Andrian Kuch

Romain Tisserand

François Durand

Tony Monnet

Jean-François Debril

J-F Debril

Postural adjustments preceding string release in trained archers

Keywords: Biomechanics, Motor Control, Performance, Archery, Balance

1.Introduction

Olympic archery is a sport requiring to shoot arrows with a recurve bow at a target located 70 meters away [START_REF] Leroyer | Biomechanical study of the final push-pull in archery[END_REF]. To achieve both accuracy and precision, elite archers are trained to stand upright as quietly as possible while aiming at the target. Indeed, natural whole-body oscillations observed during standing prior to string release may result in aiming error and therefore reduce the shooting performance [START_REF] Park | Modelling archers' scores at different distances to quantify score loss due to equipment selection and technique errors[END_REF]. But precisely shooting arrows is not only about aiming. When archers release their fingers, the string propels the arrow in the direction of the target with less than 16 ms of string and arrow contact [START_REF] Zanevsky | Bow tuning in the vertical plane[END_REF]. For modern bows, the recoil kinetic energy is estimated at around 0.2 J, which leads to a recoil velocity of about 250 mm/s [START_REF] Meyer | Applications of Physics to Archery[END_REF]. This bow recoil can potentially disturb archers' postural equilibrium and contribute to poorer shooting performance [START_REF] Zanevskyy | Archery bow stabiliser modelling[END_REF]. Yet, how elite archers deal with the consequences of this voluntary triggered movement and whether or not their postural strategy has an impact on their shooting performance remains unclear.

Postural control serves two main purposes: orientation and balance [START_REF] Horak | Postural orientation and equilibrium[END_REF]. When all forces applied on a body are balanced, it can be assumed that its postural balance is regulated by the displacement of the center of pressure (CoP) to maintain the vertical projection of the center of mass (CoM) within the base of support (BoS) [START_REF] Winter | Human balance and posture control during standing and walking[END_REF]. According to the simple and widely used inverted pendulum model [START_REF] Winter | Human balance and posture control during standing and walking[END_REF], when postural balance is challenged by motion of the CoM in one direction, the CoP has to move in the same direction and with larger amplitude to keep the CoM projection within the BoS. When the CoM motion comes from a self-generated voluntary movement, i.e. an 'internal' perturbation, the postural control strategy is thought to be pro-active [START_REF] Massion | Postural control system[END_REF][START_REF] Horak | Postural perturbations: New insights for treatment of balance disorders[END_REF]. This means that the dynamic consequences of the forthcoming perturbation are predicted and therefore integrated in the following motor response plan. Thus, the dynamic consequences of the movement perturbation on posture can be minimized through anticipatory postural adjustments (APA) [START_REF] Massion | Postural control system[END_REF][START_REF] Bouisset | A sequence of postural movements precedes voluntary movement[END_REF][START_REF] Aruin | Anticipatory postural adjustments associated with lateral and rotational perturbations during standing[END_REF], starting before the movement onset [START_REF] Massion | Movement, posture and equilibrium: Interaction and coordination[END_REF], to maintain both postural orientation and balance [START_REF] Aruin | The role of motor action in anticipatory postural adjustments studied with self-induced and externally triggered perturbations[END_REF].

In archery, a shot is generally divided into two main phases: i) an aiming phase, where the archer aims at the target while maintaining an upright quiet standing posture, and ii) a shooting phase, where the string is released and the arrow flies toward the target [START_REF] Leroyer | Biomechanical study of the final push-pull in archery[END_REF]. These phases are separated by the clicker fall. The clicker is a metallic device mounted on the bow riser to optimize bow draw length consistency [START_REF] Leroyer | Biomechanical study of the final push-pull in archery[END_REF]. Its fall emits a sound, signaling to the archer that the correct bow draw length has been reached. Archers are taught to release the string only after hearing this sound, and a shorter duration between the clicker fall and the string release was linked to increased performance [START_REF] Spratford | Postural stability, clicker reaction time and bow draw force predict performance in elite recurve archery[END_REF]. Like reacting to the clicker fall, postural stability is a feature that can be trained and improved through regular practice [START_REF] Paillard | Plasticity of the postural function to sport and/or motor experience[END_REF][START_REF] Jagdhane | The Effect of a Four-Week Balance Training Program on Anticipatory Postural Adjustments in Older Adults: A Pilot Feasibility Study[END_REF]. In standing shooting disciplines like archery [START_REF] Spratford | Postural stability, clicker reaction time and bow draw force predict performance in elite recurve archery[END_REF][START_REF] Mason | Body stability and performance in archery[END_REF][START_REF] Keast | Fine body movements and the cardiac cycle in archery[END_REF][START_REF] Sarro | Relationship between bow stability and postural control in recurve archery[END_REF][START_REF] Simsek | A comparison of the ground reaction forces of archers with different levels of expertise during the arrow shooting[END_REF], rifle [START_REF] Aalto | Postural Stability in Shooters[END_REF][START_REF] Ball | Body sway, aim point fluctuation and performances in rifle shooters: Inter-and intra-individual analysis[END_REF][START_REF] Era | Postural stability and skilled performance-a study On top-Level and naive rifle shooters[END_REF][START_REF] Ko | Skill level constrains the coordination of posture and upperlimb movement in a pistol-aiming task[END_REF] or biathlon (standing shooting) [START_REF] Sattlecker | Factors discriminating high from low score performance in biathlon shooting[END_REF], analysis of postural balance during the aiming phase showed that better performance in skilled shooters compared to less skilled counterparts could be explained by smaller postural sway characteristics. However, very few studies characterized postural balance during the shooting phase, leaving the question of how skilled shooter minimize the dynamic consequences of their shooting activity on postural stability partially unanswered. In air pistol shooting, electromyographic analysis revealed the presence of APA specific to the direction of the movement and muscular activity starting up to 100 ms before the shooting onset [START_REF] Minvielle | Study of anticipatory postural adjustments in an air pistol-shooting task[END_REF]. Yet, their mechanical contribution to postural balance have never been characterized in archery.

The goal of our study is to characterize the postural strategy used by trained archers to limit the potentially detrimental effect of the string release on postural balance and determine whether or not this has a positive influence on performance. Since trained archers release their string voluntarily, we first hypothesized that APA will be triggered in the direction of the bow recoil, to limit its dynamic impact on postural balance, and that these APA will be finely timed with the clicker fall. Second, since APA can be trained to limit the detrimental consequences of self-generated perturbation on postural balance [START_REF] Jagdhane | The Effect of a Four-Week Balance Training Program on Anticipatory Postural Adjustments in Older Adults: A Pilot Feasibility Study[END_REF], we hypothesized that APA characteristics will be mechanically more efficient in elite archers than in sub-elite archers, showing a better dynamic postural control that could partially explain performance difference between the two levels of expertise.

Material and Methods

Participants

Thirteen archers participated in this study. Six participants (3 males, 3 females, age 27.0 ± 4.7, height 174.3 ± 8.2 cm, mass 80.6 ± 27.2 kg) were recruited in the French National elite training center and were placed in the elite group (16.0 ± 1.9 years of practice), according to the following inclusion criteria: over 10 years of archery practice and at least one participation to a senior international outdoor archery competition (European Championships, World Cup stages, World Championships, Olympic Qualifiers or Olympic Games) in the year prior to the experiment. Seven participants (4 males, 3 females, age 16.5 ± 4.9, 169.2 ± 3.6 cm, mass 59.7 ± 4.8 kg) were recruited in a next-generation training center and were placed in the sub-elite group (8.0 ± 1.8 years of practice), according to the following inclusion criteria: less than 10 years of archery practice and never competed at a senior international outdoor archery competition. For both populations, the exclusion criteria was to not have suffered of any injury 6 months prior to the experiments. The experiment was approved by the department local ethics committee and conducted with the informed and written consent of all participants.

Experimental design

Each participant first completed their usual competitive warm-up routine for 15 minutes. Warm-up routines included exercises mobilizing upper limbs joints and muscles followed by taking a dozen of shots at a standard 10-ring competition target face of 122 cm diameter, located at the Olympic distance of 70 meters. During the experiment, each participant was asked to shoot 18 arrows on that same target, in the same conditions (no wind). Participants executed every shot using their own recurve bow, equipped with stabilizers and sight, as well as their own arrows. Participants started every trial with both feet on the ground, wearing shoes. Before taking a shot, participants stepped on two force plates, by placing each foot on one of the two (Figure 1A). When installed, participants could shoot one arrow at their own pace. Once the arrow reached the target, participants stepped out of the force plate and started over. The score of each arrow was collected.

Instrumentation

To compute the CoP before and during the shot, we used the ground reaction forces and torques from two force plates (600x500x50 mm, Kistler 9260AA6, Winterthur, Switzerland) sampled at 1000 Hz. The force plates were placed next to each other and oriented so that the medio-lateral axis (ML) was aligned with the shooting direction and the antero-posterior axis (AP) was the axis orthogonal to the shooting direction (Figure 1A). Signals from the force plates were retrieved using Bioware software (Kistler 2812A, version 5.3.2.9), filtered with a 3 rd order Butterworth low-pass filter with a 6 Hz cutoff frequency [START_REF] Spratford | Postural stability, clicker reaction time and bow draw force predict performance in elite recurve archery[END_REF] and used to process the global CoP coordinates according to Kistler recommendations.

FIGURE 1 ABOUT HERE

To identify characteristic instants relative to the different phases in archery, we filmed every shot with a high-speed camera (Photron FASTCAM Mini UX100 type 800K-C-16G, Tokyo, Japan) sampled at 1000 Hz. The camera was placed 2 m away from the participant, filming the bow riser on the clicker's side (Figure 1B). Force plates and the high-speed camera were synchronized using a BNC trigger port on the data acquisition system (Kistler DAQ 5691A, Winterthur, Switzerland). Each measurement started with an external wired trigger button pressed by an experimenter.

Postural stability

Temporal identification

Images from the high-speed camera were used to identify two decisive instants: the clicker falling time and the string release time. Here, the clicker falling time was identified as the first image where the clicker touched the bow riser (Figure 1B right), and the string release time was identified as the first image where the arrow tip moved towards the target. To compute each participant's reaction time, the clicker falling time was subtracted to the string release time. Postural stability of participants during the aiming phase was characterized on a period of 1 second prior to the clicker falling time [START_REF] Spratford | Postural stability, clicker reaction time and bow draw force predict performance in elite recurve archery[END_REF]. This time interval was chosen as a minimum duration prior to the clicker fall observed for all the participants between aiming at full draw and releasing. The shooting phase was considered to be the period starting after to the clicker falling time until 0.5 second after the string release time.

Postural stability during the aiming phase

During the aiming phase, six postural stability indicators were computed [START_REF] Prieto | Measures of postural steadiness: Differences between healthy young and elderly adults[END_REF]: the mean (𝑑 𝑚𝑒𝑎𝑛 ) and root mean square distance (𝑑 𝑟𝑚𝑠 ) around the mean CoP, the mean (𝑣 𝑚𝑒𝑎𝑛 ) and maximum velocity (𝑣 𝑚𝑎𝑥 ) of the CoP, in both the ML and AP axis as well as in the 𝑂𝑥𝑦 plane; the sway amplitude (𝐴) only in both ML and AP directions; and finally the 95% confidence ellipse area (𝐴 95% ) in the 𝑂𝑥𝑦 plane.

Postural stability during the shooting phase

A CoP displacement away from the target was observed in the ML shooting axis in every trial (Figure 2). This ML CoP movement onset (𝑡 0 ) was computed as the instant from the clicker falling time when the participant's sway velocity in the ML axis crossed a threshold set at two standard deviations above 𝑣 𝑚𝑒𝑎𝑛 measured during the aiming phase. To characterize the postural strategy during the shooting phase, we assessed the CoP trajectory on a period of 0.5 second after 𝑡 0 .

For analysis purposes, we used the timing of 𝑡 0 to characterize the archers' postural strategy.

If 𝑡 0 occurred prior to the string release which constitutes the perturbation, the postural strategy was labelled as "early", otherwise it was labelled as "late". Usage of either one or the other strategy was quantified in each group.

Statistical analysis

A total of 230 shots (elite 𝑛 = 107 , sub-elite 𝑛 = 123 ) were successfully recorded and analyzed. Normality of the different dependent variables was tested with a Shapiro-Wilk test.

Since those were not normally distributed, results are presented with medians and interquartile ranges unless said otherwise. For all the CoP indicators measured during the aiming phase, two variables were extracted to account for intra-individual variability. First, results of all 18 trials from one archer were averaged and then averaged among all the archers from a same group (elite 𝑛 = 6, sub-elite 𝑛 = 7). Second, the standard deviation of all 18 trials for one archer was computed, followed by the standard deviation of all archers from a same group. These two results (average and standard deviation) were then compared for each CoP indicator using a non-parametric Mann-Whitney U test between the two groups.

Reaction time and 𝑡 0 were compared using Kruskall-Wallis tests with two factors (Group: elite vs sub-elite and Strategy: early or late), with Dunn-Sidak tests as post-hoc. Differences in percentage of use of the early or the late strategy between the two groups were evaluated using a Fisher's exact test.

Since every archer had a different distribution of early and late shots, to avoid any individual influence on the group results, the CoP trajectories during the shooting phase were averaged to form one continuous timeseries representative of each archer (elite 𝑛 = 6, sub-elite 𝑛 = 7) in each strategy. Then, one dimensional statistical parametric mapping (1D SPM) [START_REF] Pataky | Region-of-interest analyses of onedimensional biomechanical trajectories: Bridging 0D and 1D theory, augmenting statistical power[END_REF], using non-parametric two sample t-tests [START_REF] Pataky | Zero-vs. one-dimensional, parametric vs. nonparametric, and confidence interval vs. hypothesis testing procedures in one-dimensional biomechanical trajectory analysis[END_REF], was conducted to analyze the influence of expertise within strategies and strategy within groups, and to identify any differences in the continuous ML CoP trajectory after 𝑡 0 . Score distributions were not normal, according to a Shapiro-Wilk test. Therefore, nonparametric Mann-Whitney U test was used to compare score performance between both groups and both postural strategies. Additionally, a Kruskal-Wallis test was performed to compare the effect of the strategy on the performance score for each group, with Dunn's test used as post-hoc with a Holm-Bonferroni correction. For all statistical tests, a 𝑝 < 0.05 was set for significance. Data processing was performed using customized routines written in

Results

Postural stability during the aiming phase

Averaged CoP indicators and their respective variability calculated during the aiming phase are presented in Table 1 with the results of their statistical comparison between the two groups. For the averaged results no differences were found between the two groups, except for 𝑑 𝑚𝑒𝑎𝑛 and 𝑑 𝑟𝑚𝑠 that were smaller in the elite group compared to the sub-elite group in both the AP axis and the 0𝑥𝑦 plane, as well as 𝐴 𝐴𝑃 (p < 0.05). For the variability results no differences were found between the two groups, except for 𝑣 𝑚𝑎𝑥 that was larger in the elite group compared to the sub-elite group in both the ML axis and the 0𝑥𝑦 plane (p < 0.05).

TABLE 1 ABOUT HERE

Postural stability during the shooting phase

Kruskal-Wallis test indicated evidence of differences between the pairs of group/strategy for reaction time (H(3) = 28.1, 𝑝 < 0.001) and 𝑡 0 (H(3) = 174.9, 𝑝 < 0.001). The elite group had a shorter reaction time compared to the sub-elite group (146 ± 12 ms versus 156 ± 15 ms, 𝑝 < 0.001). Within strategies, reaction time was shorter in the elite group compared to the subelite group for the early strategy only (145 ± 11 ms versus 157 ± 14 ms, 𝑝 = 0.02). Within groups, no difference was found for the 𝑅𝑇 between early and late strategies (both 𝑝 > 0.05).

Within groups, 𝑡 0 occurred significantly sooner in the early than in the late strategy for both the elite (23 ± 38 ms versus 163 ± 24 ms, 𝑝 < 0.001) and the sub-elite archers (119 ± 37 ms versus 180 ± 26 ms, 𝑝 < 0.001). Within strategies, 𝑡 0 occurred sooner in the elite group compared to the sub-elite group for the early strategy only (𝑝 < 0.001).

FIGURE 2 ABOUT HERE

The early and late strategy were used by every archer regardless of their level of expertise.

However elite archers used the early strategy more often than the sub-elite group (82 trials out of 107 or 77 ± 16 % versus 73 trials out of 123 or 59 ± 17 % of early shots, respectively, 𝑝 = 0.007).

The 1D SPM analysis (Figure 3) showed no difference among groups or strategies for the CoP trajectory in the ML axis for a period of 0.5 seconds after 𝑡 0 (𝑝 > 0.05).

Scores

The elite group scored higher (9.2 ± 0.8 (mean ± SD)) than the sub-elite group (8.9 ± 1.0), 𝑝 = 0.03. However, no differences were found for scores between the trials where an early strategy was used (9.0 ± 0.8) and where a late strategy was used (8.9 ± 1.1) nor when scores of early and late strategies were compared within groups (all 𝑝 > 0.05).

FIGURE 3 ABOUT HERE

Discussion

The objective of this study was to characterize the postural strategy used by trained archers to limit the potentially detrimental effect of string release on postural balance and determine whether this strategy could explain differences in performance outcome between two levels of expertise. As expected, all archers displayed APA in the shooting direction and with greater amplitude than the balance perturbation. Moreover, these APA were finely timed clicker fall, especially in elite archers but with no evident impact on scoring performance. These results suggest that the mechanical consequences of the bow recoil are finely integrated into the motor program released for the shooting phase.

Postural stability during the aiming phase

During the aiming phase, three averaged postural sway indicators (𝑑 𝑚𝑒𝑎𝑛 , 𝑑 𝑟𝑚𝑠 and 𝐴 𝐴𝑃) were smaller in the elite group compared to the sub-elite group for the AP component (Table 1). The variability analysis showed there was no difference between groups for those three indicators (Table 1), confirming the difference in the averaged results is not due to intraindividual variance. This suggests that the determinant axis for postural stability during aiming in trained archers is not the shooting direction axis but the one perpendicular to it. In a bipedal standing posture, the BoS is generally smaller in the AP than in the ML direction and actions of the mono-articular ankle muscles, such as the soleus and tibialis anterior, are mainly responsible for accurate movement of the CoP in the AP direction [START_REF] Winter | Human balance and posture control during standing and walking[END_REF]. In the ML direction, stronger hip muscles are thought to be responsible for CoP sway control [START_REF] Winter | Human balance and posture control during standing and walking[END_REF]. This can explain that even in young adults, larger postural sway characteristics are found in the AP axis than in the ML axis in quiet standing postures [START_REF] Prieto | Measures of postural steadiness: Differences between healthy young and elderly adults[END_REF]. Thus, fine postural balance control for elite archers seems to be principally controlled in the AP direction [START_REF] Ko | Skill level constrains the coordination of posture and upperlimb movement in a pistol-aiming task[END_REF][START_REF] Sattlecker | Factors discriminating high from low score performance in biathlon shooting[END_REF]. The difference between the elite group and the sub-elite group corroborates previous results showing that reduced CoP sway during aiming is positively correlated with expertise and performance in archery [START_REF] Spratford | Postural stability, clicker reaction time and bow draw force predict performance in elite recurve archery[END_REF][START_REF] Mason | Body stability and performance in archery[END_REF][START_REF] Keast | Fine body movements and the cardiac cycle in archery[END_REF][START_REF] Sarro | Relationship between bow stability and postural control in recurve archery[END_REF][START_REF] Simsek | A comparison of the ground reaction forces of archers with different levels of expertise during the arrow shooting[END_REF], akin to other shooting disciplines [START_REF] Aalto | Postural Stability in Shooters[END_REF][START_REF] Ball | Body sway, aim point fluctuation and performances in rifle shooters: Inter-and intra-individual analysis[END_REF][START_REF] Era | Postural stability and skilled performance-a study On top-Level and naive rifle shooters[END_REF][START_REF] Ko | Skill level constrains the coordination of posture and upperlimb movement in a pistol-aiming task[END_REF][START_REF] Sattlecker | Factors discriminating high from low score performance in biathlon shooting[END_REF]. Along with these studies, we support the view that elite archers perform better than non-elite archers partly because they reduce the AP oscillation of their CoP during the aiming phase.

Only one indicator (ML component of 𝑣 𝑚𝑎𝑥 ) showed a larger variability among archers from the elite group compared to the sub-elite group (Table 1). This increased variability can be explained by the onset distribution of the ML CoP movement of elite archers (t0) that sometimes occurred before the clicker falling time (Figure 2) and may have been caught as the maximal CoP velocity of the aiming phase in our analysis.

Postural stability during the shooting phase

An anticipated postural strategy finely integrating the mechanical consequences of the string release

Releasing the string frees the potential energy accumulated in the bow limbs, inducing a recoil movement of the bow [START_REF] Meyer | Applications of Physics to Archery[END_REF]. From a bow recoil velocity estimated at 0.26 m/s [START_REF] Meyer | Applications of Physics to Archery[END_REF], a string and arrow common motion time of 16 ms during the internal ballistic phase [START_REF] Zanevsky | Bow tuning in the vertical plane[END_REF] and assuming a rigid body system including both the archer and the bow, we estimated the displacement of the system CoM at approximately 4 mm in the direction opposite to the target. According to the inverted pendulum model, widely used to study the mechanics of postural control [START_REF] Winter | Human balance and posture control during standing and walking[END_REF], this recoil caused by the recoil motion of the bow may threaten the mechanical balance conditions achieved during the aiming phase. To keep balancing the pendulum, a CoP movement towards the back leg is necessary to slow down the CoM motion and restore postural balance. For all participants and in all trials, we observed that the CoP indeed moves away from the target, with an amplitude between 15 to 20 mm, which is about 8 to 10 times larger than oscillations measured during the aiming phase (1.9 mm) and 4 to 5 times larger than the estimated CoM displacement caused by the bow recoil perturbation. Besides, the ML CoP sway amplitude remains consistent within groups and strategy (Figure 2 & Figure 3). Because our participants were all trained archers, they are used to experience the mechanical consequences of string release and therefore seem able to generate APA finely matching the spatial characteristics of an expected postural perturbation [START_REF] Horak | Central programming of postural movements: Adaptation to altered support-surface configurations[END_REF][START_REF] Kaewmanee | Effect of predictability of the magnitude of a perturbation on anticipatory and compensatory postural adjustments[END_REF].

The string release is extremely short (< 16 ms of string and arrow contact [START_REF] Zanevsky | Bow tuning in the vertical plane[END_REF]). Even though the CoP is assumed to move without inertia, its motion depends on the contraction of postural muscles that have a certain latency [START_REF] Winter | Human balance and posture control during standing and walking[END_REF]. Therefore, lower-limb muscle contractions should be initiated before the shooting onset [START_REF] Minvielle | Study of anticipatory postural adjustments in an air pistol-shooting task[END_REF] so that the CoP can be closer to the maximum recoil amplitude at the string release time (between 30 and 66% here, Figure 2). In a large majority of trials (67% for all participants), 𝑡 0 preceded the perturbation and occurred only 23 ms after the clicker fall on average in elite archers, which is below any postural reflex latency response (~40 ms [START_REF] Jacobs | Cortical control of postural responses[END_REF]). The distribution of 𝑡 0 were also quite small in both groups (< 40 ms). Together, these results suggest motor commands responsible for the CoP recoil motion were preprogrammed beforehand [START_REF] Massion | Postural control system[END_REF][START_REF] Horak | Postural perturbations: New insights for treatment of balance disorders[END_REF] and released at a similar dedicated timing. By initiating a large CoP movement preceding the perturbation, our study suggests that trained archers attempt to release pre-programmed anticipatory postural commands that aim to counteract the mechanical consequences of the string release to maintain balance. The high reproducibility in the timing release of this postural strategy also suggests that archers have finely integrated [START_REF] Cavallari | The organization and control of intra-limb anticipatory postural adjustments and their role in movement performance[END_REF] the timing of the string release in their shooting motor commands.

Expertise makes archers trigger their postural strategy sooner

When the temporal delay of 𝑡 0 was removed, both groups exhibited the same postural response in terms of ML CoP movement (Figure 3). This result suggests that sub-elite archers have integrated the spatial characteristics of the bow recoil perturbation in their motor program. Both groups of archers did trigger the string release after hearing the clicker fall sound in the range of reaction times following expected auditory stimuli (120-160 ms [START_REF] Jakobs | Effects of timing and movement uncertainty implicate the temporo-parietal junction in the prediction of forthcoming motor actions[END_REF]).

However, the release of the APA did not seem to be timed with the same event in the two groups. Elite archers seemed to release their postural strategy at two distinct temporal instants (Figure 2). The shots labelled as early were released 23 ms ± 38 ms after the clicker fall, which is about 130 ms before the string release; whereas the shots labelled as late were released 163 ± 24 ms after the clicker fall, which is about only 15 ms after the string release.

With 77 ± 16 % of early shots, our results suggest elite archers aim to release their postural strategy with the clicker fall. On the contrary, sub-elite archers seemed to release the same postural strategy in terms of amplitude, but closer to the string release only. In the latter group, the shots labelled as early were released 119 ms ± 37 ms after the clicker fall, which is about 30 ms before the string release; whereas the shots labelled as late were released 180 ± 26 ms after the clicker fall, which is about 30 ms after the string release. The timing between the ML CoP movement onset in early shots and string release is much shorter than APA measured in predictable perturbation condition (100-150 ms range [START_REF] Krishnan | Early and late components of feed-forward postural adjustments to predictable perturbations[END_REF]) and its proximity to the late shots (60ms) point toward an anticipating postural strategy timed on one event, suggesting subelite archers are anticipating the string release event more than the clicker fall. This observation questions our arbitrary choice to use the perturbation as a temporal threshold to categorize early and late shots, which seems less suitable for the analysis of postural strategy in sub-elite archers than in elite archers (Figure 2). In the same way that APA are developed and improved through extensive training in sports requiring enhanced postural control such as slacklining or gymnastics [START_REF] Paillard | Plasticity of the postural function to sport and/or motor experience[END_REF], our findings suggest that elite archers have updated their internal models of postural control [START_REF] Kubicki | Balance control in aging: Improvements in anticipatory postural adjustments and updating of internal models[END_REF] whereas adolescent sub-elite archers may suffer from sensorimotor representation that are not completely mature [START_REF] Assaiante | Body schema building during childhood and adolescence: A neurosensory approach[END_REF][START_REF] Mallau | Postural strategies and sensory integration: No turning point between childhood and adolescence[END_REF] due to shorter experience [START_REF] Paillard | Plasticity of the postural function to sport and/or motor experience[END_REF] and/or biological maturation not fully completed [START_REF] John | Influence of biological maturity on static and dynamic postural control among male youth soccer players[END_REF].

As expected, the performance score was higher for the elite group compared to the sub-elite group [START_REF] Simsek | A comparison of the ground reaction forces of archers with different levels of expertise during the arrow shooting[END_REF], but scores between the early and late strategies within these groups did not show any statistical differences, despite being slightly higher for the early strategy. Therefore, an anticipatory postural strategy released sooner does not seem to have an influence in the performance score. However, in this study the participants shot only 18 arrows which is not sufficient to ascertain if those postural strategies differences would indeed affect the global score in competition [START_REF] Park | Using group sizes to optimise archery equipment[END_REF]. In a world elite event archer shoot 72 arrows just in the qualification round. According to a model proposed of losing 1.4 points per ranking place [START_REF] Park | Analysis of scores and arrow grouping at major international archery competitions[END_REF], such slightly lower score per arrow (0.2 points) can translate into the loss of 10 ranking places, which would harden the matchup for the face-off rounds.

Limitations

Studying elite level athletes comes with inherent difficulties, such as the limited number of participants and their tight schedule compatibility. Only trained archers participated in this study, which may limit the generalization of our results to all archers. Measuring the postural stability of the same group of young archers years later after full maturation of their body schema and/or biological maturation, as well as in beginner archers may better clarify how the postural strategy described here is learned and can be trained. Electromyography measurements, often used in anticipated postural adjustments studies [START_REF] Bouisset | A sequence of postural movements precedes voluntary movement[END_REF][START_REF] Aruin | Anticipatory postural adjustments associated with lateral and rotational perturbations during standing[END_REF][START_REF] Aruin | The role of motor action in anticipatory postural adjustments studied with self-induced and externally triggered perturbations[END_REF] were not considered in order to conduct the experiment without disturbing the archer's shooting sequence. Further experiments with muscular activity monitoring devices could pinpoint which muscles are pre-activated prior to the string release and complement the understanding of this anticipatory strategy.

Conclusion

By releasing pre-programmed anticipatory postural adjustments at a desired instant during their shooting sequence, trained archers seem to have fully integrated the characteristics of the balance disturbance of the string release on their whole-body upright posture. Elite archers seem to time the releasing of this anticipatory strategy with the clicker fall, whereas younger sub-elite archers time the releasing of this strategy rather with the string release instant. However, this difference in timing between the two levels of expertise did not allow to conclude as to whether releasing this strategy sooner helps increases performance.

Tables

1 Table 1 Postural stability indicators computed during the aiming phase and compared between the two groups. Abbreviations stand for: mean (𝑑 𝑚𝑒𝑎𝑛 ) and 2 root mean square distance (𝑑 𝑟𝑚𝑠 ) around the mean CoP, the mean (𝑣 𝑚𝑒𝑎𝑛 ) and maximum velocity (𝑣 𝑚𝑎𝑥 ) of the CoP, the sway amplitude (𝐴) and the 95% 3 confidence ellipse area (𝐴 95% ). ML = mediolateral (shooting) direction. AP = anteroposterior direction. *𝑝 < 0.05 displacement away from the target) between strategy (early/late -top panels) and expertise 42 (elite/sub-elite -bottom panels). Left: timeseries mean and error cloud. Right: SPM timeseries. No 43 temporal differences within strategy or expertise as there is no crossing over or under their respective 44 𝑝 value threshold (dotted lines).

  Matlab (versionR2021b, The MathWorks, Natick, USA) and the open source spm1d Matlab package 1 (version M.0.4.8). Statistical analysis was conducted using the open source statistical software JASP (version 0.16.4, Amsterdam, Netherlands).

27 Figure 1 . 33 Figure 2 . 40 Figure 3 .

 271332403 Figure 1

https://spm1d.org

Declaration of interest statement

The authors report no competing interests.


Jean-Baptiste Riccoboni

Tony Monnet

Antoine Eon

Patrick Lacouture

Jean-Pierre Gazeau

Mario Campone

Biomechanical comparison between manual and motorless device assisted patient handling: sitting to and from standing position

Keywords: Biomechanical, Caregiver, Constraint, Device, Disorders, Handling, Health, Load, Medical, Musculoskeletal, Nurse, Patient, Transferring

Background:

Occupational safety and health institutions report that caregivers are particularly at risk of developing work-related musculoskeletal disorders (WRMSDs) and patient handling is often pointed out as one of the main causes. While lots of studies addressed the use of assistive devices in order to protect caregivers, it seems that motorless stand-up lifts have not been studied yet.

Objectives: The aim of this work is to provide quantitative data about the loads in the low back area, as well as qualitative data about subjects perceptions, resulting from the use of a motorless stand-up lift and to compare them to those resulting from manual patient handling.

Methods: Nine caregivers participated to motion capture and ground reaction forces measurement sessions. These recordings were performed in three cases of handling: manual handling with one caregiver, manual handling with two caregivers, motorless device assisted handling. Forces and torques at the L5/S1 joint were then estimated through Inverse Dynamics process. A questionnaire about manual and motorless device assisted handling was also submitted.

Results: Motorless device assisted handling involved the smallest loads whereas manual handling with one caregiver involved the biggest loads.

Conclusions: Our findings suggest that, if the situation allows it, caregivers should be helped by another caregiver or use a motorless stand-up lift when handling a patient from sitting to standing position or from standing to sitting position considering the reduced loads these aids involve.

Introduction

It is now a well-known fact that people working in the healthcare sector, involved in some way in patient handling (called caregivers in the article), are particularly at risk of developing work-related musculoskeletal disorders (WRMSDs), especially at the back, and therefore to be impacted by all their usual consequences ( Bureau of Labour Statistics, 2016 ;Health and Safety Executive, 2016/17 (1) ; Health and Safety Executive, 2016/17 (2) ). Examples can be found in the works of Applegeet et al. (2007) , [START_REF] Hignett | Safety risks associated with physical interactions between patients and caregivers during treatment and care delivery in Home Care settings: A systematic review[END_REF] and [START_REF] Ribeiro | Work related musculoskeletal disorders in primary health care nurses[END_REF] .

As pointed out by [START_REF] Roffey | Causal assessment of workplace manual handling or assisting patients and low back pain: results of a systematic review[END_REF] , it is still difficult to show that workplace manual handling or assisting patients are independently causative of pain at specific areas. WRMSDs are complex problems, with many and various parameters. A few instances of these, while not directly linked to patient handling, are discussed in the articles of [START_REF] Carneiro | Work-related musculoskeletal disorders in home care nurses: Study of the main risk factors[END_REF] , [START_REF] Holman | Nurses' perceptions of how job environment and culture influence patient handling[END_REF] , [START_REF] Kay | Moments of speaking and silencing: Nurses share their experiences of manual handling in healthcare[END_REF] , [START_REF] Zodwa | Factors affecting working conditions in public hospitals: A literature review[END_REF] and [START_REF] Wulff Risør | A multi-component patient-handling intervention improves attitudes and behaviors for safe patient handling and reduces aggression experienced by nursing staff: A controlled before-after study[END_REF] .

Nevertheless, detecting and identifying the elements of patient handling leading to too much stress (compared to the acceptable values recommended by occupational safety and health institutions) then modifying them in order to decrease, nullify or avoid their related constraints (with the expectation of reducing the people exposure to WRMSDs) has been the subject of lots of studies these last years.

About detecting and identifying, questionnaires [START_REF] Mirmohammadi | A Cross-sectional Study on Work-related Musculoskeletal Disorders and Associated Risk Factors Among Hospital Health Cares[END_REF], physical [START_REF] Hwang | Wristband-type wearable health devices to measure construction workers' physical demands[END_REF]Lee et al., 2017 ;[START_REF] Mark | Working postures and physical activity among registered nurses[END_REF][START_REF] Yan | Wearable IMU-based real-time motion warning system for construction workers' musculoskeletal disorders prevention[END_REF] or physiological measurements [START_REF] Wang | Monitoring workers' attention and vigilance in construction activities through a wireless and wearable electroencephalography system[END_REF] and risk assessment methods including several relevant criteria [START_REF] Villarroya | Comparison between five risk assessment methods of patient handling[END_REF] ) are often used.

Other studies tried to determine some of the components of WRMSDs appearance: [START_REF] Mcdermott | Investigation of manual handling training practices in organisations and beliefs regarding effectiveness[END_REF] on training in patient handling, Lee et al. on availability and use of patient lifting equipment (2013) or more broadly on the importance of organizational safety culture and practices (2017) .

Some solutions are then proposed: the most popular consists of training the personnel [START_REF] Hodder | Effects of training and experience on patient transfer biomechanics[END_REF][START_REF] Marc | Reducing Patient Handling Injuries Through Contextual Training[END_REF], using assistive devices [START_REF] Alamgir | Evaluation of ceiling lifts: Transfer time, patient comfort and staff perceptions[END_REF]2009 ) or even approaches combining multiple interventions [START_REF] Haglund | Pediatric Safe Patient Handling[END_REF][START_REF] Huffman | On SHiPs and Safety: A Journey of Safe Patient Handling in Pediatrics[END_REF]2014 ;[START_REF] Krill | Empowering staff nurses to use research to change practice for safe patient handling[END_REF] even though these should be furthered [START_REF] Thomas | Interventions to reduce injuries when transferring patients: A critical appraisal of reviews and a realist synthesis[END_REF]. In a similar way, [START_REF] Waters | Patient Handling Tasks with High Risk for Musculoskeletal Disorders in Critical Care[END_REF] described high risk tasks and proposed solutions for each.

Finally there are studies that try to compare these solutions and it is in this category that our work falls, focusing on the potential benefits that mechanical assistive devices can get to the problem of WRMSDs. Even if other technologies to measure biomechanical constraints are promising, like inertial measurement units [START_REF] Bolink | Validity of an inertial measurement unit to assess pelvic orientation angles during gait, sit-stand transfers and step-up transfers: Comparison with an optoelectronic motion capture system[END_REF][START_REF] Faber | Estimating 3D L5/S1 moments and ground reaction forces during trunk bending using a full-body ambulatory inertial motion capture system[END_REF][START_REF] Koopman | Estimating the L5S1 flexion/extension moment in symmetrical lifting using a simplified ambulatory measurement system[END_REF], electrogoniometers [START_REF] Weiner | Repositioning a passive patient in bed: Choosing an ergonomically advantageous assistive device[END_REF] and computer vision [START_REF] Mehrizi | A computer vision based method for 3D posture estimation of symmetrical lifting[END_REF], most of the studies in this category have used optoelectronic motion capture system. [START_REF] Dutta | A biomechanical assessment of floor and overhead lifts using one or two caregivers for patient transfers[END_REF] , [START_REF] Garg | A biomechanical and ergonomic evaluation of patient transferring tasks: bed to wheelchair and wheelchair to bed[END_REF][START_REF] Pasqualina | Comparison of cumulative low back loads of caregivers when transferring patients using overhead and floor mechanical lifting devices[END_REF] and [START_REF] Zhuang | Biomechanical evaluation of assistive devices for transferring residents[END_REF] have worked on a couple of devices going from walking belts to overhead lifts and compared the biomechanical constraints involved in each case. [START_REF] Zhuang | Psychophysical assessment of assistive devices for transferring patients/residents[END_REF] have also done a psychophysical assessment of some assistive devices. Yet and to our knowledge, it seems that one type of device have not been studied: the motorless stand-up lifts.

The aim of this work is to provide quantitative data about the loads in the low back area, as well as qualitative data about subjects perceptions, resulting from the use of a motorless stand-up lift and to compare them to those resulting from manual patient handling.

Methods a. Subjects i. Caregivers

A group of nine caregivers, composed of seven women and two men, volunteered and were therefore selected after giving their informed consent in accordance with local ethical procedures. They were from 21 to 59 years old (mean: 38.3; standard deviation: 14.2), were from 155 to 181 centimeters tall (169.7; 9), weighed from 48 to 98 kilograms (66.1; 14.7). There were five nurses and four assistant nurses; all of them knew how to manually handle patient (at least from their initial training courses) and how to use the motorless stand-up lift; all of them were working at the Institut de Cancérologie de l'Ouest (ICO), a institute specialized in cancer treatments (both outpatient and inpatient), research and training. They had from 0.7 to 41 years of experience (17.2; 15). Seven of them suffered or had suffered from MSDs, mainly at the low back and shoulders areas. They claimed that they manually handle patients 9.5 times a day on average (with a standard deviation of 6.3) and that this require 1.6 persons on average (0.5). They used the motorless stand-up lift 0.8 times a day (1.7) and it requires 1.3 persons (0.5).

ii.

Surrogate patient

Each caregiver played the role of the handled patient (called the surrogate patient in this article) several times, meaning they had to imitate the behaviour of real patients. Their instructions were to provide the minimum efforts allowing the handling to be carried out. In addition and for the purpose of providing a point of comparison between the subjects, a male surrogate patient (called MSP) measuring 175 centimeters and weighing 63 kilograms participated in all the experiments with the same instructions. More detailed explanations will be given afterwards.

b. Setting and equipment

The experiments took place in a motion capture room at the Institut PPRIME. As these measures were not in situ , we used a seat with an adjustable (and not tracked) height in order to mimic an hospital bed in terms of functionality and clutter. This height changed between subjects.

The motorless stand-up lift used was a Vertic'Easy from HMS-VILGO ( Fig. 1 ).

Fig. 1. Motorless stand-up lift

Its dimensions was 1300mm in height, 510mm in length, 470mm in width and it weighed 16kg unloaded. The position of the handgrips and tibial support were not tracked (as determining the best parameters to handle patient was not an objective of this study) but did not changed during and between experiments, even if the subjects were told that they were allowed to change them depending on what they thought was most comfortable.

c. Measurement techniques

By measuring the three-dimensional location of body segments and the mechanical forces applied by the subject on the ground then by using Inverse Dynamics process, it is possible to calculate the joint reaction forces and torques. We also wanted to obtain several details about the subjects so we would notably be able to compare our results to those of previous studies.

i. Motion capture system

A Vicon Motion Systems Limited (Oxford, GB) motion capture system composed of 19 MX T40 cameras sampled at 100Hz, connected through Vicon Motion Systems Limited's Nexus 1.8.5, was used for all the recordings. Subjects were equipped with 52 reflective markers following a modified version of the Istituto Ortopedico Rizzoli Full-Body Marker Set [START_REF] Cappozzo | Position and orientation in space of bones during movement: anatomical frame definition and determination[END_REF][START_REF] Leardini | Multi-segment trunk kinematics during locomotion and elementary exercises[END_REF], showcased in Fig. 2 . Head and upper limbs segments had their movements recorded with a view to describe full-body postures in future studies but were not used in this one nor included in the biomechanical constraints computation.

Fig. 2. Subject equipped with markers

ii.

Ground reaction forces measuring instrument Ten M40-600x400 force platforms from SENSIX (Poitiers, FR) sampled at 1000Hz were placed at the center of the recording and calibrated area and were synchronized with the kinematics acquisitions by Vicon Motion Systems Limited's Nexus 1.8.5. Their layout allowed subjects to put their feet "naturally", in their own words, while having one foot per platform during handling.

iii.

Subject data procurement technique

A questionnaire was submitted to the subjects at the end of each session. Despite the fact that not all the questions and their answers will be described in this article, here are the main topics covered: background information (like gender, age, position, seniority…), presence or not of WRMSDs and details about it if so, quantity and perception of manual and motorless device assisted patient handlings. The objective of this questionnaire was to strengthen the analysis of the biomechanical results.

d. Handling methods

Prior to the experiments, we spent one month at the ICO premises to get a better understanding of all the tasks performed by caregivers. The following handling methods were chosen from the observations made at the ICO and from the advices of the first participating caregivers.

While the techniques employed by the subjects were highly similar, their gestures presented some variations regarding hands positions and orientations in particular. We decided to let them some freedom on this in order to obtain the most natural behaviour.

i.

Manual handling with one caregiver (MH1C)

Subjects were asked to lift alone the surrogate patients by facing them and placing their legs on each side of those of the surrogate patients. The Fig. 4a and4b illustrate this handling where the subject chose to place his hands on the back of the surrogate patient with his arms under the armpits. In Fig. 4c , the subject chose to place his hands lower on the back, one above the other.

ii.

Manual handling with two caregivers (MH2C)

When two caregivers were involved, the one equipped with the markers (called the main caregiver in this article) was on the left of the surrogate patient ( Fig. 4d and4e ) while the other was on the right. Both were free on how to lift the surrogate patient as long as the feet of the main caregiver were entirely on two different force platforms. The right arm of the main caregiver was placed under the left armpit of the surrogate patient and the right hand was on the left forearm.

iii.

Motorless device assisted handling (MDAH)

The subject faced the surrogate patient when using the motorless stand-up lift. The latter was in the middle, the tibial support on the side of the surrogate patient. The subject immobilized the device thanks to the upper handgrips ( Fig. 4f ) while the surrogate patient used the lower handgrips as a support to stand up. Pictures of a MDAH are shown in Fig. 3 . Caregivers arrived by group of three at the Institut PPRIME (all recordings took place over three days). They had been previously informed of the general conduct of the recording session, were asked to come with tight clothes and gave their consent through a signed agreement. At their arrival, we presented them more thoroughly the different steps of the day. Subjects were equipped with the markers one at a time and we proceeded every time to a few trials of manual and motorless device assisted handling before starting to record. Once the subjects felt ready, we started with manual handlings. The equipped subject (playing the role of the main caregiver) was asked to lift the two others subjects and the male surrogate patient (MSP) up alone three times each (one by one). The main caregiver was next helped by a second caregiver to perform the manual handlings of the last remaining subject and the MSP; then the second caregiver and the remaining subject switched roles. Finally, the main caregiver handled the two other subjects and the MSP with the motorless stand-up lift. Surrogate patients were sitting on a seat at a height set by the main caregiver. Each subject performed 3 trials for the 10 handling cases. An example of this description is in the Table 1 . We removed afterwards the markers from the main caregiver and positioned them on one of the two other subjects (who became the main caregiver) to restart all the procedures described above. When all the recordings for the three subjects were done, we gave them the questionnaire to fill as their last task. f. Data analyses i.

Biomechanical measurement

Kinematics and ground reaction forces data were exported as a .c3d file then imported into C-Motion's (Germantown, MD) Visual3D. Beforehand, we had built a custom kinetic model in which segments and joints are considered as rigid bodies and spherical joints respectively. Joints positions were defined as follow: midpoints of the malleoli for the ankles, midpoints of the femoral epicondyles for the knees, results of regression equations for hips adapted from [START_REF] Bell | Prediction of hip joint centre location from external landmarks[END_REF][START_REF] Bell | A comparison of the accuracy of several hip center location prediction methods[END_REF] ; the back have been divided into five segments with the six joints being from bottom to top: the fifth, third and first lumbar vertebrae, the seventh and third thoracic vertebrae and the seventh cervical vertebrae. Segments definition of volumes, mass, moments of inertia and center of gravity location were inspired by the work of Ernest P. [START_REF] Hanavan | A mathematical model of the human body[END_REF] . Associating feet to their corresponding force platforms and performing the Inverse Dynamics process, we obtained joint forces and torques for each segment in its frame of reference. These results were finally compared in the three cases (MH1C, MH2C and MDAH). To improve data readability, each trial was divided in two parts: when subjects helped the surrogate patient to stand up (called the standing up part) and when subjects helped the surrogate patient to sit (called the sitting part).

ii.

Answers to the questionnaire Answers to the questionnaire were used to know, inter alia, subject's perceptions and were compared to biomechanical measurement.

iii.

Statistical analysis

One-way ANOVAs were run for forces and torques to compare standing up and sitting parts and to compare handling methods. The significance level was 0.05.

Multiple comparisons were then performed (Tukey's test) to test for differences among pairs of means.

Results

a. Loads Fig. 5 and Fig. 6 (for sitting and standing part respectively) present forces and torques at the L5/S1 joint of one of the subjects using each handling method to handle the MSP (weight: 63kg) each time. Graphs for other subjects had similar looks. Abscissa axis is in percentage of movement completion (0 beginning and 100 end of the movement) and ordinate axis are in N or N.m.

For the sitting part, maximum values for forces and torques were close between both manual handling methods. These values however were smaller with the motorless handling method. Forces remained constant overall through handling for the three methods while torques increased through handling for the manual handlings and were nearly constant for the MDAH. When looking at the motion files, the raise in forces and torques from 0 to 5% of movement completion happened when the surrogate patient knee's started to bend. The drops in forces and torques near 80% of movement completion happened when the surrogate patient started to lie on the seat. For the standing up part, differences between handling methods in force and torque values were bigger. MH1C had the highest maximum values while MDAH had the lowest. These peak values happened around 50% of movement completion for manual handling methods. Like for the sitting part, MDAH induced nearly constant forces and torques. When looking at the motion files, the raise in forces and torques from 0 to 5% of movement completion happened when the main caregiver started to pull the surrogate patient up. When comparing sitting and standing up parts, forces and torques were significantly bigger during the standing up part with the manual handling methods and there were no significant differences with the MDAH method. Mean forces increased from 471.1N to 613.8N (F=170.9; p<0.05) with one caregiver, from 454.8N to 537.2N (F=130.6; p<0.05) with two caregivers and were the same (353.3N≈350.2N; F=1.53; p=0.22) with one caregiver and the motorless stand-up lift. Mean torques increased from 114.8N.m to 148.2N.m (F=74.53; p<0.05) with one caregiver, from 94.6N.m to 103.0N.m (F=8.69; p<0.05) with two caregivers and were the same (64. 4N.m≈68.8N.m; F=1.26; p=0.26) with one caregiver and the motorless stand-up lift. For the sitting part: changing handling method induced significant differences for forces (F=186.7; p<0.05) and torques (F=118.5; p<0.05). Multiple comparisons revealed that forces differed with a p-value of 0.04 for the MH1C/MH2C comparison and with a p-value <0.01 for the MH1C/MDAH and MH2C/MDAH comparisons. It revealed that torques differed with a p-value of <0.01 for the three comparisons. For the standing up part: changing handling method induced significant differences for forces (F=493.4; p<0.05) and torques (F=217.1; p<0.05). Multiple comparisons revealed that forces and torques differed with p-values <0.05 for the three comparisons. protection of caregiver health, better control of patient equilibrium. In favour of manual handling, their arguments were: more pleasant for the personnel and less intimidating for the patient, quicker and more adaptable to the environment, causing less cluttering.

Discussion a. MH1C versus MH2C

In light of these results and if the situation allows it, we recommend that caregivers handle patients with the help of one colleague instead of doing it alone. While the load difference is the smallest of our comparisons between handling methods, caregivers may feel a benefit over the long term because reducing the loads have a positive effect on the exposure to WRMSDs by reducing fatigue phenomenon and risk of injury. Loads were also applied less time with MH2C, as shown for instance in Fig. 5b and5d .

b. Manual methods versus MDAH Furthermore, we strongly incite caregivers to use the motorless stand-up lift considering the involved loads. This device helped to greatly reduce forces and torques on the low back area for all the subjects. Using it is even more interesting if there is no other caregiver available to help handling patients as this comparison presents the biggest difference.

c. Questionnaire

We expected these results because of what we retained from the month spent at the ICO (unpublished data). The low utilization of the motorless stand-up lift by caregivers, compared to the frequency of manual handlings as claimed in section 2.a.i, seemed to be more related to the difficulties in getting it ready for use (e. g. due to its relative unavailability) than to the difficulties of its use. This study could bring more information about its benefits and we felt that caregivers' opinion on handling equipments evolved through our work as they seemed more inclined to use them in the future.

d. Comparison with previous findings

As written in the introduction, it seems that there are no studies with an aim close to ours. We would like to remind on another note that our computations were done using the ascent method of the Inverse Dynamics process. The values presented here are composed of the effect of the subject upper body (trunk, head, upper limbs) mass and the load applied by the surrogate patient which complicate comparisons with studies using other measurement or computation methods.

That being said, our findings are going in most studies' direction: dividing the loads between caregivers and/or using assistive device help reducing loads and thus fighting against WRMSDs.

e. Limitations i. Subjects

To begin with, the number of subjects was quite low. This was mainly due to the fact that it was not possible for the ICO to send more people as it would have been difficult to ensure quality care without enough employees. Means presented in this study may vary with more subjects. Having both genders further diminish the reliability of our comparisons. The subject data (like age, size, weight, experience…) present sometimes important standard deviation and, in combination with the small pool of subject, this could affect our results in unknown ways. Experience with an assistive device, for instance, can help the caregivers to lower the loads on their low backs ( Dutta et al.;2011 ).

As more than 60% of the caregivers working at the ICO present MSDs (unpublished results), we decided to not exclude them from our selection of subjects because it would have been even more difficult to find participants and also not representative of this population. Subjects suffering or having suffered from MSDs may have behaved differently from others, which may have altered the values of the biomechanical measurements.

It should be noted that all the subjects were able to pull themselves up with the motorless stand-up lift. There are cases where real patients can not do so because they are too weakened, using a motorless stand-up lift is then not possible. Caregivers are often well aware of this fact but it is worth to mention it.

ii.

Setting and equipment

The experiments were done in a motion capture room. Despite our efforts to mimic an hospital room and the numerous trials performed before recording, environment and technical requirements may have impacted subject behaviour.

We tested only one model of motorless stand-up lift. Results may be different with other models and/or brands.

iii.

Data collection

Even if we tried to not exhaust our subjects, our data collection procedure had to be efficient in order to allow the caregivers to get back to their job as soon as possible. Therefore, fatigue may have induce an increasing bias in the values of the biomechanical measurements through the day. iv.

Handling methods

There are two important points here. First, we only test one method to handle the surrogate patient for each case of handling, each method described as the best one for its case according to the caregivers. It could be interesting to test several methods for each case and to compare them.

Second, while most of the aspects of each methods were imposed, some (like hands position) were decided by the subject. We believe that the results have not been significantly affected but further research should be done. v.

Measurement techniques

Our subjects worn shoes during the experiments because they do at the hospital. As described by [START_REF] Eytan | In-shoe center of pressure: Indirect force plate vs. direct insole measurement[END_REF] , force platforms give an approximate location of the center of pressure and therefore induce an approximation in calculations. Despite having ten force platforms and that the caregivers told they felt no constraints about the placement of their feet, instrumented force shoes could have been interesting to use. Some applications and information about this technology can be found in the work of [START_REF] Faber | Determination of joint moments with instrumented force shoes in a variety of tasks[END_REF]2018) .

As pointed out by [START_REF] Ghezelbash | Effects of motion segment simulation and joint positioning on spinal loads in trunk musculoskeletal models[END_REF] and since our biomechanical model includes posteriorly shifted joints and frictionless spherical joints, our computed spinal loads are likely overestimated. This phenomenon is even more complex when considering that there is a displacement of the center of rotation during motion [START_REF] Senteler | Sensitivity of intervertebral joint forces to center of rotation location and trends along its migration path[END_REF]2018 ). Research done by [START_REF] Nerot | Estimation of spinal joint centers from external back profile and anatomical landmarks[END_REF] could help but is yet limited to the standing position. This should not however change our findings because we worked on comparisons.

Finally, it should be noted that there are side effects when using handling equipment: some risks are discussed in the study of [START_REF] Elnitsky | Implications for patient safety in the use of safe patient handling equipment: A national survey[END_REF] and [START_REF] Menzel | The physical workload of nursing personnel: association with musculoskeletal discomfort[END_REF] talk about the problem of loads displacement to wrists and hands. Unfortunately, we did not have the means nor the time to consider this last point for the motorless device assisted handlings. [START_REF] Oakman | Workplace risk management practices to prevent musculoskeletal and mental health disorders: What are the gaps?[END_REF] tried to widen the WRMSDs issue by highlighting the gaps of modern workplace risk management practices.

Conclusions

To be helped by an other caregiver or by the use of a motorless stand-up lift when handling a patient from sitting to standing position or from standing to sitting position reduce loads in the low back area. Although sometimes difficult to obtain or apply because of several adverse conditions, these aids are well received by the caregivers. We suggest, if the situation allows it, to use them considering the reduced loads they involve.

Fig. 3 .

 3 Fig. 3. Pictures of a MDAH

Fig. 5 .

 5 Fig. 5. Forces and torques at the L5/S1 joint during the sitting part with: MH1C (a. and b.), MH2C (c. and d.), MDAH (e. and f.)

Fig. 6 .

 6 Fig. 6. Forces and torques at the L5/S1 joint during the standing up part with: MH1C (a. and b.), MH2C(c. and d.), MDAH (e. and f.)

Fig. 7 .

 7 Fig. 7. Comparison between sitting and standing up part with respect to forces and torques at the L5/S1 joint for each handling method

Fig. 8 .

 8 Fig. 8. Comparison between handling methods with respect to forces and torques at the L5/S1 joint for both trial parts

Table 1 .

 1 Example of trials recording (S1, S2, S3 are subjects; S1 is the only one equipped with markers; MSP is the male surrogate patient)

	Handling method	Caregiver(s)	S2	Surrogate patient S3	MSP
	MH1C	S1	3 trials	3 trials	3 trials
		S1 and S2	/	3 trials	3 trials
	MH2C
		S1 and S3	3 trials	/	3 trials
	MDAH	S1	3 trials	3 trials	3 trials

Acknowledgements

This project was conducted as part of a partnership between the Institut PPRIME and the Institut de Cancérologie de l'Ouest (ICO). It was funded by the ICO. We would like to thank all the persons involved in the project, with a particular attention for the caregivers, who were approachable and cooperative right from the start until the very end. Findings, conclusions, views and recommendations expressed in this article do not necessarily reflect those of the ICO. Mention of product names does not constitute endorsement by the Institut PPRIME and the ICO. This research was partially funded by the French government research program "Investissements d'Avenir" through the Robotex Equipment of Excellence: ANR-10-EQPX-44.

Subjects slightly prefered MDAH to manual handling from the point of view of both the caregiver and the surrogate patient. As we expected tight results, we asked them to explain their answers. In favour of the motorless stand-up lift, their arguments were: easier to use for the personnel, easier to perform lifts without pain and fatigue, reassuring when the patient body weight is important or when the patient physical abilities are diminished, better