Mornieux Guillaume
email: guillaume.mornieux@univ-lorraine.fr

Weltin Elmar

Pauls Monika

Rott Franz

Gollhofer Albert

Guillaume Mornieux

Influence of a full body compression suit on trunk positioning and knee joint mechanics during lateral movements

Keywords: cuttings, perturbation, trunk lateral lean, knee abduction moment, agility

Trunk positioning has been shown to be associated with knee joint loading during athletic tasks, especially changes of direction. The purpose of the present study was to test whether a full body compression suit (FBCS) would improve trunk positioning and knee joint control during lateral movements. Twelve female athletes performed lateral reactive jumps (LRJ) and unanticipated cuttings with and without the customized FBCS, while 3D kinematics and kinetics were measured. FBCS did not influence trunk positioning during LRJ and led to increased trunk lateral lean during cuttings (P < .001). However, while wearing FBCS, knee joint abduction and internal rotation angles were reduced during LRJ (P < .001 and P = .013, respectively), whereas knee joint moments were comparable during cuttings. FBCS cannot support the trunk segment during unanticipated dynamic movements. But increased trunk lateral lean during cutting maneuvers was not high enough to elicit increased knee joint moments. On the contrary, knee joint abduction and internal rotation were reduced during LRJ, speaking for a better knee joint alignment with FBCS. Athletes seeking to improve trunk positioning may not benefit from a full body compression suit.

Introduction

Lateral movements with changes of direction are common in many team sports such as soccer or handball. During cutting maneuvers, the athlete performs a complex dynamic task by quickly changing their direction of movement while securing their balance. Knee joint control is an essential requirement during such movements to prevent misalignments in the lower extremities with the consequence that the knee may be injured. [START_REF] Olsen | Injury mechanisms for anterior cruciate ligament injuries in team handball: a systematic video analysis[END_REF][START_REF] Alentorn-Geli | Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 1: Mechanisms of injury and underlying risk factors[END_REF] Non-contact anterior cruciate ligament (ACL) injury mechanism during cutting maneuvers typically involves knee external flexion moment combined with dynamic valgus rotation with the body weight shifted over the injured leg. [START_REF] Alentorn-Geli | Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 1: Mechanisms of injury and underlying risk factors[END_REF] Trunk positioning during lateral movements is of interest as it would influence the location of the body weight, and potential knee joint loads. [START_REF] Hewett | The mechanistic connection between the trunk, hip, knee, and anterior cruciate ligament injury[END_REF][START_REF] Hughes | A review of recent perspectives on biomechanical risk factors associated with anterior cruciate ligament injury[END_REF] Hewett and Myer [START_REF] Hewett | The mechanistic connection between the trunk, hip, knee, and anterior cruciate ligament injury[END_REF] proposed a knee injury model that accounted for both the trunk and hip joints, because increased trunk lateral lean is reportedly associated with larger external knee abduction moments. Greater lateral trunk position has been shown during anterior cruciate ligament injury situations in female athletes. [START_REF] Hewett | Video analysis of trunk and knee motion during non-contact anterior cruciate ligament injury in female athletes: lateral trunk and knee abduction motion are combined components of the injury mechanism[END_REF] Different laboratory studies using cutting maneuvers and lateral reactive jumps paradigms have demonstrated that increased trunk lateral lean is positively associated with increased knee abduction moments. [START_REF] Jamison | Knee moments during run-to-cut maneuvers are associated with lateral trunk positioning[END_REF][START_REF] Weltin | Influence of gender on trunk and lower limb biomechanics during lateral movements[END_REF][START_REF] Dempsey | The effect of technique change on knee loads during sidestep cutting[END_REF] Further, increased knee joint loading has been associated with increased trunk rotation away from the stance limb. [START_REF] Dempsey | The effect of technique change on knee loads during sidestep cutting[END_REF][START_REF] Frank | Trunk and hip biomechanics influence anterior cruciate loading mechanisms in physically active participants[END_REF] For instance, higher knee internal rotation moment was found for trunk rotation in the opposite direction to the cutting maneuver. [START_REF] Dempsey | The effect of technique change on knee loads during sidestep cutting[END_REF] Accordingly, any improvement of trunk positioning is of interest with regard to knee joint loading. This would be especially true for female athletes, whose incidence of ACL injuries in sports such as soccer and basketball is much higher than male athletes. [START_REF] Agel | Anterior cruciate ligament injury in national collegiate athletic association basketball and soccer: a 13-year review[END_REF][START_REF] Renstrom | Non-contact ACL injuries in female athletes: an International Olympic Committee current concepts statement[END_REF] In sport sciences, the influence of compression garments, providing passive support to different body segments during exercise has been extensively studied. [START_REF] Macrae | Compression garments and exercise. Garment considerations, physiology and performance[END_REF] It has been showed that compressive and elastic garments could reduce the range of motion of the joint. [START_REF] Bernhardt | Influence of moderate prophylactic compression on sport performance[END_REF][START_REF] Doan | Evaluation of a lower-body compression garment[END_REF] Compression shorts reduced the active range of motion during hip flexion in a non-athletic task [START_REF] Bernhardt | Influence of moderate prophylactic compression on sport performance[END_REF] and hip range of motion during a 60 m sprint. [START_REF] Doan | Evaluation of a lower-body compression garment[END_REF] Wearing shorts with directional compression was associated with reduced hip adductor activity during cutting manoeuvres. [START_REF] Chaudhari | Hip adductor activations during run-to-cut manoeuvres in compression shorts: implications for return to sport after groin injury[END_REF] In this latter study, the use of diagonal bands of fabric that only stretch along their length increased the compression level along the line of action of hip adductors. Therefore, apparel, through their compression and elasticity properties, could be used to passively support body segments and joints during various tasks. A compression suit that would functionally support the trunk over the pelvis and leg could be a solution to avoid trunk positioning in the opposite direction to the intended movement. Such apparel would work as external trunk stabilizers to improve trunk positioning during various tasks, especially lateral movements, and subsequently possibly reduce knee joint loading.

In sports, any improvement of movement control would further be relevant if associated with enhanced performance. However, when considering whole-body dynamic tasks, wearing a compression suit did not influence performance in cricket players [START_REF] Duffield | Comparison of three types of full-body compression garments on throwing and repeat-sprint performance in cricket players[END_REF] or endurance performance during running. [START_REF] Sperlich | Different types of compression clothing do not increase sub-maximal and maximal endurance performance in well-trained athletes[END_REF] Only one study has shown possible benefits of a whole-body compression garment on performance during high intensity intermittent exercises, mainly because of better muscle oxygenation. [START_REF] Sear | The effects of whole-body compression garments on prolonged high-intensity intermittent exercise[END_REF] Therefore, athletic performance is not likely to be increased when using garments.

The aim of this present study was to test whether compression garments can improve trunk and hip positioning and resultant knee joint loading, while not decreasing performance during dynamic lateral sporting movements. For that purpose, a full body compression suit with functional elastic straps was designed to improve trunk positioning and knee joint control during lateral movements. We tested the hypotheses that i) trunk and hip joint angles would be reduced with the compression suit, ii) knee joint abduction moment would be reduced with the compression suit and iii) exercise performance level would be maintained while wearing the compression suit.

Methods

According to other studies evaluating comparable dependent measures during unanticipated lateral movements [START_REF] Mornieux | Anticipatory postural adjustments during cutting manoeuvres in football and their consequences for knee injury risk[END_REF] or the influence of compression shorts on sports performance, [START_REF] Bernhardt | Influence of moderate prophylactic compression on sport performance[END_REF] a sample size of 13 participants was estimated relevant for statistical analysis.

The rationale for focusing on females was their relative higher risk of sustaining non-contact ACL injuries. [START_REF] Agel | Anterior cruciate ligament injury in national collegiate athletic association basketball and soccer: a 13-year review[END_REF] Among the 13 female athletes recruited to participate to the study, one was injured during a training session with her team during the investigation period and could not take part in the measurements. Therefore, 12 female athletes (body height = 171 ± 4 cm; body mass = 59 ± 4 kg; age = 22 ± 2 years) who were familiar with lateral movements (soccer, basketball and handball) completed the entire study. Participants did not have a previous history of serious knee injury or any current knee pain. Written consent was obtained prior to study participation. The study was approved by the local ethics committee (approval 72/14) and was conformed to requirements stipulated in the Declaration of Helsinki.

During a first visit, participants' body dimensions were scanned (BodyKinectizer, adidas IT Innovation, Herzogenaurach, Germany), in order to customize the full body compression suit. During a second visit at our laboratory, participants performed different lateral movements and performance tests under two different apparel conditions, either while wearing the customized full body compression suit (FBCS) with 2x thermoplastic polyurethane (TPU) layers or with standard sport shorts and bras (Control). FBCS was a prototype specially designed for the present study in order to provide support for the trunk over the pelvis segment in three dimensions. For that purpose, a mean compression level of 5.2 mmHg was achieved and the 2x TPU layers design linked the upper body with lower limbs across the pelvis in the back (Figure 1). The compression level was first determined for elastic and TPU fabrics separately, according to their relative stretch and then averaged over the whole suit. At specific points, the compression level was even greater, for instance at the low waist and thigh lines (8.3 and 6.7 mmHg, respectively). By determining the compression exactly and creating a customized suit for each subject, identical apparel conditions were guaranteed for all subjects, and the possible effectiveness of the compressive suit was measurable. [START_REF] Troynikov | Factors influencing the effectiveness of compression garments used in sports[END_REF] To determine to what extent wearing FBCS would influence trunk range of motion, trunk passive flexibility in frontal and sagittal planes was measured according to the literature. [START_REF] Yoshida | The effect of kinesio taping on lower trunk range of motions[END_REF] In addition, trunk rotation passive flexibility was evaluated while participants lay on their backs with the right bent knee crossing over the thigh of the left extended leg. Participants remained relaxed and kept their back and both shoulders in contact with the floor, while the distance between the floor and knee was measured. Reduced mean trunk passive flexibility over all participants was obtained with FBCS compared to Control for flexion (2.2 cm) and rotation (2 cm), but was comparable for lateral flexion.

The two conditions were randomly assigned over all participants. According to pilot measurements, participants were given 30 minutes to become accustomed to the suit. During this period, standardized exercises were carried out based on runs, jumps and cuttings. When subjects started the measurement with FBCS, another 30 minutes practice was given to them prior to the Control condition.

Firstly, participants performed perturbed lateral reactive jumps (LRJ) to focus on the influence of FBCS on movement control during a functional task associated with high trunk lateral lean in the frontal plane. From a standing position, subjects performed a lateral reactive movement by jumping to their left side and then pushing off with their left leg to get back to their original position. Jump distance was set at 124% of the individual leg length. In order to induce lateral perturbations, subjects performed LRJ on a motor-driven plate, which could translate in the horizontal (medio-lateral) direction. The specially designed motor-driven plate (University of Sport Science, Freiburg, Germany) constituted of two independently moving plates (1.15 m x 0.3 m), each driven by a separate motor (AKM51G, Danaher Motion, Düsseldorf, Germany). Details of this material have been previously described in the literature. [START_REF] Mornieux | Changes in leg kinematics in response to unpredictability in lateral jump execution[END_REF] LRJ were performed, either without any perturbation (stable condition), or with a perturbation to the left (sliding condition) or to the right (counteracting condition) as soon as the foot made initial contact with the plate. Each perturbation consisted of a 10 cm translation in either direction over 250 ms, inducing a peak velocity of 0.55 m.s -1 after 120 ms.

Unpredictability was introduced using these three different landing modalities (stable, sliding, counteracting) that were unknown prior to movement execution. Perturbation was triggered by breaking a light beam (M18 series, Panasonic Electric Works Europe AG, Holzkirchen, Germany), 0.04 m above the plate, to compensate for the delay in activation of the motor-driven plate.

Secondly, participants were asked to perform either cutting maneuvers to 45°, crossovers to -20° or straight runs on a force plate (BP600900, AMTI®, Watertown, USA), after approaching the force plate (FP) at 4 ± 0.2 m.s -1 . The direction of the movement was indicated in a randomized order by a light signal occurring 650 ms before the left foot contacted the FP. The visual light stimulus was triggered automatically during the approach run via a light switch (M18 series, Panasonic Electric Works Europe AG, Holzkirchen, Germany) placed 3.7 m from FP. According to a light cue delay set individually, the 650 ms time could be achieved.

The three lights were placed at eye level in the directions of the three tasks, 7 m away from the FP. The 45° cutting maneuver to the right was the only movement to be further analyzed due to its relevance when investigating performance and injury aspects of changing direction tasks.

Finally, three different performance tests were conducted to evaluate agility with the 5-10-5 yards test, jump height, i.e. leg muscle power, with counter movement jumps (CMJ) and core strength with a cable lift test. This latter test targets the action of hips, torso rotators, upper back, chest and shoulders while turning the trunk away from the strength machine as the hands first pull the rope cable handle towards the chest and then push up and away. The rationale for these tests is the importance of agility, leg muscle power and core strength for high performance during soccer, basketball or handball games. The good reliability of jump height measurements during CMJ has been reported in the literature (intraclass correlation coefficient (ICC) = 0.9), [START_REF] Moir | Intersession reliability of vertical jump height in women and men[END_REF] as well as for the 5-10-5 yards agility test (ICC > 0.87). [START_REF] Stewart | Reliability, factorial validity, and interrelationships of five commonly used change of direction speed tests[END_REF] The time to accomplish the agility test was measured manually. Two CMJ were performed on the FP. The cable lift test was conducted twice at 70% of one-repetition maximum.

All participants were familiar with such movements due to their sport practice and were given enough practice trials to feel comfortable with all task modalities prior to the measurements.

Three-dimensional leg kinematics were recorded using reflective skin markers ( 14 mm) placed on anatomical landmarks of the subjects' trunk (three markers), pelvis (four markers) and left thigh (three markers), shank (four markers) and foot (six markers). Markers were captured with a 12-camera motion analysis system (Vicon MX TM , VICON Motion Systems Ltd.®, Oxford, UK) with a sampling frequency of 200 Hz. A static trial with the subject standing in a pre-defined neutral position was used to define the kinematic model and to calculate segment lengths and joint centers. The kinematic modelling was performed with the motion analysis software (Vicon Nexus 1.6.1, VICON Motion Systems Ltd.®, Oxford, UK). Joint kinematics in three rotational degrees of freedom were determined using an yxz Euler rotation sequence of the respective segment coordinate system according to the global reference system defined in Vicon. This rotation sequence was around the flexion/extension axis first, then around the abduction/adduction axis followed by the rotation around the external/internal axis. This marker placement and kinematic modelling were used in previously published articles. [START_REF] Weltin | Influence of gender on trunk and lower limb biomechanics during lateral movements[END_REF][START_REF] Mornieux | Anticipatory postural adjustments during cutting manoeuvres in football and their consequences for knee injury risk[END_REF] Marker trajectory and ground reaction force signals were both filtered with a low pass Butterworth filter (4 th order, 15 Hz cut-off frequency) prior to calculating external joint moments with a standard inverse dynamics approach. Three-dimensional kinematic data of the trunk segment and hip joint were analyzed to understand the influence of FBCS on trunk positioning. Three-dimensional kinematics and kinetics of the knee joint were further analyzed to determine the influence of FBCS on knee joint stability and loading. Both kinematics and kinetics were measured during cutting maneuvers, but only kinematics was observed during LRJ. These variables were analyzed at the time of their peak values during the first half of ground contact during LRJ, to give some insights into how the apparel condition influences the range of motion of the different body segments. However, during cutting maneuvers, the different variables were determined at the time of peak knee abduction moment, as this is a relevant parameter associated with knee injury. [START_REF] Hewett | Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study[END_REF] Trunk lateral lean and trunk axial rotation were defined relative to the global coordinate system, i.e. deviations from the vertical axis and from the anterior-posterior axis, respectively.

Both were positive for an orientation in the new movement direction (i.e. to the right direction).

Hip rotation, knee rotation and knee rotation joint moment were defined as positive for an external rotation.

The time to accomplish the 5-10-5 yards test served as outcome measure for agility.

CMJ height was calculated based on the flight time method. During cable lift testing, the vertical acceleration signal of the accelerometer (University of Sport Science, Freiburg, Germany) placed on the top of the weight stack was filtered with a low pass Butterworth filter (4 th order, 20 Hz cut-off frequency) and then integrated to obtain velocity. Based on Newton's second law, lift force was calculated. Finally, power was determined by means of the product of force and velocity.

The selected parameters were averaged across eight trials for the different LRJ modalities and the 45° cutting maneuver, while the time during the 5-10-5 yards test was reported, and the highest CMJ and mean peak power value over two cable lift trials were calculated. These parameters served as the basis for the statistical analysis (Statistica 12, StatSoft, Inc.®, Tulsa, OK, US). All results are presented as group mean ± standard deviation (SD). After having confirmed that data followed a normal distribution (Shapiro-Wilk W), sphericity was verified by means of the Mauchly's test. The influence of the apparel condition on dependent variables for LRJ modalities was calculated using a two-way analysis of variance (General Linear Model) with repeated measures to test for main effects of and interactions between apparel (Control vs. FBCS) and LRJ modality (stable vs. sliding vs. counteracting). If a statistically significant LRJ modality main effect was observed, Bonferroni corrected paired comparisons determined where the differences occurred. The influence of the apparel condition (Control vs. FBCS) on the dependent variables for the cutting maneuver and performance tasks was analyzed using paired two-tailed t-tests. The magnitude of the changes was assessed with effect size (ES) and 95% confidence interval (CI). The ES was evaluated by means of partial eta squared (η 2 p) values and were considered small (.01 -.06), medium (.06 -.14) and large (> .14) according to Cohen [START_REF] Cohen | Statistical power analysis for the behavorial sciences[END_REF] . The level of significance was set at P < .05.

Results

There was a significant main effect of the apparel condition on hip rotation, knee abduction and rotation during LRJ (Table 1). Hip joint internal rotation was increased with FBCS (P = .019), while knee joint abduction and internal rotation were reduced (P = .002 and P < .001, respectively). Statistical analysis revealed a significant main effect of the LRJ modality on trunk forward lean and trunk lateral lean (P = .025 and P = .003, respectively), hip flexion, abduction and rotation (P < .001 for all), knee flexion and rotation (both P < .001).

However, no interaction effect (apparel condition x LRJ modality) was reported. Specifically, pairwise comparisons revealed that stable, sliding and counteracting modalities yielded significantly different values from each other for trunk lateral lean (P < .004 for all), hip flexion (P < .001 for all), hip rotation (P < .003 for all), knee flexion (P < .001 for all) and knee rotation (P < .003 for all). Trunk flexion was significantly greater during counteracting than both stable (P < .02) and sliding (P < .001). Hip abduction was significantly greater during sliding than both stable (P < .001) and counteracting (P < .001).

During the cutting maneuver, trunk forward lean and axial rotation were not influenced by the apparel condition at the time of peak knee abduction moment, while trunk lateral lean was significantly increased by FBCS (P < .001). Hip joint internal rotation was higher for FBCS than Control (P = .003). Knee joint angles and moments, especially the knee abduction moment,

were not influenced by the apparel condition (Table 2). The ensemble averages of trunk lateral lean and knee joint abduction moment over the stance phase of the cutting maneuver are presented (Figure 2).

Performance parameters were not significantly influenced by the apparel condition. The sprint time measured during the 5-10-5 yards test was comparable between the two apparel conditions (5.6 ± 0.3 s for both conditions; P = .90, 95% CI [-0.15, 0.13], η 2 p = .002). CMJ height was not significantly different between Control and FBCS conditions (0.28 ± 0.05 m for both; P = .40, 95% CI [-0.01, 0.01], η 2 p = .07). Mean peak power produced by the participants during the cable lift did not differ significantly (P = .55, 95% CI [-6, 11], η 2 p = .03) when performing the movement in the Control condition (282 ± 69 W) or with FBCS (280 ± 68 W).

Discussion

The main findings of the study were as follows: 1) FBCS did not reduce trunk angles during lateral movements, with even an increased trunk lateral lean during cutting maneuvers;

2) hip internal rotation was increased with FBCS; 3) knee joint moments were not reduced with FBCS; and 4) performance level was kept at the same level.

Perturbed lateral reactive jumps yielded significantly different segment and joint configurations at the time of their peak values. A detailed analysis of the influence of LRJ modality at the time of initial contact has already been provided in the literature. [START_REF] Weltin | Effect of gender on trunk and pelvis control during lateral movements with perturbed landing[END_REF] But during perturbed lateral reactive jumps, trunk positioning was not influenced by the compression garment. It seems that despite a suit design enabling a reduced passive trunk range of motion, FBCS cannot support the trunk segment during an active dynamic movement. Thus, the trunk segment was not supported either during cutting maneuvers. On the contrary, trunk lateral lean was even significantly increased by 4° while wearing FBCS during 45° cutting maneuvers.

While all participants showed greater trunk lateral lean with FBCS than without apparel, five subjects reached particularly large values around 15° with FBCS. Although literature demonstrated that compression garments could reduce the active range of motion of the hip joint during a non-athletic task [START_REF] Bernhardt | Influence of moderate prophylactic compression on sport performance[END_REF] and during sprinting, [START_REF] Doan | Evaluation of a lower-body compression garment[END_REF] a more neutral trunk positioning through apparel could not be achieved during unanticipated lateral movements. It might be possible that participants would rely on the support of the suit according to the compression and elasticity properties felt by the participants, thereby reducing their active trunk muscle control. But the lack of apparel support would bring participants to this higher trunk lateral lean in the frontal plane during cutting maneuvers.

The specific FBCS linked shoulders with knees across the pelvis in the back thanks to the elastic straps design. But at the hip joint level, only hip internal rotation was altered. The hip internal rotation observed in the present study is consistent with literature during lateral reactive jumps [START_REF] Weltin | Effect of gender on trunk and pelvis control during lateral movements with perturbed landing[END_REF] and cutting manoeuvres. [START_REF] Beaulieu | Lower limb muscle activity and kinematics of an unanticipated cutting manoeuvre: a gender comparison[END_REF] However, increased peak internal rotation values with FBCS might rather reflect a lack of strength in the external hip rotators in female athletes, [START_REF] Leetun | Core stability measures as risk factors for lower extremity injury in athletes[END_REF] exacerbated by the additional tension of the compression garment.

It appears that external passive support with the aim of enhancing trunk positioning during dynamic lateral movements classically encountered during team sports is difficult to achieve with apparel. At this point, further improvements to the design of the compression suit does not seem possible as several years of research and development were needed to yield this current comfortable full-body suit, which enables reduced passive trunk flexibility through the combined compression and elasticity properties. In addition, garments were customized to each participant in the present study. Further increasing the compression level of the suit to further support body segments, i.e. by reaching typical values of compression socks > 15 mmHg, [START_REF] Ali | Physiological effects of wearing graduated compression stockings during running[END_REF] does not seem possible given that i) the straps used already increase the stiffness of the suit and ii) the higher compression level would reduce the comfort level necessary to perform wholebody dynamic movements.

Trunk and hip control alterations with FBCS could have consequences at the knee joint level. Indeed, higher trunk lateral lean during cutting maneuvers while wearing FBCS might be associated with increased knee joint loading. [START_REF] Jamison | Knee moments during run-to-cut maneuvers are associated with lateral trunk positioning[END_REF][START_REF] Weltin | Influence of gender on trunk and lower limb biomechanics during lateral movements[END_REF] In present study, higher trunk lateral lean had no deleterious influence on the knee joint, as no significant difference in knee abduction moment was evident between FBCS and control conditions. Increased peak internal hip rotation angles with FBCS might reflect a lack of strength in the external hip rotators [START_REF] Leetun | Core stability measures as risk factors for lower extremity injury in athletes[END_REF] and further a critical knee joint position (position of no return), where the hip moves more rapidly into adduction and internal rotation and the knee collapses in valgus. [START_REF] Ireland | Anterior cruciate ligament injury in female athletes: epidemiology[END_REF] But the consequences at the knee joint level during lateral reactive jumps were rather a reduction of the abduction and internal rotation angles whatever the landing condition with FBCS which speaks in favor of better knee joint alignment. Thus, alteration of trunk positioning during lateral movements with FBCS was not associated with changes in knee joint stability.

This result does not support the theoretical model proposed by Hewett and Myer 3 and is not in agreement with experimental results, where increased trunk lateral lean was associated with increased knee joint abduction moment. [START_REF] Jamison | Knee moments during run-to-cut maneuvers are associated with lateral trunk positioning[END_REF][START_REF] Weltin | Influence of gender on trunk and lower limb biomechanics during lateral movements[END_REF] This finding underlines the fact that trunk positioning only partially influences knee joint loading, and that relatively small trunk angle variations (4°), albeit significant, may not always have an effect on knee abduction moment, especially when considering the large variability of this knee joint loading parameter.

It is worth noticing that FBCS had no influence on any of the performance parameters.

The same level of agility while wearing the compression suit or in the control condition during the 5-10-5 yards test is consistent with the literature. [START_REF] Bernhardt | Influence of moderate prophylactic compression on sport performance[END_REF][START_REF] Duffield | Comparison of three types of full-body compression garments on throwing and repeat-sprint performance in cricket players[END_REF][START_REF] Davies | The effects of compression garments on recovery[END_REF] Although improvement of jumping performance with a compression garment has been reported in the literature, [START_REF] Doan | Evaluation of a lower-body compression garment[END_REF][START_REF] Kraemer | Influence of compression garments on vertical jump performance in NCAA Division I volleyball players[END_REF] it is interesting to note that jump height was not impaired with the compression suit targeting trunk and pelvis positioning rather than the lower limbs in the present study. Finally, core strength during the cable lift task was kept at the same level when performed with FBCS. Accordingly, FBCS can be used by athletes without risk of compromising performance.

This study has some limitations that need to be considered. The moderate reliability of knee kinetics during dynamic tasks such as cutting tasks would suggest the need for a greater sample size than 12 participants. [START_REF] Sankey | How reliable are knee kinematics and kinetics during side-cutting manoeuvres ?[END_REF] In the present study, measured variables were assessed over eight trials to ensure representative values for each participant. But the inter-subject variability was pretty high for joint kinetics and the associated statistical power low (< 0.2). Thus, future studies would possibly need a larger sample size to tease out effects on knee joint kinetics or at least enable to discuss the lack of difference in knee joint loading. Another potential limitation was the use of a cable lift task. While this test seems to be relevant to evaluate core strength, the validity and reliability of this measure would need to be assessed to determine its measurement error. This will allow evaluating the importance of observed differences from a statistical perspective as well as in terms of its practical implication.

Even if compression and elasticity properties were combined with this customized full body compression suit, trunk kinematics could not be favorably influenced. Achieving external support of trunk positioning in female athletes through apparel seems to be difficult during unanticipated lateral movements. However, a full body compression suit did not impair any of the performance parameters and even yielded a better alignment of the knee during lateral reactive jumps.

Funding: this work was supported by the adidas AG via a research grant. the authors have no conflict of interest to declare.

Tables

 Table 1 Mean± SD values of the different kinematic variables during lateral reactive jumps (LRJ) performed in Control and full body compression suit (FBCS) conditions.

The influence of the apparel condition is reported using P, 95% confidence interval (95%CI) and effect size (ES) with partial eta squared (η


Y Doumbia
email: yaya.doumbia@centralesupelec.fr

T Malica

D Wolfersberger

K Panajotov

M Sciamanna

Nonlinear dynamics induced by optical injection with a frequency comb

published or not. The documents may come

Nonlinear dynamics induced by optical injection with a frequency comb

Introduction

Semiconductor lasers are extensively used in scientific fields such as biology, chemistry, and medicine. A lot of research has been devoted to the improvement of the laser properties to extend its applications. One of important field of research is the locking of the intrinsic oscillation frequency of a laser by an external oscillator. The competition between the intrinsic oscillation frequency and the external frequency can lead to several dynamics, including optical injection locking, periodic dynamics, and chaos. These rich and complex waveforms can find applications in several field such as optical communication [START_REF] Lin | Stability of period-one (P1) oscillations generated by semiconductor lasers subject to optical injection or optical feedback[END_REF] and physical security based on optical chaos [START_REF] Sakuraba | Tb/s physical random bit generation with bandwidth-enhanced chaos in three-cascaded semiconductor lasers[END_REF]. Recently the injection of a single-mode laser with a periodic signal (frequency comb) has attracted much attention [START_REF] Doumbia | Optical injection dynamics of frequency combs[END_REF][START_REF] Doumbia | Nonlinear dynamics of a laser diode with an injection of an optical frequency comb[END_REF][START_REF] Gavrielides | Comb injection and sidebands suppression[END_REF][START_REF] Shortiss | Harmonic frequency locking and tuning of comb frequency spacing through optical injection[END_REF]. Furthermore, the nonlinear dynamics of a single mode diode laser with a frequency comb is not widely known.

In this work, we will do an in-depth analysis of a single-mode semiconductor laser optically injected with an asymmetric frequency comb by varying the injection parameters and the initial comb properties. We observe a time-periodic dynamic which correspond to a new frequency comb with the same repetition rate as the injected comb. The number of resulting comb lines can be tailored with the injection parameters and the comb properties. An electric signal modulation is generated by an Arbitrary Waveform Generator (AWG) (Tektronix AWG 700002A) and sent to the RF port of a Mach-Zehnder (MZ) Modulator. The polarization controllers (P.C) allows to align the laser polarization with the input of the MZ modulator and to control the polarization of the injected light. Optical comb with 3 optical frequency lines is generated in the output of the MZ modulator. The comb's optical power is controlled using a variable optical attenuator (VOA). The injected laser is a Distributed feedback (DFB) laser emitting at ∼1549.7 nm. The fiber circulator is arranged to provide isolation for the injected laser.

Model description, experimental setup

The injected laser dynamics is analyzed with a high-resolution optical spectrum analyzer BOSA 400. The temporal output is measured with a real-time digital oscilloscope (OSC). A photodiode (PD) is placed at the input of the oscilloscope to convert optical signal to an electrical signal. Figure 1(b) shows the optical spectrum of the injected comb. This figure is obtained for a fixed comb spacing Ω=5 GHz. The difference between the power of the central comb line and the sides comb lines is around 12 dB.

Results and discussion

Conclusion

In conclusion, we have demonstrated experimentally the nonlinear dynamics of a single mode semiconductor laser under frequency comb injection. The injected laser output shows several dynamics including selective amplification (not show) and an unlocked time-periodic dynamic corresponding to a new frequency comb. The number of comb lines increase with the injection parameters.

Figure 1 (Figure 1 :

 11 Figure 1(a) shows the experimental setup for the optical injection with a frequency comb. The output of a continuous-wave (CW) tunable laser (Yanista Tunics T100S) is first amplified Erbium-Doped Fiber Amplifier (EDFA)

Figure 2

 2 Figure 2 shows experimental optical spectra of a single mode semiconductor laser under frequency comb injection for fixed comb spacing to Ω=2GHz, and injection strength of k=0.36. These figures are obtained for detunings of Δν=-7.4 GHz (a) and Δν =-10.3 GHz (b). When varying the detuning frequency, the laser output display a timedynamic corresponding to a new frequency comb with the same comb spacing as the injected comb. Several new frequency comb regions are observed in the plane if injection parameters. The new combs in Fig.2(a) and 2(b) are obtained in different region for larger detuning frequency corresponding to several times the injected comb spacing. The number of output comb lines increases with the injection strength and when varying the detuning frequency from positive to negative value. The new frequency comb regions far from injection locking region have a larger number of output comb lines.

Figure 2 :

 2 Figure 2: Optical spectra of unlock time-periodic dynamics for fixed comb spacing to Ω=2GHz and injection strength to k=0.36. (a) and (b) are obtained for detuning of Δν=-7.4 GHz and Δν=-10.3 GHz, respectively.


Y Doumbia
email: yaya.doumbia@centralesupelec.fr

T Malica

D Wolfersberger

K Panajotov

M Sciamanna

Broad

published or not. The documents may come

Introduction

The injection locking properties of semiconductor lasers have been analyzed since more than forty years :first, as a way to control the coherence properties of the injected laser, but more recently, as a technique to tailor specific nonlinear dynamics including optical chaos [1], time-periodic self-pulsation [2], and also dissipative solitons in large aperture laser diodes [3]. These nonlinear dynamics found applications in various fields such as: the physical security based on optical chaos, optical sensing, and radio-over fiber communications. Much less studied is the dynamics of a single mode laser diode injected by a multimode laser diode. This configuration has raised recent interest within the context of optical frequency combs. In this Letter, we make an in-depth analysis of single mode semiconductor laser dynamics induced by optical injection of a frequency comb by varying both the injection parameters and the comb properties. The number of the resulting comb modes can be tailored by varying the injection parameters and the initial comb properties (number of modes and the comb spacing).

Results and discussions

We shall discuss the emergence of complex dynamics from the injection locked solution. Figure 1 for example shows a detailed mapping of the parameter region corresponding to time-periodic dynamics in the plane of the injection parameters. The region shaded in blue correspond to the time-periodic dynamics of the slave laser output. Time-periodic dynamics could either correspond to injection locking -in which case the slave laser output power oscillates periodically at the frequency corresponding to the injected comb mode spacing -or to an unlocked dynamic that corresponds to a new comb solution and which we call "comb x" in the mapping. The region shaded in red correspond the non-periodic dynamics of the slave laser which we identify as chaotic dynamics bifurcating from injection locked solution. We are not interested here by the peculiar chaotic properties of the slave laser dynamics but, by the parameter range in which the slave laser shows an optical comb dynamics and the corresponding comb properties. Interestingly, the unlocked comb dynamics of the slave laser extends to a much broader range of injection parameters than the injection locking solution. Figures 1 (a) and 1 (b) correspond to 3comb modes and 7-comb modes injection respectively. The region around zero detuning is the injection locking region. We identify as many injection locking regions as the number of the comb modes; labelled 'IL' in the mapping. In both cases, we observe two regions of unlocked time-periodic dynamics (see regions labelled 'comb1' and comb2 in the mapping). The areas corresponding to the new comb solution extend both with the injection strength and with the detuning. We also notice that when increasing the master laser number of comb modes, i.e., comparing Figure 1 (b) to Figure 1 (a), the areas corresponding to the new comb solution extend more to the negative detuning as the injection strength increases. It is also worth noting that the comb1 region connect to the injection locking region, hence, suggesting that this new comb solution indeed bifurcates from the injection locking solution.

Tailoring the comb properties by varying the injection parameters is better seen in Fig. ~\ref{fig:fig4}, where, we plot the full-width at half maximum (FWHM) of the pulsing dynamics of the slave laser output when varying the injection strength within the parameter range corresponding to the unlocked slave laser comb solution in the two unlocked regions of the Figure 3 (a). The pulse FWHM decreases when the injection strength increases. Figure (b) and(c) then analyze the FWHM of the slave laser pulse output when increasing the injection strength κ. Figure (c) present the FWHM for 3 detuning values of 0.7 GHZ (red line), 0 GHz (blue line) and 0.7 GHZ (brown line) for the case of comb2. In both cases (comb1 and comb2), the FWHM decreases when κ increases. Furthermore, FWHM is smaller for the negative detuning than for the zero or positive detuning.

Conclusion

In summary, we show that it is possible to generate a broadened frequency comb using injection locking technique.

Increasing the injection strength, the slave laser gets locked, chaotic dynamics is observed. Most importantly, new comb solutions take place in the unlocked time-periodic dynamics. The pulse width of the mode-locked slave laser dynamics is controlled by injection parameters. In particular, the slave laser pulse width decreases when increasing the injection strength and when moving the master laser towards more negative detuning.

Figure 1 :

 1 Figure 1 : Numerical mapping of semiconductor laser subject to optical injection with frequency comb for Left: 3 comb modes and Right: 7 comb modes Comb spacing = 5 GHz.

Figure 2 :

 2 Figure 2: Control of comb properties. (a) slave laser output in the unlocked comb1 and comb 2 solutions for Δν0=0.2 GHz, κ=0.202 (green) and κ =0.6 (red).(b) FWHM of the comb1 solution when increasing κ. (c) same as (b) for the comb2 solution also comparing the cases for Δν0=-0.7 GHz (red line), Δν0=0 GHz (blue line) and Δν0=0.7 GHz (brown line).


Guillaume Mornieux
email: guillaume.mornieux@univ-lorraine.fr

Elmar Weltin

Craig Friedman

Monika Pauls

Shad Forsythe

Albert Gollhofer

Mornieux Guillaume

Weltin Elmar

Friedman Craig

Pauls Monika

Influence of a Functional Core Stability Program on Trunk and Knee Joint Biomechanics in Female Athletes During Lateral Movements

Keywords: cuttings, perturbation, trunk lateral lean, knee abduction moment, agility

come

INTRODUCTION

Epidemiological studies have demonstrated that female athletes have a relative higher risk of sustaining non-contact anterior cruciate ligament (ACL) injuries than male athletes during sports such as soccer or basketball [START_REF] Agel | Anterior cruciate ligament injury in national collegiate athletic association basketball and soccer: a 13-year review[END_REF][START_REF] Renstrom | Non-contact ACL injuries in female athletes: an International Olympic Committee current concepts statement[END_REF]. As a countermeasure, preventive neuromuscular training (PNMT) interventions have been designed to reduce knee joint injuries, especially ACL ruptures in female athletes [START_REF] Myer | The influence of age on the effectiveness of neuromuscular training to reduce anterior cruciate ligament injury in female athletes[END_REF][START_REF] Sugimoto | Specific exercise effects of preventive neuromuscular training intervention on anterior cruciate ligament injury risk reduction in young females: meta-analysis and subgroup analysis[END_REF]. It has been generally accepted that PNMT significantly reduces ACL injuries, especially when emphasis is put on specific training exercises based on strengthening and trunk neuromuscular control [START_REF] Sugimoto | Specific exercise effects of preventive neuromuscular training intervention on anterior cruciate ligament injury risk reduction in young females: meta-analysis and subgroup analysis[END_REF].

During athletic movements, trunk neuromuscular control is of interest since it influences trunk load and motion and therefore knee joint loads [START_REF] Hewett | The mechanistic connection between the trunk, hip, knee, and anterior cruciate ligament injury[END_REF][START_REF] Hughes | A review of recent perspectives on biomechanical risk factors associated with anterior cruciate ligament injury[END_REF]. Hewett and Myer [START_REF] Hewett | The mechanistic connection between the trunk, hip, knee, and anterior cruciate ligament injury[END_REF] proposed a knee injury model that accounted for both the trunk and hip joints, since increased trunk lateral lean is reportedly associated with higher external knee abduction moments. Moreover, a greater lateral trunk position has been shown during anterior cruciate ligament injury situations in female athletes [START_REF] Hewett | Video analysis of trunk and knee motion during non-contact anterior cruciate ligament injury in female athletes: lateral trunk and knee abduction motion are combined components of the injury mechanism[END_REF]. In a prospective study over 3 years, Zazulak et al. [START_REF] Zazulak | Deficits in neuromuscular control of the trunk predict knee injury risk: a prospective biomechanicalepidemiologic study[END_REF] demonstrated greater trunk displacement after a sudden force release in athletes who subsequently sustained a knee injury. The association between trunk positioning and knee joint loads has been investigated during changes of direction or cutting maneuvers, since these complex dynamic tasks represent one of the most common playing situations that could lead to a non-contact ACL injury [START_REF] Alentorn-Geli | Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 1: mechanisms of injury and underlying risk factors[END_REF][START_REF] Koga | Mechanisms for noncontact anterior cruciate ligament injuries: knee joint kinematics in 10 injury situations from female team handball and basketball[END_REF]. During cutting maneuvers, increased trunk lateral lean has been demonstrated to be positively associated with increased knee abduction moments [START_REF] Dempsey | The effect of technique change on knee loads during sidestep cutting[END_REF][START_REF] Jamison | Knee moments during run-to-cut maneuvers are associated with lateral trunk positioning[END_REF][START_REF] Weltin | Influence of gender on trunk and lower limb biomechanics during lateral movements[END_REF]. Furthermore, increased knee joint loading has been associated with increased trunk rotation away from the stance limb [START_REF] Dempsey | The effect of technique change on knee loads during sidestep cutting[END_REF][START_REF] Frank | Trunk and hip biomechanics influence anterior cruciate loading mechanisms in physically active participants[END_REF]. The redirection of the center of mass towards the new movement direction seems to be a key factor, as demonstrated by Donnelly et al. [START_REF] Donnelly | Optimizing whole-body kinematics to minimize valgus knee loading during sidestepping: Implications for ACL injury risk[END_REF], when producing an optimized whole-body kinematics solution to reduce the knee joint abduction moment. This is also in agreement with David et al. [START_REF] David | Identification and risk estimation of movement strategies during cutting maneuvers[END_REF], who described a cutting movement strategy with forefoot striking in combination with pre-orientation of the upper body towards the new movement direction to reduce knee joint loads. Accordingly, any improvement in trunk and pelvis positioning during cutting maneuvers is of interest with regard to knee joint loading. Thus, core stability [START_REF] Hibbs | Optimizing performance by improving core stability and core strength[END_REF][START_REF] Kibler | The role of core stability in athletic function[END_REF] should be additionally targeted in existing PNMT interventions aimed at modifying cutting maneuver biomechanics [START_REF] Pappas | Do exercises used in injury prevention programmes modify cutting task biomechanics? A systematic review with meta-analysis[END_REF].

Although literature has reported that quasi-static core stability (CS) training could significantly reduce the vertical ground reaction force (3) and change leg kinematics and joint moments [START_REF] Pfile | Different exercise training interventions and drop-landing biomechanics in high school female athletes[END_REF] during drop jump landings, such biomechanical improvements were not found after quasi-static CS training during cutting maneuvers [START_REF] Jamison | Randommized Controlled Trial of the effects of a trunk stabilization program on trunk control and knee loading[END_REF]. In this latter study, 6 weeks of CS training did not improve core strength, trunk control or knee joint loading during cutting maneuvers relative to resistance training. Moreover, a 6-week dynamic CS program [START_REF] Whyte | Effects of a dynamic core stability program on the biomechanics of cutting maneuvers: a randomized controlled trial[END_REF] did not influence trunk or pelvis kinematics or knee moments during unanticipated cuttings. In addition, a 4-week training program combining lateral movements and perturbation influenced trunk and pelvis rotation in the same manner as a standard plyometric training, and no influence was found on knee joint moments during unanticipated cuttings [START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF]. Therefore, enhanced dynamic and functional CS training is needed to effectively improve trunk and pelvis control towards the new movement direction during unanticipated cutting maneuvers.

However, knee joint loads, and especially knee joint abduction moment, as a relevant parameter associated with knee injury [START_REF] Hewett | Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study[END_REF], seems not to be influenced by such training, although such training should be beneficial for athletic performance. Indeed, it has been reported in the literature that the endurance [START_REF] Jamison | Randommized Controlled Trial of the effects of a trunk stabilization program on trunk control and knee loading[END_REF][START_REF] Watson | Dance, balance and core muscle performance measures are improved following a 9-week core stabilization training program among competitive collegiate dancers[END_REF] and strength [START_REF] Prieske | Neuromuscular and athletic performance following core strength training in elite youth soccer: role of instability[END_REF][START_REF] Watson | Dance, balance and core muscle performance measures are improved following a 9-week core stabilization training program among competitive collegiate dancers[END_REF] of core muscles was improved and specific athletic performance parameters were increased [START_REF] Prieske | Neuromuscular and athletic performance following core strength training in elite youth soccer: role of instability[END_REF][START_REF] Watson | Dance, balance and core muscle performance measures are improved following a 9-week core stabilization training program among competitive collegiate dancers[END_REF][START_REF] Weston | Isolated core training improves Core stability training program and lateral movements 22 sprint performance in National-Level Junior[END_REF] after CS programs.

The purpose of the present study was to evaluate the influence of a 5-week dynamic and functional CS training program on trunk and pelvis control and knee joint control during dynamic lateral sporting movements, as well as athletic performance. We tested the hypotheses that i) trunk and pelvis kinematics would be better orientated towards the new movement direction after CS training; ii) the knee joint abduction moment would remain at the same level during cuttings after CS training; and iii) jump performance, agility, core endurance and power would be enhanced following CS training.

METHODS

Experimental Approach to the Problem

This study proposed a training intervention over 5 weeks to determine the effects of a dynamic and functional core stability program on cutting biomechanics and athletic performance. Female participants were recruited according to their relative higher risk of sustaining non-contact ACL injuries [START_REF] Agel | Anterior cruciate ligament injury in national collegiate athletic association basketball and soccer: a 13-year review[END_REF]. Although no control group was used in the present study, it was assumed that the results provided by Weltin et al. [START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF] [START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF], in addition to their comparable testing procedure. Indeed, the methodology used to assess biomechanics, the perturbed lateral reactive jump and cutting maneuver conditions, and the dependent biomechanical variables were exactly the same in this latter study as in the present one.

Pre and post training measurements evaluated three-dimensional trunk and pelvis kinematics during perturbed lateral reactive jumps, and additionally the knee joint moment during 45° cutting maneuvers. Core endurance and power as well as jump performance and agility were also evaluated.

Subjects

An a priori power analysis was conducted to estimate the sample size based on the measured improvement in the intervention group (effect size of 0.6 for trunk kinematic data) of recent published research [START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF]. It was estimated that 19 participants were needed to achieve 80% statistical power with an alpha level of 0.05. This sample size is in line with other studies [START_REF] Jamison | Randommized Controlled Trial of the effects of a trunk stabilization program on trunk control and knee loading[END_REF]. Twenty-two females were initially enrolled in the study to account for possible dropouts.

All participants were interviewed individually to assess eligibility for participation. They had to meet the following inclusion criteria: at least five years of active participation in team sports with high exposure to running, jumping and cutting tasks, no additional core stability or balance training, no history of anterior cruciate ligament injury, trunk or back injury, neurological disorders or any current pain. Participants trained at least three times a week (up to five times) and played at different levels (1 st to 5 th division). Three participants withdrew from the study due to injuries (two ankle ligament injuries and one hamstring muscle tear) that occurred during their regularly scheduled season practices and matches, resulting in more than two missed training sessions in the present intervention. Accordingly, 19 female athletes were included in the analysis (22 ± 3 years, 172 ± 7 cm, 61 ± 7 kg, 9 ± 4 years sport experience).

The study was approved by the local ethics committee (approval number: 390/13) and conformed to the requirements stipulated in the Declaration of Helsinki. All participants were informed about the procedures, purpose, benefits and possible risks associated with the experimental set-up prior to signing an institutionally approved informed consent document to participate in the study.

Procedures

Intervention. All participants took part in regularly scheduled season practices and matches, in addition to attending the present core stability (CS) program three times per week for five weeks (15 sessions). The present CS training (Table 1) involved 15 training sessions. Each session lasted for 30min and was based on warm-up exercises, medicine ball throws, a cable lift task, and different variations of lateral shuffles. These specific exercises are relevant with respect to cutting maneuver biomechanics, since i) they are dynamic, ii) they target the core but also involve the whole-body, and iii) they are functional with respect to the nature of a cutting maneuver, involving trunk and pelvis segment rotations while pushing medially and upwards with the stance leg into the new movement direction. Together with a progressive increase in the intensity and the introduction of perturbation in the latter sessions, the present CS training should be more effective than previously tested programs seeking to improve cutting maneuver biomechanics [START_REF] Jamison | Randommized Controlled Trial of the effects of a trunk stabilization program on trunk control and knee loading[END_REF][START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF][START_REF] Whyte | Effects of a dynamic core stability program on the biomechanics of cutting maneuvers: a randomized controlled trial[END_REF]. Participants were given instructions and illustrations about the exercises and were supervised by athletic coaches to ensure proper and effective execution of the movement techniques. Particular attention was paid to the rotation of the hip during the medicine ball throw. Indeed, hips should lead the movement followed by the upper body. The ball should be held at the hip level opposite the wall, and the foot should rotate together with the hip. Focus was set on turning the shoulders and torso, as well as combining familiar movements of upright rows and an incline press during the cable lift.

During shuffles, athletes had to keep their weight forward to maintain an upright torso position with respect to the vertical axis in the frontal plane and to point their toes forwards.

The motor learning principles behind the training procedure referred mainly to the internal focus of attention and explicit learning during the first 6 training sessions. Then, an external focus of attention and implicit learning were introduced by focusing on the medicine ball ("trying to launch the ball from the hip") or performing shuffles using mirror drills with partners. Moreover, differential learning was initiated by having a variety of movement patterns targeting the trunk and pelvis segments re-orientation throughout the intervention and changing the environment (different training partners, resistances, and perturbations). Firstly, participants performed perturbed lateral reactive jumps (LRJ) to focus on movement control during a functional task associated with high trunk lateral lean in the frontal plane.

From a standing position, subjects performed a lateral reactive movement by jumping to their left side and then pushing off with their left leg to get back to their original position. In order to induce lateral perturbations, subjects performed LRJ on a motor-driven plate, which could move in the horizontal (medio-lateral) direction. The specially designed motor-driven plate (University of Sport Science, Freiburg, Germany) was comprised of two independently moving plates (1.15 m × 0.3 m), each driven by a separate motor (AKM51G, Danaher Motion, Düsseldorf, Germany). LRJ were performed either without any perturbation (stable condition) or with a perturbation to the left (sliding condition) or to the right (counteracting condition) as soon as the foot made initial contact with the plate. Each perturbation consisted of a 10 cm translation. Unpredictability was introduced using these three different landing modalities (stable, sliding, and counteracting) that were unknown prior to movement execution. The perturbation was triggered by breaking a light beam (M18 series, Panasonic Electric Works Europe AG, Holzkirchen, Germany). Details of this material have been previously described in the literature [START_REF] Mornieux | Changes in leg kinematics in response to unpredictability in lateral jump execution[END_REF].

Secondly, participants were asked to perform either cutting maneuvers to 45°, crossovers to -20°, or straight runs on a force plate (BP600900, AMTI®, Watertown, USA) after approaching the force plate (FP) at 4 ± 0.2 m.s -1 . The direction of the movement was indicated in a randomized order by a light signal occurring 650 ms before the left foot contacted the FP.

The visual light stimulus was triggered automatically during the approach run via a light switch (M18 series, Panasonic Electric Works Europe AG, Holzkirchen, Germany). The 45° cutting maneuver to the right was the only movement further analyzed due to its relevance when investigating performance and injury aspects of changing direction tasks. Details of these procedures have been previously described in the literature [START_REF] Mornieux | Influence of a full-body compression suit on trunk positioning and knee joint mechanics during lateral movements[END_REF]. The test-retest reliability of these specific biomechanical evaluations were additionally tested for nine participants providing an intraclass correlation coefficient (ICC) value for the different dependant variables. The reliability of the trunk, pelvis, and knee kinematics during LRJ was good, with ICC values respectively above 0.94, 0.89 and 0.87 when averaged over the three conditions. During cuttings, the reliability of the trunk and pelvis kinematics was good (ICC > 0.81 and 0.85, respectively), and the ICC values for knee joint moments were 0.88 (flexion), 0.72 (abduction), and 0.52 (external rotation). Based on the ICC values, the standard error of measurement (SEM) was determined to provide further valuable information in order to interpret significant effects detected during LRJ and cuttings with the present repeated measures design.

Finally, core endurance and power, jump performance, and agility were evaluated. Holding times during a prone plank and a left side plank were used to evaluate core endurance. Time was stopped when participants could not hold the plank anymore. Jump performance was evaluated by broad and lateral jumps, agility with the 5-10-5 yards test and core power with a cable lift test. This latter test targets the action of the hips, torso rotators, upper back, chest and shoulders while turning the trunk away from the strength machine as the hands first pull the rope cable handle towards the chest and then push up and away (Figure 1).

Figure 1 about here

The rationale for all these tests was the importance of agility, leg muscle strength, core endurance and power for high performance in team ball sports. The best out of two broad jumps and two lateral jumps to the right were determined from the distance measured. The time taken to accomplish the agility test was measured manually. The cable lift test was conducted three times at 70% of the one-repetition maximum. The vertical acceleration signal of an accelerometer (University of Sport Science, Freiburg, Germany) placed on the top of the weight stack was filtered with a low pass Butterworth filter (4 th order, 20 Hz cut-off frequency) and then integrated to obtain velocity. Lift force was calculated based on Newton's second law. Finally, core power was determined as the product of force and velocity. The good reliability for lower back stabilization exercises (ICC > 0.96), as well as for the 5-10-5 yards agility test (ICC > 0.87) have been reported in the literature [START_REF] Mcgill | Endurance/time for low back stabilization exercise : clinical targets for testing and training from a normal database[END_REF][START_REF] Stewart | Reliability, factorial validity, and interrelationships of five commonly used change of direction speed tests[END_REF]. The test-retest reliability of this specific cable lift task across the nine participants was additionnally evaluated (ICC = 0.67).

All participants were familiar with such movements due to their sport practice and were given enough practice trials to feel comfortable with all task modalities prior to the measurements. Data analysis. Three-dimensional leg kinematics were recorded using reflective skin markers ( 14 mm) placed on anatomical landmarks of the subjects' trunk (three markers), pelvis (four markers), left thigh (three markers), shank (four markers), and foot (six markers). The markers were captured with a 12-camera motion analysis system (Vicon MX TM , VICON Motion Systems Ltd.®, Oxford, UK) with a sampling frequency of 200 Hz. A static trial with the subject standing in a pre-defined neutral position was used to define the kinematic model and to calculate segment lengths and joint centers. The kinematic modeling was performed using the motion analysis software (Vicon Nexus 1.6.1, VICON Motion Systems Ltd.®, Oxford, UK). Joint kinematics in three rotational degrees of freedom were determined using an yxz Euler rotation sequence of the respective segment coordinate system according to the global reference system defined in Vicon. This rotation sequence was around the flexion/extension axis first, then around the abduction/adduction axis, followed by rotation around the external/internal axis. This marker placement and kinematic modeling were used in previously published articles [START_REF] Mornieux | Anticipatory postural adjustments during cutting manoeuvres in football and their consequences for knee injury risk[END_REF][START_REF] Weltin | Influence of gender on trunk and lower limb biomechanics during lateral movements[END_REF].

The marker trajectories and ground reaction force signals were both filtered with a low pass Butterworth filter (4 th order, 15 Hz cut-off frequency) prior to calculating external joint moments with a standard inverse dynamics approach. Three-dimensional kinematic data of the trunk and pelvic segments were analyzed to understand the influence of CS training on core control. The three-dimensional kinematics and kinetics of the knee joint were further analyzed to determine the influence of CS training on knee joint stability and loading. Both kinematics and kinetics were measured during cutting maneuvers, but only kinematics was observed during LRJ. These variables were analyzed at the time of their peak values during the first half of ground contact during LRJ to give some insight into how CS training influences the range of motion of the different body segments. However, during cutting maneuvers, the different variables were determined at the time of peak knee abduction moment. The trunk and pelvis kinematics were defined relative to the global coordinate system, i.e., deviations from the vertical axis for trunk forward and lateral leans, and for pelvis forward and medial tilts, and from the anterior-posterior axis for trunk and pelvis right Core stability training program and lateral movements 12 axial rotation. These parameters were positive for an orientation in the new movement direction (i.e., a medial lean and rotation in the right direction). Knee rotation parameters were positive for an external rotation.

Statistical Analyses

The selected parameters were averaged across eight trials for the different LRJ modalities and the 45° cutting maneuver, whereas time for the 5-10-5 yards test was reported and the highest CMJ and mean peak power value over three cable lift trials were calculated. These parameters served as the basis for the statistical analysis (Statistica 12, StatSoft, Inc.®, Tulsa, OK, US).

All results are presented as group mean ± standard deviation (SD). After having confirmed that the data followed a normal distribution (Shapiro-Wilk W), sphericity was verified by means of a Mauchly's test. The influence of CS training on dependent variables for LRJ modalities was calculated using a two-way analysis of variance (General Linear Model) with repeated measures to test for the main effects of and interactions between CS training (PRE vs. POST) and LRJ modality (stable vs. sliding vs. counteracting). If a statistically significant LRJ modality main effect was observed, Bonferroni corrected paired comparisons determined where the differences occurred. The influence of CS training (PRE vs. POST) on the dependent variables for the cutting maneuver and performance tasks was analyzed using paired two-tailed t-tests. The magnitude of the changes was assessed with effect size (ES) and a 95% confidence interval (CI). The ES was evaluated by means of Cohen's d values for repeated measures designs. The ES was considered trivial (0-0.19), small (0.20-0.49), medium (0.50-0.79), or large (>0.80) according to Cohen [START_REF] Cohen | A power primer[END_REF]. The level of significance was set at p < 0.05.

RESULTS

Core stability training program and lateral movements 13

There was a significant main effect of CS training on trunk lateral lean (p = 0.04), trunk axial rotation (p = 0.002), pelvis medial tilt (p = 0.002), and pelvis axial rotation (p < 0.001) during LRJ, but no significant influence at the knee joint level (Table 2). SEM values for trunk lateral lean and rotation were 0.7° and 1.4°, respectively, while the pelvis medial tilt and rotation had SEM values of 1.3° and 1.7°, respectively.

Table 2 about here

Statistical analysis revealed a significant main effect of the LRJ modality on trunk forward lean and trunk lateral lean (both p < 0.001), pelvis medial tilt (p < 0.001), knee flexion, abduction, and rotation (p < 0.001, p = 0.04 and p < 0.001, respectively). Significant interaction effects (CS training × LRJ modality) were found only for trunk axial rotation (p = 0.01) and knee joint abduction (p = 0.03). Specifically, pairwise comparisons revealed that stable, sliding, and counteracting modalities yielded significantly different values from each other for trunk forward lean (p < 0.007 for all), trunk lateral lean (p < 0.001 for all), pelvis medial tilt (p < 0.002 for all), knee flexion (p < 0.001 for all), knee abduction (p < 0.05 for all), and knee rotation (p < 0.001 for all).

During the cutting maneuver (Table 3), trunk forward lean and axial rotation were not influenced after CS training, whereas trunk lateral lean was significantly increased (p = 0.03).

Pelvis kinematics was not significantly influenced by CS training. Knee joint flexion moment decreased significantly (p = 0.01) after CS training, whereas the knee joint rotation moment increased (p = 0.02). However, the knee joint abduction moment remained at the same level after CS training. SEM values were 1.6° for trunk lateral lean, and 0.28 Nm.kg -1 and 0.08 Nm.kg -1 for knee joint flexion and rotation moments, respectively. Table 3 about here All core endurance (prone and side planks), jump performance, agility, and core power parameters were significantly improved after CS training (Table 4).

Table 4 about here

DISCUSSION

The main findings of the study were as follows: 1) CS training increased trunk and pelvis rotation in the new movement direction during lateral reactive jumps; 2) lateral trunk lean was increased during lateral movements after CS training; 3) the knee joint abduction moment was not influenced by CS training; and 4) CS training improved core endurance and power, jump performance, and agility.

Perturbed lateral reactive jumps yielded significantly different segment and joint configurations. This influence of LRJ modality has already been described in the literature [START_REF] Mornieux | Influence of a full-body compression suit on trunk positioning and knee joint mechanics during lateral movements[END_REF][START_REF] Weltin | Effect of gender on trunk and pelvis control during lateral movements with perturbed landing[END_REF]. Moreover, during perturbed lateral reactive jumps, the trunk and pelvis positioning were influenced by CS training. Indeed, trunk lateral flexion and pelvis lateral tilt were significantly increased after CS training, and both right axial rotations of these segments were enhanced after training. Five weeks of dynamic and functional core stability training resulted in a better positioning of the core segments in the transversal plane with increased rotation towards the new movement direction during lateral reactive jumps, which is in agreement with results found by Weltin et al. [START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF] after a training program combining plyometric exercises with perturbation compared to plyometric training alone. However, core segments were significantly less orientated towards the new movement direction in the frontal plane.

Core stability training program and lateral movements

Although significant, these changes in trunk lateral flexion and pelvis lateral tilt were rather small given the standard error of the measurements (0.7° and 1.3°, respectively). During unanticipated cutting maneuvers, the better orientation of trunk and pelvis segments in the transversal plane seen during LRJ was lacking, which differs from the results of Weltin et al. [START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF]. In the frontal plane, pelvis medial tilt was kept at the same level and only a small 2° increase in trunk lateral flexion (with respect to 1.6° SEM) was observed after CS training.

Core stability improvements during lateral movements after dynamic and functional core stability training were limited and, therefore, our first hypothesis was mainly rejected.

Although the better trunk and pelvis orientation towards the new movement direction seen in the transversal plane during LRJ might possibly reflect a reduced knee injury risk [START_REF] David | Identification and risk estimation of movement strategies during cutting maneuvers[END_REF][START_REF] Dempsey | The effect of technique change on knee loads during sidestep cutting[END_REF][START_REF] Donnelly | Optimizing whole-body kinematics to minimize valgus knee loading during sidestepping: Implications for ACL injury risk[END_REF][START_REF] Frank | Trunk and hip biomechanics influence anterior cruciate loading mechanisms in physically active participants[END_REF], this was lacking during unanticipated cutting maneuvers. The increased trunk lateral lean during lateral movements after CS training could be seen as a negative effect on knee joint loads [START_REF] Dempsey | The effect of technique change on knee loads during sidestep cutting[END_REF][START_REF] Jamison | Knee moments during run-to-cut maneuvers are associated with lateral trunk positioning[END_REF][START_REF] Weltin | Influence of gender on trunk and lower limb biomechanics during lateral movements[END_REF]. However, in the absence of an increased knee joint abduction angle and moment, this could also reflect a strategy to move the center of mass towards the new direction during the stance phase [START_REF] Patla | Online steering: coordination and control of body center of mass, head and body reorientation[END_REF]. Nevertheless, training programs seeking to reduce trunk lateral lean during cutting maneuvers should instead focus on a specific technique training that is more effective [START_REF] Dempsey | Changing sidestep cutting technique reduces knee valgus loading[END_REF]. However, the present dynamic and functional CS training influenced core stability during lateral movements to a larger extent than previous SC programs [START_REF] Jamison | Randommized Controlled Trial of the effects of a trunk stabilization program on trunk control and knee loading[END_REF][START_REF] Whyte | Effects of a dynamic core stability program on the biomechanics of cutting maneuvers: a randomized controlled trial[END_REF].

Despite some core stability alterations after CS training, the knee joint abduction moment was kept at the same level during unanticipated cutting maneuvers. This is in line with previous studies [START_REF] Jamison | Randommized Controlled Trial of the effects of a trunk stabilization program on trunk control and knee loading[END_REF][START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF][START_REF] Whyte | Effects of a dynamic core stability program on the biomechanics of cutting maneuvers: a randomized controlled trial[END_REF] and confirms our second hypothesis. However, other consequences at the knee joint level were found during unanticipated cutting maneuvers with a lower knee flexion moment and higher knee external rotation moment. Although significant, the increased knee external rotation moment (0.01 to 0.08 Nm.kg -1 ) was small given its standard error of measurement and compared to the larger influence on knee joint flexion (a reduction from 0.73 to 0.26 Nm.kg -1 ), which would probably mean an overall knee joint load reduction after CS training. During lateral reactive jumps, knee joint stability remained at the same level after CS training.

The present dynamic and functional CS training influenced all tested performance parameters as hypothesized. The significant enhanced core muscle endurance and power are in line with the literature [START_REF] Jamison | Randommized Controlled Trial of the effects of a trunk stabilization program on trunk control and knee loading[END_REF][START_REF] Prieske | Neuromuscular and athletic performance following core strength training in elite youth soccer: role of instability[END_REF][START_REF] Watson | Dance, balance and core muscle performance measures are improved following a 9-week core stabilization training program among competitive collegiate dancers[END_REF]. Jump performance and agility were also increased by training. Therefore the present CS exercises can be implemented by athletes in their regular training to improve performance and underlines the importance of implementing dynamic movements, added resistance and functional exercises during training, as described by Hibbs et al. [START_REF] Hibbs | Optimizing performance by improving core stability and core strength[END_REF]. This study has some limitations that need to be considered. First, the lack of any control group might reduce the impact of the results found for the intervention group. However, the training was designed according to evidence from the literature based on a model for potential performance benefits [START_REF] Hibbs | Optimizing performance by improving core stability and core strength[END_REF] and a framework for ACL injury prevention [START_REF] Donnelly | An anterior cruciate ligament injury prevention framework: incorporating the recent evidence[END_REF]. Moreover, the testing procedure used in the present study was comparable to another training study with two different interventions [START_REF] Weltin | Effects of perturbation or plyometric training on core control and knee joint loading in women during lateral movements[END_REF], which helped with the interpretation of the present findings. The sprint time during the agility test was measured manually, which might decrease accuracy of the measurement. However, according to the substantial improvement of all other performance parameters, we are confident of our conclusion about the enhanced athletic performance after core stability training. Finally, the motor learning principles behind the training procedure referred mainly to an internal focus of attention and explicit learning.

Future research in this field might consider i) directing the subject's attention to the effects of the movements on the environment, ii) strengthening the implicit learning, and iii) promoting self-controlled learning in order to optimize the training intervention and therefore improve participants' functional performance [START_REF] Gokeler | Principles of motor learning to support neuroplasticity after ACL injury: implications for optimizing performance and reducing risk of second ACL injury[END_REF].

In conclusion, a 5-week dynamic and functional core stability training program improves core muscles endurance and power improvement, as well as increasing jump performance and agility. This might explain the better orientation of the trunk and pelvis towards the new movement direction during lateral reactive jumps, especially in the transversal plane.

However, this improved core stability was not observed during a more challenging and dynamic task, i.e., unanticipated cuttings, during which knee joint abduction remained at the same level after the training.

PRACTICAL APPLICATIONS

The practical application of this research will allow strength and conditioning professionals to setup training programs with relevant exercises targeting the enhancement of core stability. By using different variations of lateral shuffles, medicine ball throws, or cable lift tasks, as described in the present study, it is possible to increase core endurance and power, as well as jump performance and agility of trained individuals. These exercises can be performed in every gym with very simple sport equipment. Ultimately, these improvements are likely to enable athletes to perform at a better level with a reduced risk of knee joint injury. The influence of the core stability training intervention is reported using p, 95% confidence interval (95%CI) and effect size (ES). **, *** indicates significant differences between PRE and POST with p < 0.01 and p < 0.001, respectively.

Table 4

Mean ± SD values of the different core endurance and athletic performance variables before (PRE) and after (POST) the core ctability training intervention.

The influence of the core stability training intervention is reported using p, 95% confidence interval (95%CI) and effect size (ES). *, **, *** indicates significant differences between PRE and POST with p < 0.05, p < 0.01 and p < 0.001, respectively.

  would constitute reference data to interpret the influence of CS training. The rationale for this was the relatively similar training volume used in their study (three times per week over four weeks) for two relevant training programs, i.e., a modified plyometric training program originally designed by Chimera et al. (4) and a plyometric training program with perturbation called PPT training in the Weltin' study

FigureFigure 1 TablesTable 1 Table 3

 113 Figure

Figure 1

 1 Figure 1

Table 1

 1 about here Testing. Participants performed different lateral movements and performance tests prior to CS training (PRE) and after the CS training intervention (POST). Post testing was scheduled in order to match conditions of the PRE measurements, in terms of day of the week, time of the day, and sleep.

ACKNOWLEDGMENTS

We would like to thank the participants for their time and commitment throughout this study.

The authors report no conflict of interest and state that the results of the present study do not constitute endorsement of the product by the authors or the NSCA.

Core stability training program and lateral movements

CS training sessions