch and to process nanocomposite films. Native waxy maize was gelatinized directly in presence of water, plasticizer and cellulose.

Film conditioning. Starch, as well as cellulose, are highly hygroscopic materials. The structure and therefore the properties of these materials are strongly related to the water content. [31][32][33][34][35][36] The moisture content of the nanocomposite films was achieved by conditioning the samples at room temperature in dessicators at controlled humidities containing saturated salt solutions. Six relative humidity (RH) conditions at 20-25°C were used, namely 0, 35, 43, 58, 75 and 98%. The saturated salt solutions were P 2 O 5 , CaCl 2 .6H 2 O, K 2 CO 3 .2H 2 O, NaBr.2H 2 O, NaCl and CuSO 4 .5H 2 O, respectively. Conditioning was achieved for at least two weeks to ensure the equilibration of the water content in the films with that of the atmosphere (stabilization of the sample weight). Thermogravimetric Analysis. Thermogravimetric analysis was used to accurately determine the water content of the films conditioned at different relative humidities. The measurements were achieved with a Perkin Elmer TGA7 instrument. Few milligrams of the sample were heated from room temperature up to 130°C at 5°C/min under nitrogen flow (flow rate 20 ml/min). The temperature was subsequently stabilized for 1 h. The loss of weight, ascribed to the water content, was measured for different water activities (water activity = % conditioning relative humidity/100) of the samples.

Water uptake. The kinetic of water absorption was determined for all compositions.

The specimens used were thin rectangular strips with dimensions of 10 mm  10 mm  1 mm.

The films were therefore supposed to be thin enough, so that the molecular diffusion was considered to be one-dimensional. Samples were first dried overnight at 100°C. After weighing, they were conditioned at 20-25°C in a desiccator containing sodium sulfate to ensure a RH ratio of 98%. The conditioning of samples in high moisture atmosphere was preferred to the classical technique of immersion in water, because starch is very sensitive to liquid water and can partially dissolve after long time exposure to water. The samples were removed at specific intervals and weighed using a four-digit balance. The water content or water uptake (WU) of the samples was calculated as follows:

100 0 M ) 0 M t (M WU(%)    (1)
Where M t and M 0 are the weights of the sample after t min exposure to 98% RH and before exposure to high moisture content, respectively.

The mean moisture uptake of each sample was calculated for various conditioning times (t). The mass of water sorbed at time t, (M t -M 0 ), can be expressed as : 37

                    2 2 2 0 n 2 2 0 t L 4 t ) 1 n 2 ( D exp ) 1 n 2 ( 8 1 M M M (2)
where, M  is the mass sorbed at equilibrium, 2L the thickness of the polymeric film and D the diffusion coefficient. At short times, Equation (2) can be written as :

2 / 1 2 / 1 0 t t D L 2 M M M           (3)
At (Mt-M 0 /M  )  0.5, the error in using equation (3) instead of equation (2) to determine the diffusion coefficient is on the order of 0.1%. [START_REF] Vergnaud | Liquid Transport Process in Polymeric Materials: Modeling and Industrial Applications[END_REF] Microscopies. Transmission electron microscopy (TEM) observations were achieved with a Philips CM200 electron microscope operating at 80 kV. A drop of a dilute suspension of cellulose whiskers was deposited and allowed to dry on a carbon-coated grid, previously irradiated with a UV lamp.

Scanning electron microscopy (SEM) was performed to investigate the morphology of the nanocomposite films with a JEOL JSM-6100 instrument. The specimens were frozen under liquid nitrogen, then fractured, mounted, coated with gold/palladium on a JEOL JFC-1100E ion sputter coater, and observed. SEM micrographs were obtained using 7 kV secondary electrons.

Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (DSC)

was performed with a Perkin-Elmer DSC7 equipment, fitted with a cooler system using liquid nitrogen. It was calibrated with an indium standard. Conditioned samples were placed in pressure-tight DSC cells and at least three individual measurements were made to ensure reproducibility. Each sample was heated from -100 to +250°C at a heating rate of 10 °C/min. The melting temperature (Tm) was taken as the peak temperature of the melting endotherm while the glass transition temperature (Tg) was taken as the inflection point of the specific heat increment at the glass-rubber transition.

Contact angle measurements. Contact angle measurements were achieved in order to

evaluate the selective affinity of the natural polymers used (amylopectin and cellulose) with the plasticizers (water and glycerol). Solid pure amylopectin and tunicin whisker films were obtained by evaporation. Drops of glycerol or water were deposited on the solid polymeric surface. The contact angles were measured with a CCD camera and processed by an image analysis video card which calculated  (contact angle) automatically using an image analysis setup. This image analyzer determines the diameter, D, and the height, h, of the solvent droplet in order to evaluate the contact angle following Eq. ( 4).

) h 2 ( 2 tan         (4)
Measurements were performed by static mode on the starch surface and by dynamic mode on the cellulose surface. [START_REF] Jallu | TAGA Proceedings[END_REF][START_REF] Lanet | TAGA Proceedings[END_REF] The Owens-Wendt's approach [START_REF] Owens | [END_REF] was used to estimate the surface energy (polar and dispersive components) of the amylopectin and cellulose surface. It is worth noting that the results are only rough estimate of these values because, this method generally requires several (up to three) test solutions. For our experiments, only two solutions, water and glycerol, were used.

X-ray diffraction. Wide-angle X-ray scattering (WAXS) patterns were measured in reflection with a diffractometer using a static detector (SIEMENS D500). Conditioned films were mounted on a poly(methyl methacrylate) (PMMA) hollow support and sealed with a thin aluminum foil in order to preserve moisture conditions during the experiment as explained elsewhere. 31 Samples were exposed for a period of 20 s for each angle of incidence using a Cu  X-ray source with a wavelength of 1.5406 Å operating at 40 kV and 20 mA. The angle of incidence was varied between 8° and 40° by steps of 0.05°. Periodical distances (d) of the main peaks were calculated according to Bragg Law.

Results and discussion

Morphological characterization of tunicin whiskers. A transmission electron

micrograph obtained from a dilute suspension of tunicin whiskers is presented in figure 1a.

The suspension is constituted of individual cellulose fragments consisting of slender parallelepiped rods that have a broad distribution in size. These fragments have a length ranging from 500 nm up to 1-2 µm and they are almost 10 nm in width. The average aspect ratio (L/d, L being the length and d the diameter) of these whiskers is therefore around 50-200. It was estimated elsewhere 13,14 to be close to 70.

Morphological characterization of nanocomposite films.

In order to evaluate the morphology of the tunicin whiskers/plasticized starch materials, examination of the surface of fractured films was carried out using SEM. Figures 1b, 1c and1d show the fractured surface of unfilled plasticized starch and nanocomposite films filled with 6.2 and 25 wt% cellulose whiskers, respectively. By comparing the micrographs showing the surface of fracture of unfilled starch and of composites, it is easy to identify cellulose whiskers. In fact, tunicin whiskers appear like white dots. Their concentration is a direct function of the cellulose composition in the composite. These shinny dots correspond to the transversal sections of the cellulose whiskers. Their diameter was determined by SEM microscopy and were around 62  2 nm. This value is much higher than the whiskers diameter. This results from a charge concentration effect due to the emergence of tunicin whiskers from the observed surface. It is worth noting the homogeneous distribution of the filler (cellulose whiskers) within the matrix.

The good dispersion level of the filler within the matrix should result in optimal and improved mechanical performance of the composites as commented in the second part of the present article. 19 Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was used to determine the water content of the various samples. Figure 2 displays the evolution of the desorbed water content versus water activity upon temperature conditions imposed during TGA experiments and described in the experimental section for both unfilled and filled with tunicin whiskers plasticized starch. We ascertain that for all the samples, the desorbed water content increases as the water activity of conditioning saturated salt solutions (a w ) increases, but following different behaviors.

Three well separated zones are displayed in Figure 2. At low water activity (0 < a w < 0.35), the water content increases slightly and the curves corresponding to the different loading levels are very similar and tend to merge into a single curve. The water content is lower than 10 wt% whatever the composition may be. At intermediate water activity (0.35 < a w < 0.75), the water content increases more rapidly. This is ascribed to the fact that for this water activity range the glass-rubber transition of the plasticized starch matrix probably becomes lower than room temperature at which the samples were conditioned. The concomitant increase in the free volume generates an increased mobility of water molecules within the entangled amylopectin network. In this water activity range again, no obvious difference of behavior is observed as a function of the cellulose whiskers content and the water content varies from 10 to 25 wt% as the water activity varies from 0.35 up to 0.75.

However, for highly filled samples the water content seems to be lower than for poorly filled ones. This observation is more pronounced for a w = 0.75. This can be ascribed to either an increase of the crystallinity or to an increase of the glass-rubber transition temperature of starch material in the presence of cellulose whiskers. In both cases, it should result in a decrease of the mobility of water molecules. At high water activity (a w > 0.75), the desorbed water content continues to increase more rapidly and the previously difference reported between highly and poorly filled composites tones down.

Water Uptake. In sorption kinetics experiments, the mass of sorbed penetrant is measured as a function of time. The water uptake during exposure to 98% RH of the various cellulose whiskers/plasticized starch composites versus time was evaluated. It was observed that each composition absorbed water during the experiment. The diffusivity of water is strongly influenced by the microstructure of the material, such as the porosity that can develop during drying and also by the water affinity of the polymer components. 42 Moreover, the addition of plasticizers generally increases gas, water and solute permeability of the film. 43 The change in weight during conditioning at 98% RH is plotted against time in Figure 3.

These swelling data are means of several trials and the reliability of measurements was very good. Two well separated zones are displayed in Figure 3. At lower times (zone I : t < 100 hours), the kinetic of absorption is fast, whereas at extended times the kinetic of absorption is slow and leads to a plateau (zone II). In zone I, no clear trend is observed with respect to the cellulose whiskers content. In zone II, the water uptake reaches a plateau, which corresponds to the water uptake at equilibrium.

The water uptake at equilibrium versus cellulose composition is plotted in figure 4. It is observed that unfilled starch absorbs around 62% water. It corresponds to  1.6 g of water per gram of starch. The water uptake at equilibrium decreases as the tunicin whisker content increases. It is only  40% for the 25 wt% tunicin whiskers filled composite. Therefore, the swelling of the material is reduced in the presence of cellulose whiskers within the plasticized starch system. Similar results were reported with cellulose microfibrils filled starch. 11,12 This phenomenon was ascribed to the formation of a microfibril network, which prevented the swelling of the starch and therefore its water absorption. However, it is worth noting that the structure of cellulose whiskers completely differs from the microfibrils one. The former occur as rigid and geometrically well-defined rods, whereas the latter consist of soft and roughly individualized hairy-shaped fillers. The decreased water sensitivity of cellulose filled starch should consequently result, at least partially, from another phenomenon. This could include a decrease of the amylopectin chains mobility, resulting from an increase of the glass-rubber transition temperature or an increase of the crystallinity.

The water diffusivity or diffusion coefficient, D, of water in the starch-based material was estimated using Eq (3). The plots of (M t -M 0 )/  as a function of (t/L 2 ) 1/2 were performed for all the compositions and for (M t -M 0 )/    The diffusion coefficients were calculated from the slope of these plots. The water diffusion coefficients of the unfilled matrix and reinforced composites are collected in Table 1. The unfilled plasticized starch matrix displays the highest diffusion coefficient. Adding cellulose whiskers within the starch matrix results first in a decrease of D value from 1.76×10 -9 cm 2 .s -1 for unfilled starch up to 1.47×10 -9 cm 2 .s -1 for the 6.2 wt% filled system. This observation agrees with previous results obtained for cellulose microfibrils/starch composites. This phenomenon was ascribed to the presence of a three-dimensional intertwined cellulose microfibrils network within the matrix, resulting from the establishment of strong hydrogen bonds between cellulose microfibrils which can develop during the evaporation step. This network tends to stabilize the starch matrix when it is submitted to strong moisture conditions. At higher loading level, the previous trend is not clear and an inverse dependence with the whiskers addition is observed.

Thermal Analysis. Differential scanning calorimetry (DSC) measurements were performed on plasticized starch matrix and related tunicin whiskers filled composites conditioned at various relative moisture contents.

Starch/glycerol matrix. Figure 5 shows the DSC traces of glycerol plasticized starch matrix conditioned at 0 and up to 75% RH. All samples display two distinct ill-defined (as least for this heat capacity scale) specific heat increments. Expended views (not shown) of the apparently flat low temperature DSC traces were performed to precisely analyze these events.

The temperatures associated to the midpoints of these two calorimetric transitions are plotted in Figure 6 (filled circles) as a function of the water content.

The low temperature specific heat increment is located between -47 and -98°C depending on the moisture content. A relaxation process in glycerol plasticized potato starch was observed by Lourdin et al. 44 in this temperature range using dynamic mechanical analysis. It was assigned to the combination of a secondary relaxation of starch and the main relaxation of the water-glycerol mix. Secondary relaxation processes have been observed by several authors in a range of gelatinized and granular solid starches. They were assigned to either an increase in mobility of water in starch, 45 or to small motions of the chain backbone and rotation of methylol groups. 36 However, it is worth noting that secondary relaxations are not detectable by DSC. It was reported that at high glycerol content (12% and up), this relaxation process was mainly due to glycerol. 45 We suggest to assign this low temperature transition to the glass-rubber transition of glycerol-rich domains.

The high temperature specific heat increment is observed from 27 to -13°C depending on the moisture content. It is ascribed to the glass-rubber transition of amylopectin-rich domains. The waxy maize starch-glycerol matrix appears therefore to be a complex system composed of glycerol-rich and amylopectin-rich domains. The average composition of these two distinct domains can be estimated, at least for water-free material, using the Fox equation.

Taking the extrapolated value of Orford et al. 46 for Tg ( 500 K) of starch, and the experimental value for Tg of glycerol as determined from DSC measurements ( 194 K), it leads to glycerol-rich domains containing around 20 wt% amylopectin and to amylopectinrich domains composed of about 65 wt% amylopectin. These values have to be compared to the mean value, assuming a homogeneous distribution of glycerol within starch, of 67 wt% amylopectin. This obvious aberration clearly shows that the Fox equation is unsuitable for this polymer/solvent system. This inadequacy results from expected strong interactions between both components. Dynamic mechanical measurements performed on this system in the second part of this paper 19 support the idea that this heterogeneous system should be visualized as a blend composed of glycerol-rich domains included in an amylopectin-rich matrix.

Both events strongly depend on the water content and their temperature decreases as the moisture content increases, displaying a classical plasticizing effect of water. This phenomenon tends to stabilize at high moisture content, because no significant evolution of both transitions is observed between 15 and 25% water content.

For low water content plasticized starch (up to 35% RH), the flat shape of the DSC trace (Figure 5) is an indication of the amorphous state of the material. At increasing moisture content, an endothermal peak appears. Its temperature position first increases with water content, from  130°C for samples conditioned at 43% RH up to  155°C for samples conditioned at 58% RH. At increasing moisture content, it tends to stabilize. This first order transition is attributed to the melting of water-induced crystalline amylopectin domains.

Indeed, it is well known that during storage gelatinized starch can convert from a noncrystallized form to a crystalline form. This event, known as retrogradation, results from the reassociation of amorphous starch or starch with a low degree of ordering into a more ordered state. This phenomenon includes the formation of short-range ordering such as the formation of single and double helices, gelation, the formation of entanglements or juncture points, and the crystallization of aggregates of helical structures. This reorganization and crystallization of the amylopectin molecules is favored by the plasticization effect induced by water. 47,48 This increase in crystallinity of starches when submitted to increased moisture conditions was also supported by X-ray diffraction experiments. 31,49,50 At high water contents, the amylopectin is thought to form both inter-as intramolecular double helices. The displacement of the endothermal peak towards higher temperatures when the water content increases is probably due to the formation of larger crystal domains as a result of increased mobility of amorphous chains. In addition, increasing crystallinity of the amylopectin lowers the mobility of the amylopectin resulting in a reinforcement of the network by the formation of physical crosslinks and a stabilization of the retrogradation phenomenon.

Tunicin whisker/plasticized starch composites. The thermal behavior of tunicin whiskers-based composites was also characterized by DSC measurements. The DSC traces again show two glass-rubber transitions as, already, observed for the starch matrix. The temperature associated to the midpoints of these two calorimetric transitions are plotted in Figure 6 as a function of water content for the different compositions. From the knowledge of the thermal behavior of the unfilled glycerol plasticized starch matrix, the low and high temperature events were associated to the glass-rubber transition of glycerol-and amylopectin-rich domains, respectively.

Tg of the glycerol-rich fraction decreases as the moisture content increases, similarly to what was observed for the unfilled plasticized matrix. For the amylopectin-rich fraction, two distinct trends were observed depending on the tunicin whiskers content. At low loading level (up to 3.2 wt% tunicin whiskers), the classical plasticization effect of water is reported and Tg decreases as the water content increases following the behavior of the unfilled matrix. For higher cellulose content (6.2 wt% and up), Tg of amylopectin-rich domains significantly increases as the moisture content increases. The apparent unalteration of the evolution of Tg of the glycerol-rich fraction versus water content in the presence of cellulose whiskers results most probably from the fact that these domains occur as inclusions in the continuous phase constituted of amylopectin-rich domains. Therefore, the tunicin whiskers are most probably in direct contact with amylopectin-rich domains rather than glycerol-rich domains when dispersed in the plasticized starch matrix.

Three phenomena could explain the antiplasticization effect of amylopectin-rich domains in the presence of cellulose whiskers.

i) The first one is due to the likely strong affinity of amylopectin molecules with the reactive cellulose surface. Both components exhibit a high density of hydroxyl groups. This coupling effect could result in a restricted molecular mobility of amylopectin molecules in contact with the whiskers surface. Owing to the very high specific surface of tunicin whiskers, this hindered mobility could be strong enough to affect the global flexibility of the starch matrix.

ii) The second explanation could be the selective partitioning of glycerol within the material in the presence of cellulose whiskers. One can imagine that glycerol can present higher affinity for the cellulose surface than for the starch-based matrix. A migration of the main plasticizer from the amylopectin-rich domains towards the filler/matrix interface could result, decreasing the plasticizing efficiency of the glycerol for the starch matrix. This phenomenon should result in an increase of Tg and could be emphasized in moist conditions.

The selective partitioning of glycerol in polyvinyl alcohol (PVA) and hydroxypropyl methylcellulose (HPMC) blends was reported elsewhere. 51 iii) Another explanation could be the most likely different crystallization conditions of the glycerol-water-starch system in the presence of the omnipresent cellulose surface in tunicin whiskers-rich composites. A transcrystallization phenomenon was reported for poly(hydroxyoctanoate) in the presence of tunicin whiskers. 18 If such a transcrystallization occurs in the tunicin whisker/starch composites, one can think that it could result in a restricted mobility of amorphous amylopectin chains in the vicinity of the crystallite coated filler surface, because crystalline domains of amylopectin act as physical crosslinks. This hindered molecular mobility of amorphous chains in the amylopectin-rich domains should be emphasized with the water content as a result of the water-induced crystallization.

Though the three mentioned explanations could be involved simultaneously in the observed increase of Tg of the amylopectin-rich fraction of the plasticized starch matrix, an experimental feature supports the last one. Figure 7 shows the DSC traces of moist tunicin whisker/plasticized starch composites (58% RH conditioned samples) for different loading levels. Similarly to the unfilled matrix an endothermic peak attributed to the melting of waterinduced crystallites grows as the moisture content increases. This melting endotherm is observed whatever the whiskers content may be. The melting temperatures of these endothermic peaks are collected in Table 2 for all the samples. The heats of fusion were not calculated because of the strong dubiousness for the determination of the baseline.

It is worth noting that the melting endotherm of filled materials exhibits a shoulder on the low temperature side (Figure 7). This shoulder was also observed for 75% RH conditioned samples. This splitting of the melting endotherm results from the presence of a bimodal distribution of crystallite size. Cellulose most probably acts as a nucleating agent for amylopectin, producing a transcrystalline region around the cellulose whiskers. Orientated crystallization of amylose from a solution on cellulose was previously observed. 52,53 It was shown that the lamellar crystals of amylose grew exclusively on the cellulose to give a "shishkebab" morphology, consisting of a regular system of edge-on amylose crystals organized perpendicular to the cellulose microfibrillar direction. Such a behavior was assigned to a row nucleation phenomenon rather than to a true epitaxial growth. However, the high viscosity of amylopectin-rich domains surrounding the tunicin whiskers limits this phenomenon and restricts the growth of the "shish kebab" structure.

Contact angle measurements. The contact angle technique was used in order to

quantitatively characterize the affinity of water and glycerol for the amylopectin and cellulose phases. The contact angles measured according to the technique described in the experimental part are collected in Table 3 for the two liquids used (water and glycerol) and the two surfaces (amylopectin and tunicin whisker films). Contact angle values clearly show that both glycerol and water display a higher affinity for the cellulose whisker film surface than for the amylopectin one. The surface energy, as well as the polar and dispersive component values, of the two films were calculated according to the Owens-Wendt's approach 41 (Table 4). For both substrates, the polar component is much higher than the dispersive one. This is ascribed to the high density of hydroxyl groups of both polysaccharides. The surface energy is higher for cellulose than for amylopectin.

These observations tend to show that the localization of the two plasticizers (glycerol and water) is most obviously not homogeneous in the tunicin whiskers/starch composites.

They probably redistribute within the matrix, diffusing towards the cellulose surface. This relocalization decreases the plasticizing effect of glycerol and water in the bulk amylopectin matrix. The main consequence of this redistribution should be the shift of the glass-rubber transition of the amylopectin-rich phase towards higher temperatures in the composites. The accumulation of plasticizer in the cellulose/amylopectin interfacial zones could, in turn, improve the ability of amylopectin chains to crystallize leading to the formation of a transcrystalline zone around the whiskers. which crystallinity should not be changed when dispersed in the matrix, this new peak arises most probably from amylopectin. This specific crystallinity occurs, or at least is detectable, in amylopectin only in highly moist conditions and in the presence of a high tunicin whiskers content.

Wide

In order to yield the specific character of this peak, a modeling of the X-ray diffractograms was performed from the combination of the diffractograms of the pure parent components. A simple mixing rule was used to build up the theoretical diffractograms. Both experimental and theoretical wide angle X-ray diffraction patterns are shown in Figure 10a, 10b, 10c, and 10d for highly filled nanocomposites (25 wt% tunicin whiskers), conditioned at 35, 43, 58 and 75% RH, respectively. The theoretical curve fits very well the experimental data at 35% RH (Figure 10a). At higher moisture content (43 and 58% RH, Figures 10b and10c), the fit is satisfactory, except for the cellulose peak located around 2 = 14.6°, for which the predicted magnitude is much higher than the experimental one. This probably results from the different orientation of tunicin whiskers within the film obtained from only cellulose and from the composite. The whiskers distribution is most likely random in the composite contrarily to the reference whisker films, which is composed of in-plane oriented filler. The same trend is reported between the experimental and predicted X-ray diffraction patterns of composites conditioned at 75% RH (Figure 10d), but the most significant difference between these two diffractograms is the total absence of the peak at 2 = 21.15° (d = 4.2 Å) in the predicted diffractogram. Similar differences between experimental and predicted X-ray diffraction patterns were obtained for the glycerol plasticized starch filled with 16.7 wt% tunicin whiskers.

This observation could probably be interpreted as an interfacial effect in relation with the shoulder observed on the low temperature side of the melting endotherm by DSC for similar conditions. This again could be interpreted as a transcrystallization phenomenon of amylopectin on the surface of the cellulose whiskers. The whiskers could act as nucleation points, and owing to the steric obstacles due to the high whiskers content, distorsion of amylopectin crystallites occurs. However, the experimental evidence of the transcrystallinity can not be displayed by classical techniques, such as optical microscopy because of the dimensions of the tunicin whiskers.

Regarding the nature of the crystallized structure giving the diffraction peak at 2 = 21.15°, it is doubtful. However, it is well known that amylopectin crystallizes readily and forms complexes with glycerol detectable by X-ray diffraction or other methods. The peak at 2 = 21.15° could probably be assigned to a glycerol-starch V structure, but it corresponds to not any reported structure. Since the V form is a single helix, it appears that the 4.2 Å is not from the double helix B form. Further experiments are necessary to confirm this hypothesis.

Conclusions

Nanocomposite materials were obtained from glycerol plasticized waxy maize starch as the matrix and a suspension of tunicin -an animal cellulose-whiskers as a model reinforcing phase. The unfilled matrix appears as a complex heterogeneous system composed of glycerolrich domains dispersed in an amylopectin-rich continuous phase. Each phase exhibits its own glass-rubber transition, for which the temperature decreases as the moisture content increases owing to the plasticizing effect of water. This lowering of Tg induces the crystallization of the matrix at room temperature (retrogradation phenomenon) when the water content increases.

Significant changes occur in the composite systems when tunicin whiskers are homogeneously dispersed in this complex matrix. All results lead to the conclusion that both plasticizers (glycerol and water) redistribute within the matrix, diffusing towards the cellulose surface. This relocalization effect decreases the plasticizing effect of glycerol and water in the

D

  -Angle X-ray Scattering (WAXS). The nanocomposite films resulting from the casting and evaporation of gelatinized starch were characterized by WAXS. X-ray diffractograms were collected for the different water contents and loading levels in order to determine the evolution of the crystallinity. Starch/glycerol matrix. Wide angle X-ray diffraction patterns of the unfilled plasticized starch matrix conditioned at 35 and up to 75% RH are presented in Figure 8. The diffractogram recorded for a film of pure cellulose obtained from the evaporation of a tunicin whiskers suspension is added in figure 8. At low moisture content, the starch film shows no diffraction peak and displays typical behavior of a fully amorphous polymer. It is characterized by a broad hump located around 2 = 18°. This result agrees with DSC measurements. As the water content increases, the amorphous broad hump shades off progressively and three ill-defined diffraction peaks, which grow with moisture content, are observed. Their angular locations around 2 = 17.2, 22.1 and 23.9° are typical of B-starch structure. 54 The crystalline regions consist therefore of double helices with a loose packing density in the unit cell, water being an integral part of this polymorph. The d-values associated to these peaks are 5.20, 4.01 and 3.72 Å, respectively. The semicrystalline nature of moist plasticized waxy maize starch films evidenced by WAXS experiments agrees with DSC results. The tunicin whiskers film displays three well-defined peaks around 2 = 14.6, 16.4 and 22.7°. The dvalues associated to these peaks are 6.06, 5.40 and 3.91 Å, respectively. They are typical of cellulose I. The relative magnitude of these peaks depends on the whiskers orientation in the film. Tunicin whisker/plasticized starch composites. Diffractograms of the highly moist (75% RH) nanocomposite materials are shown in Figure 9. Diffraction patterns of unfilled plasticized starch and tunicin whiskers films are added as references. The diffractograms of the various cellulose/starch composites consist in a superimposition of the diffractograms of the two parent components balanced by the composition. However, it is worth noting that for the films filled with 16.7 and 25 wt% of cellulose whiskers, a new well-defined diffraction peak located around 2 = 21.15° (corresponding to the vertical line) is observed, whose intensity increases with cellulose content. The d-value associated to this peak is 4.2 Å. This diffraction peak arises neither from the unfilled plasticized starch nor from the tunicin. Because tunicin whiskers are well-defined objects,

  bulk amylopectin matrix, resulting in an increase of the Tg of amylopectin-rich domains. The accumulation of plasticizer in the cellulose/amylopectin interfacial zones improves the ability of amylopectin chains to crystallize leading to the formation of a possible transcrystalline zone around the whiskers. These specific crystallization conditions have been evidenced at high moisture content and high whiskers content (> 16.7 wt%) by DSC and WAXS. It is displayed through a shoulder on the low temperature side of the melting endotherm and the observation of a new peak in the X-ray diffraction pattern. This transcrystalline zone could originate from a glycerol-starch V structure. This inherent restricted mobility of amylopectin chains most likely accounts for the lower water uptake of cellulose/starch composites for increasing filler content.

FiguresFigure 1 .

 1 Figures

  Figure 1. (a) Transmission electron micrograph from a dilute suspension of tunicin whiskers.

Figure 2 .

 2 Figure 2. Comparison of water content determined from TGA experiments versus water

Figure 3 .

 3 Figure 3. Water uptake during conditioning at 98% RH versus time for glycerol plasticized

Figure 4 .

 4 Figure 4. Maximum relative water uptake, or water uptake at equilibrium, during

Figure 5 .

 5 Figure 5. DSC thermograms of glycerol plasticized waxy maize starch for various moisture

Figure 6 .

 6 Figure 6. Glass-rubber transition temperatures associated to the midpoints of the transitions

Figure 7 .

 7 Figure 7. DSC thermograms of 58% RH conditioned tunicin whisker/glycerol plasticized

Figure 8 .

 8 Figure 8. Wide-angle X-ray diffraction patterns of glycerol plasticized waxy maize starch for

Figure 9 .

 9 Figure 9. Wide-angle X-ray diffraction patterns of 75% RH conditioned tunicin whisker/

Figure 10 .

 10 Figure 10. Experimental and predicted wide-angle X-ray diffraction patterns of 25 wt% filled

Table contents

 contents

	Whiskers content	Water diffusion coefficient
	(wt%)	(cm 2 /s x 10 9 )
	0	1.76
	3.2	1.53
	6.2	1.47
	16.7	1.69
	25	1.59

Table 1 .

 1 Water diffusion coefficients in cellulose whiskers/plasticized starch composites conditioned at 98% RH.

	Whisker Content (wt%)		Relative Humidity (%)
		0	35	43	58	75
	0	-	-	132.6	156.9	156.2
	3.2	-	-	132.2	158.4	156.2
	6.2	-	-	131.2	165.1	155.9
	16.7	-	-	132.0	160.0	156.7
	25	-	-	134.7	169.5	158.4

Table 2 .

 2 Melting temperatures (°C) of tunicin whiskers/plasticized starch nanocomposite films conditioned at different moisture contents.

Acknowledgements

The authors gratefully acknowledge Roquette S.A. for supplying waxy maize starch, Dr. 22 H. Chanzy for stimulating discussions, Dr. M. Paillet and Y. Minot for their help in film processing, Mr. L. Fouchet for his help in contact angle, Mrs. D. Dupeyre and Mr. R. Vuong for their help in SEM and TEM microscopy, respectively, and Zebda for moral support. The authors are indebted to ADEME (Agence Française de l'Environnement et de la Maîtrise de l'Energie) for financial support (ADEME/CNRS convention # 99 01 033).

Substrate films

Water Glycerol Amylopectin 64 72

Cellulose whiskers 24 55


François Potdevin
email: francois.potdevin@univ-lille2.fr

Olivier Vors

Aurore Huchez

M Lamour

Keith Davids

Christophe Schnitzler

M Huchez

K Lamour

C Davids

Schnitzler

How can video feedback be used in physical education to support novice learning in

Keywords: Feedback, pedagogy, videobased technology, learning, self-regulation

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

HAL Id: hal-01977966 https://hal.archives-ouvertes.fr/hal-01977966 Submitted on 4 Jul 2019

Introduction

Feedback is inextricably linked to processes of learning and teaching [START_REF] Bangert-Drowns | The Instructional Effect of Feedback in Test Like Events[END_REF] and its use during the teaching process has been the focus of many studies [START_REF] Georges | Les feedback à l'école: un gage de régulation des comportements scolaires[END_REF]. The research specificity of feedback in physical education (PE) lies in its ensuing effect on learning and performance of motor skills. Feedback may be defined as the return of performance information occurring within a behavioural regulation loop, where error detection and correction are essential to motor learning [START_REF] Mulder | Delayed Sensory Feedback in the Learning of a Novel Motor Task[END_REF][START_REF] Schmidt | Motor Control and Learning: A Behavioral Emphasis[END_REF]. Literature investigating feedback has become extremely rich since the establishment of the cybernetic approach to learning [START_REF] Wiener | Cybernetics, or Control and Communication in the Animal and the Machine[END_REF]. Subsequently, a large amount of empirical research in the field of motor learning has emerged over 50 years, providing rich insights on the role of feedback on performance, learning and behaviour change (e.g. [START_REF] Bilodeau | Motor Skills Learning[END_REF][START_REF] Bilodeau | Information Feed-Back[END_REF][START_REF] Brunelle | L'efficacité de l'intervenant dans l'enseignement de l'activité physique[END_REF][START_REF] Piéron | Recherche de critères d'efficacité de l'enseignement d'habiletés motrices[END_REF][START_REF] Brunelle | Analyse des feedback émis par des maîtres de l'enseignement de la danse moderne[END_REF][START_REF] Brunelle | Analyse du climat pédagogique par le biais des réactions des éducateurs physiques en situation d'enseignement[END_REF][START_REF] De Knop | Effectiveness of Tennis Teaching[END_REF].

Various types of feedback have been identified in pedagogical research in PE setting, such as augmented feedback [START_REF] Fishman | Augmented Feedback[END_REF], information feedback [START_REF] Newell | Therapeutic Intervention as a Constraint in Learning and Relearning Movement Skills[END_REF], congruent feedback [START_REF] Rink | Effective Instruction in Physical Education[END_REF], aligned developmental feedback [START_REF] Cohen | The Effectiveness of Aligned Developmental Feedback on the Overhand Throw in Third-Grade Students[END_REF] or interrogative feedback [START_REF] Swalus | Feedback en cours d'apprentissage de tâches motrices et leur perception par les élèves[END_REF][START_REF] Driouch | Les feed-back émis par les enseignants lors des situations d'enseignement-apprentissage[END_REF]. Research in motor learning and sport pedagogy reports that feedback has been found to enhance the acquisition of fine and gross motor skills (see [START_REF] Young | Augmented Kinematic Feedback for Motor Learning[END_REF][START_REF] Wrisberg | Sport Skill Instruction for Coaches[END_REF][START_REF] Schmidt | Motor Learning and Performance -A Situation-Based Learning Approach[END_REF]) and indicated that it is one of the most powerful instructional variables affecting skill learning.

More recently, technological progress has led sports pedagogists and physical educators to reexamine strategies for providing movement-related feedback and experiment with new learning aids based more particularly on use of video feedback (VFB; [START_REF] Rucci | Three Types of Kinematic Feedback and the Execution of the Hang Power Clean[END_REF]. VFB can be defined as the playback to a learner of his/her own (static and dynamic) image in action. It is an extrinsic or augmented source of feedback [START_REF] Schmidt | Motor Control and Learning: A Behavioral Emphasis[END_REF], since it involves additional information related to one's own actions that are not available without the use of an external aid. It differs from 'intrinsic' feedback, which represents information that is detectable without external aids. VFB can be used to guide the actions of learners who find it difficult to interpret intrinsic feedback or who have less stable movement patterns [START_REF] Swinnen | Information Feedback for Motor Skill Learning: a Review[END_REF][START_REF] Hodges | The Role of Video in Facilitating Perception and Action of a Novel Coordination Movement[END_REF].

The role of VFB in motor learning has been investigated by two different theoretical frameworks over the last two decades. According to [START_REF] Swinnen | Information Feedback for Motor Skill Learning: a Review[END_REF], in one approach, the role of augmented feedback has been undertaken in investigations of movement parametrization involving specific timing or force requirements. Concepts of information processing theory have been used to explain its role in a regulation loop to calibrate or reinforce the use of a general motor programme [START_REF] Schmidt | A Schema Theory of Discrete Motor Learning[END_REF]. Since the conceptualization of [START_REF] Newell | Motor Skill Acquisition[END_REF] and [START_REF] Handford | Skill Acquisition in Sport: Some Applications of an Evolving Practice Ecology[END_REF], in an ecological dynamics approach to skill acquisition, an increased interest in the learning of segmental coordination has been developed to understand the role of augmented feedback. According to Al-Abood, Davids, and Bennett ( 2001), the ecological approach considers VFB as a type of instructional constraint which guides a learner during the search for functional task solutions in specified areas of a perceptual work motor space. In this theoretical framework, a constraint is considered as a key task variable which can be manipulated in learning design to help the learner in his/her exploration of innovative movement solutions. More recently, the non-linear pedagogy approach has suggested the need to consider feedback, not to prescribe movement solutions, but to encourage exploration of learning strategies to exploit natural self-organization processes that emerge during practice [START_REF] Renshaw | A Constraints-led Perspective to Understanding Skill Acquisition and Game Play: A Basis for Integration of Motor Learning Theory and Physical Education Praxis?[END_REF][START_REF] Chow | Nonlinear Pedagogy in Skill Acquisition: An Introduction[END_REF]. In both theoretical frameworks, VFB is considered as an essential strategy for facilitating the acquisition of new motor skills by facilitating learners' adaptations during practice.

In this respect, numerous studies have demonstrated the effectiveness of VFB in the acquisition of various sports skills over relatively short learning periods, such as the golf swing [START_REF] Guadagnoli | The Efficacy of Video Feedback for Learning the Golf Swing[END_REF], flip turns in swimming [START_REF] Hazen | A Videotaping Feedback Package for Improving Skills of Youth Competitive Swimmers[END_REF], gymnastics [START_REF] Winfrey | Effects of Self-Modeling on Self-Efficacy and Balance Beam Performance[END_REF][START_REF] Mérian | Le Feedback Vidéo en Education Physique Scolaire[END_REF], soccer skills [START_REF] Ziegler | The Effects of Attentional Shift Training on the Execution of Soccer Skills: A Preliminary Investigation[END_REF], high jump [START_REF] Mérian | Le Feedback Vidéo en Education Physique Scolaire[END_REF], diving [START_REF] Thow | Comparison of Modes of Feedback on Glide Performance in Swimming[END_REF], hang power clean in weightlifting [START_REF] Rucci | Three Types of Kinematic Feedback and the Execution of the Hang Power Clean[END_REF], spike jump in volleyball [START_REF] Parsons | Modifying Spike Jump Landing Biomechanics in Female Adolescent Volleyball Athletes Using Video and Verbal Feedback[END_REF] and hurdling [START_REF] Palao | The Impact of Video Technology on Student Performance in Physical Education[END_REF]. While results have highlighted the effectiveness of providing VFB on motor learning, the way it was used in studies varied depending on learning contexts. Since the study of [START_REF] Kernodle | Information Feedback and the Learning Multiple-Degree-of-Freedom Activities[END_REF], results from research have shown that a combination of VFB, attentional information (focusing on a specific point of the movement) and verbal instructions represents a most functional pedagogical strategy for optimizing search activities during learning [START_REF] Janelle | Maximising Performance Effectiveness Through Videotape Replay and a Self-Controlled Learning Environment[END_REF][START_REF] Rucci | Three Types of Kinematic Feedback and the Execution of the Hang Power Clean[END_REF].

However, in the extant literature, important questions remain on the amount of feedback required for optimizing learning. While increasing the quantity of feedback promotes learning [START_REF] Wulf | Reduced Feedback Frequency Enhances Generalized Motor Program Learning But Not Parameterization Learning[END_REF], going beyond a certain limit leads to the opposite effect [START_REF] Wulf | Reducing Knowledge of Results About Relative Versus Absolute Timing: Differential Effects on Learning[END_REF]. Relative reduced frequency feedback (delivery of feedback after every two or more attempts) is as effective for learning as total frequency [START_REF] Lee | On the Role of Knowledge of Results in Motor Learning: Exploring the Guidance Hypothesis[END_REF][START_REF] Winstein | Reduced Frequency Knowledge of Results Enhances Motor Skill Learning[END_REF][START_REF] Sparrow | Performance on Trials Without Knowledge Results (KR) in Reduced Relative Frequency Presentations of KR[END_REF]. According to [START_REF] Wulf | Understanding the Role of Augmented Feedback: the Good, the bad, the Ugly[END_REF], total frequency feedback can create dependence on extrinsic feedback in the long term by inhibiting the development of a learner's capacity to interpret intrinsic informations. [START_REF] Wulf | Understanding the Role of Augmented Feedback: the Good, the bad, the Ugly[END_REF] showed that relative frequency of feedback every five attempts was more effective than total frequency feedback.

How can PE specialists make sense of this laboratory-based research to enhance their everyday practice? Providing PE teachers with an increased number of digital tablets has led them to create learning aids based on presentation of VFB [START_REF] Gubacs-Collins | The Mobile Gymnasium[END_REF][START_REF] Kretschmann | Effect of Physical Education Teachers' Computer Literacy on Technology Use in Physical Education[END_REF]. Nevertheless, studies seeking to measure the impact of these aids in real-life PE teaching programmes are rare in comparison with sport settings [START_REF] Ste-Marie | Observation Interventions for Motor Skill Learning and Performance: an Applied Model for the Use of Observation[END_REF][START_REF] Palao | The Impact of Video Technology on Student Performance in Physical Education[END_REF]. Reasons for this void in the literature may include the lowest number of students in sport training groups or that athletes and coaches theoretically have greater levels of investment in specific skill improvements [START_REF] Guadagnoli | The Efficacy of Video Feedback for Learning the Golf Swing[END_REF][START_REF] Smith | Biomechanics Feedback for Rowing[END_REF], whereas PE teachers may emphasize different aims such as motor, cognitive, social, moral, spiritual or cultural development [START_REF] Sallis | Physical Education's Role in Public Health[END_REF]. Additional disincentives for PE teachers to assess the efficacy of VFB may relate to time-consuming pressures or economic issues [START_REF] Norris | Examining 25 Years of Technology in U.S. Education[END_REF][START_REF] Weir | The Use of Digital Video in Physical Education[END_REF].

Yet, several studies have shown the potential of using VFB in PE teaching to improve the effectiveness of demonstrations [START_REF] Lhuisset | The Influence of Live vs. Video-Model Presentation on the Early Acquisition of a new Complex Coordination[END_REF] for enhancing skill learning, knowledge and game understanding [START_REF] Blomqvist | Comparison of Two Types of Instruction in Badminton[END_REF][START_REF] Koekoek | Aligning Digital Video Technology with Game Pedagogy in Physical Education[END_REF]. Studies seeking to assess the specific effect of VFB on motor skill acquisition in a PE setting at different education levels have shown its effectiveness when it was coupled with teacher feedback [START_REF] Mérian | Le Feedback Vidéo en Education Physique Scolaire[END_REF][START_REF] Uhl | Dartfish Video Analysis in Secondary Physical Education: A Pilot Study[END_REF][START_REF] Potdevin | Le feedback vidéo en EPS: une double stratégie de progrès et de motivations. Le cas de l'Appui Tendu Renversé en classe de 6ème[END_REF][START_REF] Amara | Effect of Video Modeling Process on Teaching/Learning Hurdle Clearance Situations on Physical Education Students[END_REF][START_REF] Kretschmann | Employing Tablet Technology for Video Feedback in Physical Education Swimming Class[END_REF]. No effects have been observed when VFB was provided without the instruction as well [START_REF] Madou | Integrating Live Delayed Video Feedback Using Mobile Devices into a Real Life Physical Education Setting[END_REF].

To our knowledge, the few studies, which have sought to explore the impact of VFB on the learning experiences in PE setting have examined perceptions of learning using qualitative approaches [START_REF] Palao | The Impact of Video Technology on Student Performance in Physical Education[END_REF]. [START_REF] Kretschmann | Employing Tablet Technology for Video Feedback in Physical Education Swimming Class[END_REF] used a semi-structured interview methodology with students of 10 years of age, suggesting that they found VFB helpful for the learning process in swimming. With the same methodology, O 'Loughlin, Chroinin, and O'Grady (2015) showed that VFB positively influenced self-reported motivation, self-assessment and engagement when learning basketball skills in students aged 9-10 years. Also, [START_REF] Casey | Using Digital Technology to Enhance Student Engagement in Physical Education[END_REF] showed the effectiveness of using VFB in enhancing engagement with disaffected Year 7 students who developed greater depth of knowledge about throwing and catching skills. Others studies have confirmed a positive effect of VFB on motivation during PE learning [START_REF] Weir | The Use of Digital Video in Physical Education[END_REF][START_REF] Potdevin | Le feedback vidéo en EPS: une double stratégie de progrès et de motivations. Le cas de l'Appui Tendu Renversé en classe de 6ème[END_REF][START_REF] Backaberg | Video-supported Interactive Learning for Movement Awarness -A Learning Model for the Individual Development of Movement Performance Among Nursing Student[END_REF]. According to Deci andRyan's Self-Determination Theory (SDT, 1985, 1991), information provided by VFB enhances perceived control of actions to be implemented and positively influences intrinsic motivation, which is vital for successful learning [START_REF] Horn | The Influence of Teacher-Coach Behavior on the Psychological Development of Children[END_REF][START_REF] Horn | Leadership Effectiveness in the Sport Domain[END_REF]. Self-assessment tasks have been identified as a key pedagogy to enhance student achievement and motivation [START_REF] Hallam | Evaluation of Project 1 of the AiFL Development Programme: Support for Professional Practice in Formative Assessment[END_REF][START_REF] Cauley | Formative Assessment Techniques to Support Student Motivation and Achievement[END_REF] by supporting learners' regulation of their own learning. To our knowledge, no study has explored the multiple effects of VFB on skill acquisition, self-assesment competencies and motivation using quantitative data under the task constraints of a structured, school-based PE programme.

The aim of this study was, therefore, to assess the effects of a methodology combining VFB, attentional information and verbal instructional constraints on the learning of a gymnastics skill, motivation during learning and student self-assessment ability. The assessment took place in lessons undertaken during an actual school PE programme under typical teaching conditions. We sought to examine whether the use of VFB would impact positively on motor learning, self-assessment and motivation in children during learning in PE lessons.

Methods

Participants

Two classes of Year 7 pupils from the same French secondary school took part in the study during their gymnastics PE lessons. The two classes of students were considered by their teachers to be autonomous and motivated during PE lessons. VFB was offered to one class who acted as the experimental group, composed of 18 pupils (10 girls and 8 boys, age = 12.4 ± 0.5 years old). The other class (control group) included 25 pupils (12 girls and 13 boys, age = 12.6 ± 0.4 years old). During the investigation, two pupils from each group were not present for one lesson. Informed consent was obtained from the students and the family of each participant concerning the nature of the research and the use of video images during lessons for the purposes of studying effects on learning. The Ethics Committee of the French Ministry of Education approved the research project on the condition that the study did not disrupt teaching or timetabling within the school day.

Protocol

Both classes followed the same lessons plan over a period of five weeks at the rate of one 2-hour lesson per week. This sequence represented the normal exposure to PE classes in the school timetable for participants. During each of the five lessons, participants carried out the same warm-up routine and exercises. They then performed an identical number of attempts per exercise (15 attempts per exercise) to ensure a similar frequency of practicing the specific actions. Pupils were divided into groups of four to five for each exercise, and each group took turns to perform all of the suggested exercises. Five different working zones were organized around the centre of the gymnasium, so that the teacher was able to supervise activity in each of them, when standing near the VFB zone. After the pupils had completed their 15 trials, they were required to sit and wait for a signal to go to the next working zone. Written instructions informed the pupils about the study and about that task they were required to perform in each zone. The pupils also had to put a mark on a board after each trial and assess their performance according to the task instructions. The lesson was organized so that each pupil had the time and opportunity to perform every exercise. At the same time, the methodology allowed the teacher to pay more attention to the five students in the VFB zone.

The front handstand flat back exercise was part of each lesson and represented the only exercise where the participant's body was turned upside down. All students had no scholar or gymnastics club previous experience of activities that involved placing the body into a vertically aligned position. The aim of this exercise was for pupils to vertically align their bodies in an inverted vertical position (arms-trunk-legs), before letting themselves fall onto their back, keeping their bodies aligned until they hit the mat. During each lesson, pupils in both groups attempted the exercise 15 times. In other words, each participant experienced 75 attempts over the five-week period.

Pupils in the experimental group were provided with VFB for this specific exercise (Figure 1) during all five lessons. An intermittent feedback frequency schedule was implemented by the teacher (feedback provided after every five trials, rather than after every trial to allow participants to use intrinsic feedback for the first four trials). Feedback provision was as follows: at the end of the fifth attempt, each pupil was asked to answer the following question 'Do you think you were in a straight line during this attempt?' He/she was given 20 seconds to answer the question after being moved away from the group. The pupil then received VFB on his/her performance while watching it on a computer screen. The teacher froze the image just as the hips projected a vertical line with the shoulder and captured the angle (arms-trunk) as the pelvis was vertically aligned with the shoulders. The teacher then discussed the pupil's response with him/her, before providing technical advice on how to achieve the task goal. Following the feedback session, the pupil made four more attempts without VFB, then received VFB for the second time after the 10th attempt. This time, he/she was asked an additional question: 'Was your attempt better than the last time you watched it?' This procedure was repeated up to the 15th attempt, when the pupil received VFB for the third time and had to answer the two questions. The control group followed the same protocol, but only the teacher had access to the video and did not show it to the pupils. The teacher provided only verbal feedback to the participants during learning experiences.

Material

A tripod-mounted video camera (Sportcam/webcam DV 16) was connected to a laptop (Packard Bell) using a USB cable and transmitted live images to the screen. The video analysis software Kinovea was used to freeze-frames and to visually capture and measure the arm-trunk angle of each participant in the experimental group when performing the required action. The video camera was placed 3 m from the area on the floor where the student would lay place his/her hands when performing the action. The camera captured sagittal views of the participants who were required to put their hand in a 50 cm × 70 cm marked surface on the floor to limit parallax effects of image observers.

Data collection

In order to assess progress in motor skills, the arm-trunk angle (hand-shoulder-hip) was digitally video-recorded and measured in the sagittal plane just as the hips formed a vertical line with the shoulders during the 5th, 10th and 15th attempt for the pupils in both groups. In previous work, [START_REF] Potdevin | Le feedback vidéo en EPS: une double stratégie de progrès et de motivations. Le cas de l'Appui Tendu Renversé en classe de 6ème[END_REF] successfully used this angle value in order to assess motor learning in this specific task for beginner pupils aged 12 years. Unobtrusive markers at the wrist, shoulder and hip were fixed on the participant. The camera was positioned to film the participant in a sagittal plane. The arm-trunk angle was defined by these three markers and measured by two experimenters. The mean of these three attempts was calculated for each participant in each lesson.

Motivation was assessed using the Situational Motivation Scale questionnaire (SIMS; [START_REF] Guay | On the Assessment of Situational Intrinsic and Extrinsic Motivation: The Situational Motivation Scale (SIMS)[END_REF] during lessons 1 and 5 for both groups. This instrument identifies the three dimensions of motivation: intrinsic, extrinsic (identified and external) and amotivation.

Self-assessment ability corresponds here to the ability to perceive one's body in action. It is measured by the ability to judge one's own performance and progress. As already mentioned, this self-evaluation process required pupils to answer a question in each lesson after the fifth attempt: 'Do you think you were in a straight line during this attempt?' as well as an additional question after the 10th and 15th attempts: 'Was your attempt better than the last time you watched it?' Finally, participants' self-assessment ability was evaluated via the five answers given each lesson, where each correct answer was awarded a point (resulting in a score out of five points).

Statistical analysis

Inter-rater reliability between the two experimenters for the measurement of 'arm-trunk angle' was tested using the intraclass correlation coefficient (ICC) according to the recommendations of [START_REF] Shrout | Intraclass Correlation: Uses in Assessing Rates Reliability[END_REF].

Mean and standard deviation values were calculated for each lesson and for each group for the 'arm-trunk angle' and 'self-assessment ability' variables; and at the first and the fifth lesson for each group for the different psychometric scores (intrinsic motivation, identified regulation, external regulation and amotivation). When normal Gaussian distribution and sphericity of the data were verified by Shapiro-Wilk's and Mauchley tests, a two-way ANOVA (group × time) and a Bonferroni post-hoc test were used. Otherwise, the Scheirer Ray Hare test (group × time) and a Wilcoxon posthoc test with Bonferroni corrections were used.

All statistical procedures were performed using the STATISTICA software. For all post-hoc significant differences, effect size (ES) was measured according to Cohen's scale (1992): absolute ES values of <0.2 represent small treatment differences, approximately 0.5 values represent moderate treatment differences, and >0.8 represent large treatment differences The statistical significance levels were fixed at p < .05, p < .01 and p < .001).

Results

Significant improvement in motor skill performance from the fifth lesson onwards and enhanced self-assessment ability from the second lesson was observed in the experimental group. Similarly, a drop in amotivation scores between the first and the fifth lesson was revealed in the experimental group only.

Arm-trunk angle progression

The ICC value between measurements of the two experimenters was 0.98 and mean differences were 2.9 ± 2.7 degree. Arm-trunk angle progression for each pupil can be seen in Figure 2. Statistical analysis showed significant interaction effects (group × time; F (4, 148) = 3.45; p < .05) for the armtrunk angle. Significant differences were shown for the experimental group between the fifth lesson and all other lessons. Between lessons 4 and 5, the arm-trunk angle increased significantly from 146.6 ± 16.9 degrees to 161.2 ± 14.2 degrees (p < .001; ES = 0.94).

Self-assessment ability

Changes in vertical alignment self-assessment scores can be seen in Figure 3. Statistical analysis revealed significant group-time interaction effects (H (4, 148) = 173.19, p < .001). Post-hoc analysis showed significant paired differences for the experimental group only, with self-assessment scores being significantly higher for lesson 5 than for lesson 4 (p < .01, ES = 0.94) and lesson 1 (p < .001, ES = 2.51) and between lesson 1 and lesson 2 (p < .01, ES = 1.79).

Changes in motivation scores

Motivation scores are presented in Table 1. Results showed significant interaction effects (group × time) for intrinsic motivation (H (1, 40) = 69.4; p < .01) and for amotivation (F (1, 40) = 12.55, p < .001). Amotivation decreased significantly for the VFB group between lesson 1 and lesson 5 (3.06 ± 1.42 vs. 2.12 ± 0.62, p < .001, ES = -0.89).

Discussion

The aim of this study was to assess the effects of a VFB-based learning aid implemented in a series of five lessons in a PE programme to evaluate effects on the learning experiences. In that way, we evaluated the learning of the gymnastics skill, motivation during learning and self-assessment ability in real-life teaching conditions, rather than an experimental laboratory.

Significant development of motor skills and self-assessment ability

Results showed significant progress in motor skills between the first and fifth lessons for the experimental group. Arm-trunk angle values in the first lesson were consistent with data reported by [START_REF] Potdevin | Le feedback vidéo en EPS: une double stratégie de progrès et de motivations. Le cas de l'Appui Tendu Renversé en classe de 6ème[END_REF], confirming the novice level of the participants. No significant changes were observed in the arm-trunk angle between lessons 1 and 4. But there was a substantial increase in this angle between lessons 4 and 5 (ES = 0.94), suggesting the nonlinearity of the transitions in the learning process between lessons. This result is consistent with numerous studies highlighting the nonlinear nature of motor skills progression during learning, with periods of stability and sudden transitions emerging throughout [START_REF] Nourrit | On Discontinuities in Motor Learning: a Longitudinal Study of Complex Skill Acquisition on a Ski-Simulator[END_REF][START_REF] Teulier | The Nature of the Transition Between Novice and Skilled Coordination During Learning to Swing[END_REF]Delignières, Figure 3. Changes in self-assessment scores for both groups between lessons 1 and 5. *Represents a statistically significant difference (p < .05) between two lessons only for VFB group. Teulier, and [START_REF] Delignières | L'apprentissage des habiletés motrices complexes : des coordinations spontanées à la coordination experte[END_REF]. The evidence suggests that VFB acted as a key augmented informational constraint to drive the transition in motor learning. As for the control group, the arm-trunk angle values did not show any significant changes, and these results reinforce the role of VFB in optimizing motor learning (compared to traditional use of verbal instructions only) over a short period in a formal PE programme. The progress observed in the experimental group in a school PE lesson context is consistent with the findings of several experimental studies using a combination of VFB and verbal instructions for the rapid acquisition of complex skills [START_REF] Erbaugh | Role of Visual Feedback in Observational Motor Learning of Primary-Grade Children[END_REF][START_REF] Boutmans | Video feedback: Useful or Not in Physical Education Classes?[END_REF][START_REF] Kernodle | Information Feedback and the Learning Multiple-Degree-of-Freedom Activities[END_REF][START_REF] Boyce | How Should Feedback Be Delivered?[END_REF][START_REF] Janelle | Maximising Performance Effectiveness Through Videotape Replay and a Self-Controlled Learning Environment[END_REF][START_REF] Guadagnoli | The Efficacy of Video Feedback for Learning the Golf Swing[END_REF][START_REF] Mérian | Le Feedback Vidéo en Education Physique Scolaire[END_REF][START_REF] Potdevin | Le feedback vidéo en EPS: une double stratégie de progrès et de motivations. Le cas de l'Appui Tendu Renversé en classe de 6ème[END_REF]. Nevertheless, the results appear to be at odds with those of [START_REF] Rothstein | Bridging the Gap: Application of Research on Videotape Feedback and Bowling[END_REF] and [START_REF] Salmoni | Knowledge of Results and Motor Learning: A Review and Critical Reappraisal[END_REF], which pointed to the need for learners to have reached a certain level of competency before VFB could be effective in optimizing their learning. While the initial level of participants in this study was low, they progressed quickly (in 75 attempts), thus demonstrating that VFB could act as a powerful augmented informational constraint, which shortens the motor learning process in a PE context. Unlike [START_REF] Guadagnoli | The Efficacy of Video Feedback for Learning the Golf Swing[END_REF] and [START_REF] Rothstein | Bridging the Gap: Application of Research on Videotape Feedback and Bowling[END_REF], the results here likewise showed that learners need not train for a long time with VFB for the latter to contribute to motor learning, even in the case of young children as learners.

The rapid improvement in the self-assessment ability which occurred at lesson 2 (ES = 1.79) showed that pupils in the experimental group were quick to associate available intrinsic feedback linked to proprioception as they turned upside down with extrinsic information related to VFB. These results appeared to be consistent with studies by [START_REF] Winfrey | Effects of Self-Modeling on Self-Efficacy and Balance Beam Performance[END_REF], which demonstrated that female gymnasts aged between 8 and 13 developed self-grading abilities on the beam when they were given VFB. Under the task constraints of elementary school PE teaching, [START_REF] Hamlin | Motor Competency and Video Analysis[END_REF] showed that VFB could help students to analyse their own performances if criteria were provided to help student to structure their evaluations with concrete expectations [START_REF] Mcmillan | Student Self-Assessment: The key to Stronger Student Motivation and Higher Achievement[END_REF]. Our study with VFB was associated with an attentional focus on the quantified arm-trunk angle was aligned with these principles. Finally, our findings, based on self-assessment scores, were congruent with previous qualitative research supporting the role of VFB in the self-assessment process [START_REF] O'loughlin | Digital Video: The Impact on Children's Learning Experiences in Primary Physical Education[END_REF][START_REF] Kretschmann | Employing Tablet Technology for Video Feedback in Physical Education Swimming Class[END_REF].

A new insight from our study indicates the rapidity of performance progress when using the selfassessment task in the experimental group. This ability to rapidly exploit the VFB-based learning aid may be explained due to several reasons. The first lies in the use of VFB in an intermittent scheduling on a 20% basis. This 'one in five attempts' scheduling avoided dependence and provided opportunities for pupils to also exploit intrinsic information [START_REF] Wulf | Understanding the Role of Augmented Feedback: the Good, the bad, the Ugly[END_REF], from valuable sources such as proprioception when turning upside down [START_REF] Schmidt | Optimizing Summary Knowledge of Results for Skill Learning[END_REF]. It also allowed them to continue their learning in an autonomous way, even when VFB was not provided (in this case, for four out of five attempts). Conducting the self-assessment task, every five attempts most likely generated an attentional focus on perceived sensations when turning upside down in order to answer, as accurately as possible, the question 'Do you think your attempt was better than the last time you watched it?' Furthermore, this type of feedback, using freeze-frames and measuring the arm-trunk angle, is one that beginner-level pupils appear to be able to exploit. Simplifying feedback in this way appeared to contribute to reducing reliance on conscious cognitive control of the movement when identifying the important information in VFB and to enhancing its impact on learning and perception of the body in action [START_REF] Hegarty | The Roles of Mental Animations and External Animations in Understanding Mechanical Systems[END_REF][START_REF] Mayer | When Static Media Promote Active Learning: Annotated Illustrations Versus Narrated Animations in Multimedia Instruction[END_REF]. Furthermore, requesting the pupil's self-assessment immediately prior to VFB was good practice because it is likely to increase the pupil's attention capacity for watching the video and listening to the teacher's technical instructions and advice.

A gap between self-assessment-related progress (lesson 2) and that of motor skills enhancement (lesson 5) should be noted for the experimental group. This result appears to be consistent with the various theories on learning stages, which differentiate the cognitive stage where the learner becomes aware of what has to be done to succeed by consciously processing the information, from the associative stage where the learner works on the different parts of the movement in an attempt to perform the task successfully [START_REF] Fitts | Human Performance[END_REF][START_REF] Schmidt | Motor Control and Learning: A Behavioral Emphasis[END_REF]. The findings of the current study showed, initially (in lesson 2), how the VFB group succeeded in exploiting the augmented information from VFB in order to enhance awareness of their own vertical position. Second, the use of VFB allowed them gradually to regulate their actions to significantly change their vertical alignment (by lesson 5).

Changes in motivation

Amotivation scores fell significantly for the experimental group between lesson 1 and lesson 5. According to [START_REF] Ryan | Self-determination Theory and the Facilitation of Intrinsic Motivation, Social Development, and Well-Being[END_REF], amotivation represents a complete lack of intrinsic and extrinsic motivation and is conveyed by a total absence of self-determination and willpower during task completion. According to this theoretical perspective, environments that generate a lack of three types of essential needsautonomy, action effectiveness and peer-group affiliationrepresent environments that are likely to create amotivation. In this study, the amotivation profile of the pupils in the experimental group dropped significantly in the space of five lessons, despite an initially low score after the first lesson (3.06 ± 1.42). According to [START_REF] Ntoumanis | An Idiographic Analysis of Amotivation in Compulsory School Physical Education[END_REF], the reasons proposed for amotivation in disengaged pupils (aged 14 and 15 years) during PE lessons are linked to three factors: learned helplessness, non-consideration of their interests and needs and the learning context. In the case of the latter, [START_REF] Ntoumanis | A Review of Motivational Climate in Physical Activity[END_REF] have highlighted the fact that a so-called mastery learning climates, in which pupils feel able to progress by themselves, make it possible to avoid amotivation. The VFB learning aid, and the way it was implemented in this study, may have provided a context in which pupils felt they were playing an active role in their own learning. According to [START_REF] Shepard | The Role of Assessment in a Learning Culture[END_REF], VFB combined with a self-assessment task can increase students' responsibility for their own learning and make relationships with teacher more collaborative. This could have been achieved by effectively allowing them to engage more in their own learning by continually readjusting their motor performance during learning by comparing their perceptions with the reality of the video image.

A most important aspect of this engagement process, supported by VFB, was the creation of specific learning targets in collaboration with the teacher. According to [START_REF] Kingston | The Application of Goal Setting in Sport[END_REF], the multiple-goal approach (such as using self-assessment and motor alignment goals) has the advantage that the potential negative effect of failing to achieve a target level of performance can be buffered by achieving other performance goals. Moreover, the constraints of this learning environment appear to meet the need for the development of competence through more precise assessment of progress. Yet, the pupils' progress related to their vertical alignment performance did not become apparent until lesson 5. It would be interesting, in a future study, to study the motivational dynamics, lesson by lesson and week by week, in order to identify the effects of real progress on the different dimensions of motivation. It may also be the case that rapid progress in the selfassessment task also impacted the motivation profile of the experimental group with significant progress occurring as early as the second lesson, as opposed to the control group, which showed no progress in this aspect of the task.

As far as intrinsic motivation is concerned, results revealed considerably different development between the experimental group (ES = 0.26) and the control group (ES = -0.31) as showed by the significant interaction group × time (H (1, 40) = 69.4; p < .01). Post-hoc tests, however, failed to highlight any statistically significant difference between lesson 1 and lesson 5 for both groups. This result refutes our initial expectation that VFB would provide information, which would increase intrinsic motivation in learning. Factors explaining this absence of significant progress may be linked to the limited autonomy pupils were given in accessing VFB. For teaching and class management purposes, the teacher in this study wholly managed VFB, and pupils could not choose what they watched or when they received it. In that respect, scientific evidence suggests that freedom of choice in the use of feedback fosters engagement and intrinsic motivation during learning [START_REF] Janelle | Maximising Performance Effectiveness Through Videotape Replay and a Self-Controlled Learning Environment[END_REF][START_REF] Aiken | The Effects of Self-Controlled Video Feed-Back on the Learning of the Basketball Set Shot[END_REF][START_REF] Fairbrother | Self-controlled Feedback Facilitates Motor Learning in Both High and Low Activity Individuals[END_REF][START_REF] Patterson | Self-controlled KR Schedules: Does Repetition Order Matter?[END_REF][START_REF] Hung | Exploring the Effects of Integrating the IPad to Improve Students' Motivation and Badminton Skills: A WISER Model for Physical Education[END_REF]. Future studies should take this important aspect of learning into account by giving pupils greater freedom in using VFB, allowing each participant the opportunity to access visual feedback on performance during learning whenever he/she wanted it.

The use of a self-assessment task coupled with VFB in PE teaching

In the French educational system, syllabi for learning programmes (including PE) are set nationally from kindergarden to senior high school and structured around the notion of key competences. The use of Information and Communication Technology (ICT) is widely promoted, which makes VFB an appealing tool to develop pupil competencies. A competency can be defined as an integrated and stable network of knowledge and know-how, with normative behaviours, procedures and types of reasoning [START_REF] Escalié | Helping Students Build Competences in Physical Education: Theoretical Proposals and Illustrations[END_REF]. In order to develop these competencies, lesson plans often aim to integrate knowledge, skills and attitudes using a problem-solving approach. Competence-based teaching is believed to foster the transfer of learning from school to everyday life (De-Juanas Oliva, del Pozo, and Franco 2016). In the French PE curriculum, this competence-based approach is operationalized by integrating motor skills acquisition with methodological (method and tools for learning) and social (shape the individual and the citizens) competencies. In that respect, pupils have to develop these global competencies, as well as acquiring skills in different sports over relatively short periods of teaching (in general, 6-8 weeks). The results of our study reinforced the point that the use of self-assessment in a VFB task context helps learners to improve both their motor skills and their methodological competencies. It provides evidence to show that competency related to motor skill and self-assessment can be developed simultaneously in a short period of time.

Several studies [START_REF] Weir | The Use of Digital Video in Physical Education[END_REF][START_REF] Palao | The Impact of Video Technology on Student Performance in Physical Education[END_REF]) have pointed out the reasons why VFB was not being used enough in PE contexts. According to these researchers, teachers often felt that VFB is time-consuming and detrimental to students' use of practice time. Our study suggests that this kind of sheltered workshop organization might partly solve the problem, allowing teachers to safely oversee 75 skill attempts per person in five lessons while at the same time supervising the rest of the class.

Limitations and perspectives

A possible limitation of this study, requiring future confirmation, is the absence of retention tests. Given that permission to conduct the study was granted on the condition that the yearly activity schedule for PE lessons was not disrupted, it was impossible to plan a gymnastics lesson two weeks after the end of the course in the school timetable. A future study could monitor performance in vertical alignment, self-assessment and motivation two weeks after the end of the gymnastics course to observe whether significant differences between the two groups persisted. Additionally, a mixed method design, with qualitative data from semi-structured interviews with sub-samples of participants, would also help investigators understand participants' perception of VFB during the learning process and how relations with the teacher or others students could be influenced. The results of this study should also be interpreted carefully since the groups tested here were composed of novices in the gymnastic skill studied. Nevertheless, some pupils could have had previous experiences of activities that involved placing the body in a vertical reverse position or using VFB during their leisure activities. Recording overall extra-curricular gymnastic and VFB experiences for each participant in future studies is recommended to counter this possible limitation. According to the expertise reversal effect [START_REF] Kalyuga | The Expertise Reversal Effect[END_REF][START_REF] Kalyuga | Expertise Reversal Effect and its Implications for Learner-Tailored Instruction[END_REF][START_REF] Khacharem | Improving Learning From Animated Soccer Scenes: Evidence for the Expertise Reversal Effect[END_REF], levels of learner expertise may modulate the effectiveness of such means for enhancing learning. Caution should, therefore, be exercised in generalizing these results, depending on learner levels.

Last, this type of organization could be promoted with class-groups who display a fair level of autonomy in their schoolwork. Our setting allowed the teacher to supervise the entire class and focus on the regulation of the VFB workshop at the same time.

Future pedagogical and research challenges consist of examining the ways students could use VFB technology with more self-regulation and less reliance on teacher interventions. Recent studies, using digital tablets supporting self-regulation by the students, showed very good effects in the learning process and motivation in the acquisition of badminton skills and game strategies [START_REF] Hung | Exploring the Effects of Integrating the IPad to Improve Students' Motivation and Badminton Skills: A WISER Model for Physical Education[END_REF] or in learning to swim [START_REF] Kretschmann | Employing Tablet Technology for Video Feedback in Physical Education Swimming Class[END_REF]). Yet, as shown by [START_REF] Cohen | The Effectiveness of Aligned Developmental Feedback on the Overhand Throw in Third-Grade Students[END_REF], teachers might face challenges to provide an adequate level of self-regulated feedback to every kind of unexpected motor outcome. To overcome this problem, [START_REF] Post | Self-control Over Combined Video Feedback and Modelling Facilitates Motor Learning[END_REF] used a split-screen replay with a video model compared with the VFB in the same frame. Results in a laboratory context showed significant effects on motor learning, motivation and perceived competencies. Testing this innovative proposal in a more ecological context is worth pursuing, providing the potential to further improve students' learning experiences.

As mentioned in several studies (e.g. [START_REF] Weir | The Use of Digital Video in Physical Education[END_REF][START_REF] Palao | The Impact of Video Technology on Student Performance in Physical Education[END_REF], one barrier to enhance the use of new technology in PE teaching and improve pupil learning experiences is linked to lack of confidence from the teachers related to their own pedagogical-technology competency. In that respect, an important challenge in teacher training concerns the use of new technology by student teachers. In particular, the challenge concerns the sharing of pedagogical experiences about the use of ICT in different PE teaching contexts, as proposed, for example, by [START_REF] Casey | Rethinking the Relationship Between Pedagogy, Technology and Learning in Health and Physical Education[END_REF]. The current study hopefully helped to answer not only the 'how', but also the 'why' question, by promoting evidence-based grounds for use of VFB, thus justifying the need to analyse effectiveness of new pedagogical strategies using this tool.

Conclusion

Literature on the contribution of feedback in motor learning is extremely rich, but typically studied in controlled laboratory contexts during experiments. Focusing on its use in real-life teaching conditions implies being fully conversant with the different dimensions of feedback and the multiple effects it can have depending on the learning stage. The results of this study showed how using a simplified VFB-based learning aid, coupled with a self-assessment task, in real-life teaching conditions during an ongoing PE programme contributed to enhancing motor skills, self-assessment ability and motivation profiles over a short period of time in novices. As highlighted by [START_REF] Dutta | Global Information Technology Report. Living in a Hyperconnected world[END_REF], the question is not whether new technologies should be used or not. The scientific challenge is to try out the various technological solutions with the aim of making them levers of success in PE programmes to enhance the learning experience of individuals.

Disclosure

No potential conflict of interest was reported by the authors.

Figure 1 .

 1 Figure 1. Description of VFB on performance of a front handspring to flat back landing.

Figure 2 .

 2 Figure 2. Progression of vertical arm-trunk angles for each participant between lessons 1 and 5. *Represents a statistically significant difference (p < .05) between two lessons only for VFB group.

Table 1 .

 1 Changes in motivation scores between lessons 1 and 5.

		VFB group		Control group
		Lesson 1	Lesson 5	ES	Lesson 1	Lesson 5	ES
	Intrinsic motivation	5.16 ± 1.83	5.51 ± 1.32	0.26	4.96 ± 2.06	4.31 ± 2.11	-0,31
	Identified regulation motivation	4.88 ± 1.49	4.66 ± 1.70	-0.13	4.54 ± 1.88	4.37 ± 2.04	-0.09
	External motivation	4.63 ± 1.48	4.94 ± 0.84	0.26	4.24 ± 1.76	4.46 ± 1.70	0.13
	Amotivation	3.06 ± 1.42	2.12 ± 0.62 a	-0.89	3.51 ± 1.95	3.96 ± 1.68	0.25

a Represents a statistically significant difference between lesson 1 and lesson 5 for the same group; ES represents effect size values.

Funding

This study received the support of the Graduate School of Teaching and Education of Lille Nord de France. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.