Despite the higher incidence of anterior cruciate ligament injuries in pediatric female populations, limited research has investigated sex-differences in youth biomechanics. Furthermore, research involving jump mechanics typically requires participant to follow a set protocol, such as sticking the landing. To reduce variability and improve reliability, trails where participants fail to meet the required protocol are discarded; however, significant clinical findings may be elucidated from these trials. The purpose of this thesis was to provide a complete biomechanical analysis of unanticipated single-leg drop-jump landings in youth athletes. Thirty-two healthy youth athletes completed unanticipated single-leg drop-jump landings on their dominant limb. Trials where participants shifted foot position or touched the ground with the contralateral leg were categorized as failed. Drop-jump landings were time-normalized using landmarks within the drop-jump task. Statistical parametric mapping (SPM) determined time-varying sex-differences in muscle onset time, co-activation, kinematics and kinetics. Wilcoxon signed-rank tests and paired sample t-tests compared lower-limb kinematics, centre-of-mass excursion and muscle activation amplitudes during the successful and failed landings. A logistic regression model was also fit to predict the likelihood of a successful landing. SPM identified significantly greater trunk flexion angle in males during the deceleration, flight, and landing phase of the drop-jump. Greater quadriceps-gastrocnemius co-activation was identified during the flight phase in female participants and independent sample t-test identified longer muscle onset time in the vastus lateralis of male participants. When comparing failed and successful landings greater hip abduction and less external rotation angles were observed during the successful trials. In addition, greater preparatory muscle activation was observed in the rectus femoris and semitendinosus during the flight phase of the failed landings. A logistic regression model, which included eight kinematic and neuromuscular variables, offered a training classification accuracy of 70% and a leave-one-out cross-validation accuracy of 65%. In conclusion, females land in a more erect posture and may be less effective at dissipating landing forces. In addition, greater co-activation and shorter pre-activations of the lower limb musculature may indicate a less effective muscle activation strategy in females. Furthermore, hip kinematics and the surrounding musculature play an important role in controlling successful and failed unanticipated landings. The variables included in the logistic regression model indicate which key factors are linked to landing a jump successfully. Training modalities aimed at improving landing mechanics should therefore focus on modifying these variables.

Women are four to eight times more likely to injure their anterior cruciate ligament (ACL) compared to men. It is most commonly injured through a non-contact mechanism during game time situations. During landings, women display valgus collapse, where a less active gluteus medius muscle (GMed) may be unable to control the internal rotation of the thigh, causing an increase in knee joint abduction angle, augmenting the risk of ACL injury. This study’s purpose was to determine the difference between 12 male and 12 female athletes in muscle activity, specifically the GMed, and the 3D kinematics and kinetics of the lower-limb during drop jump landings from three heights; maximum vertical jump height, tibial length, and a commonly used height of 40cm. Results showed that females had greater hip adduction and knee abduction angles compared to men. The GMed activity showed no significant differences between sexes at each drop jump height.

Women are four to eight times more likely to injure their anterior cruciate ligament (ACL) compared to men. It is most commonly injured through a non-contact mechanism during game time situations. During landings, women display valgus collapse, where a less active gluteus medius muscle (GMed) may be unable to control the internal rotation of the thigh, causing an increase in knee joint abduction angle, augmenting the risk of ACL injury. This study’s purpose was to determine the difference between 12 male and 12 female athletes in muscle activity, specifically the GMed, and the 3D kinematics and kinetics of the lower-limb during drop jump landings from three heights; maximum vertical jump height, tibial length, and a commonly used height of 40cm. Results showed that females had greater hip adduction and knee abduction angles compared to men. The GMed activity showed no significant differences between sexes at each drop jump height.

Women are four to eight times more likely to injure their anterior cruciate ligament (ACL) compared to men. It is most commonly injured through a non-contact mechanism during game time situations. During landings, women display valgus collapse, where a less active gluteus medius muscle (GMed) may be unable to control the internal rotation of the thigh, causing an increase in knee joint abduction angle, augmenting the risk of ACL injury. This study’s purpose was to determine the difference between 12 male and 12 female athletes in muscle activity, specifically the GMed, and the 3D kinematics and kinetics of the lower-limb during drop jump landings from three heights; maximum vertical jump height, tibial length, and a commonly used height of 40cm. Results showed that females had greater hip adduction and knee abduction angles compared to men. The GMed activity showed no significant differences between sexes at each drop jump height.

Ce travail de thèse visait à déterminer l’influence de la durée et du niveau d’entraînement en endurance sur les mécanismes de régulation de la fatigue neuromusculaire et d’une stratégie de modulation de ces mécanismes sur la performance motrice. Ce travail de thèse consistait également à élaborer une nouvelle méthode d’entraînement permettant d’optimiser les stimuli responsables des adaptations résultantes d’un programme de conditionnement physique. Les résultats principaux issus de ces travaux sont présentés dans un résumé plus détaillé en fin de manuscrit. / The present work aimed to determine the influence of exercise duration and endurance training level on the mechanisms underlying neuromuscular fatigue regulation and the effect of a deceptive strategy on exercise performance. This thesis also aimed to elaborate a new training method that would optimize exercise-induced training stimuli that are known to trigger complementary physical abilities adaptations. The main outcomes of these works are detailed in an expanded abstract at the end of the manuscript.