The continual increase in anterior cruciate ligament (ACL) injuries in sports makes it the most common ligament injury and leads the athlete down a difficult road with reconstruction surgery (ACLR) and months of rehabilitation. Specifically, females are at a greater risk of both primary and secondary ACL injuries compared to males. The purpose of this research was to understand the differences in movement and loading variability between ACLR and healthy athletes during unilateral and bilateral landings while utilizing limb symmetry to understand between group differences. It was hypothesized that females with an ACLR would have greater variability compared to males with an ACLR and healthy female athletes. 40 ACLR and 67 healthy athletes were asked to complete seven stop jumps and 25 ACLR and 30 healthy athletes completed seven single hop trials to assess intra-subject variability. The stop jump task utilized embedded force plates and motion capture technology while the single hop task used loadsol® in-shoe force sensors. The measures studied with the stop jump included posterior and vertical ground reaction force (GRF), knee/hip abduction/adduction angles, and loading rate. The single hop measures included peak force, loading rate, and impulse. To assess variability and limb symmetry, coefficient of variation (CV) and the limb symmetry index (LSI) were calculated for each of outcome measure. A linear mixed effects model was completed in JMP (SAS Institute Inc., Cary, NC) with p<0.05 to see the effects of group, sex, and limb. During the stop jump task, the ACLR athletes showed higher variability for both posterior GRF (p<0.001), posterior GRF LSI (p<0.001), and loading rate (p=0.027) compared to controls. Females with an ACLR had higher variability in vertical GRF (p<0.001) and vertical GRF symmetry (p=0.029) compared to HC females. Additionally, females with an ACLR had higher variability in the vertical GRF (p=0.033) when compared to males with an ACLR. Knee abduction angle (p=0.024) showed males with an ACLR to have higher variability compared to females with an ACLR. For the single hop task, there was a significant difference between sex for loading rate (p<0.001), loading rate LSI (p=0.004), impulse (p=0.006), and impulse LSI (p=0.001) with males producing a higher mean CV compared to females in all measures regardless of group. Overall, these results support the hypothesis that group and sex differences exist and that females with an ACLR will have higher variability and asymmetrical movements than male ACLR patients and healthy females during bilateral landings, which could lead to increased injury risk. In unilateral landings, the results suggest that females are landing with less variability compared to males. With increased variability on the surgical limb of an athlete with an ACLR, it is likely that the athlete will have a more successful return to sport as they can react and adapt to changes in landing during sports. Future work should report reinjury rates to investigate the potential role of movement variability in injury risk and potentially determine variability thresholds for injury risk. The evaluation of bilateral and unilateral landings revealed the need to include both landing tasks in return to sport testing as well as a limb symmetry metric to understand an athlete's functional readiness to react to changing conditions during sports related movement. / Master of Science / Anterior cruciate ligament (ACL) injuries continue to rise in all sports and result in athletes having to undergo reconstruction surgery (ACLR) and months of rehabilitation if they want to compete at a high level again. Specifically, females have a greater risk of suffering both a primary and secondary ACL injury compared to males. Movement differences have been previously researched in hopes of decreasing injury risk. Variability, or the differences between one trial to the next, is a way to visualize the athlete's ability to adapt following injury. The ideal level of adaptability is still unclear as too much variability can be seen as unstable while too little can be seen as unforgiving. The purpose of this project was to better understand movement and loading variability between ACLR and healthy athletes during one legged (unilateral) and two legged (bilateral) landings. Differences between limbs were analyzed to understand whether one leg was favored over the other. Every participant completed seven stop jump trials which required them to run forward, jump off one leg and landing with two legs followed by a maximum vertical jump; another different group completed a series of seven single leg hop tasks during which they started on one-leg and jumped forward as far as they could while maintaining their balance. The measures studied with the stop jump included peak vertical and posterior force, frontal plane knee and hip angles, and loading rate. The single hop measures included peak force, loading rate, and impulse. The loading rate is defined as the ratio of peak force divided by the time it takes the athlete to reach peak force following initial contact. Impulse is the area under the force time curve and provides insight into the athlete's ability to dissipate load. The force data from each trial performed were collected using force plate technology and loadsol® shoe inserts. Variability was calculated using the coefficient of variation (CV) which is the ratio between the standard deviation and the mean value across the trails. The limb symmetry index (LSI) was calculated as the ratio between the surgical/non-dominant and the non-surgical/dominant limbs. During a stop jump, female patients with an ACLR showed greater variability in peak force and force LSI revealing asymmetric landing compared to healthy females. Additionally, females with an ACLR had greater variability compared to males with an ACLR in peak force. The ACLR group had greater variability in posterior force, posterior force symmetry, and loading rate compared to the athletes in the healthy group. During a single hop, males had higher greater variability in loading rate, loading rate limb difference, impulse, and impulse limb difference compared to females. These results suggest that all females have a more unstable and asymmetrical landing compared to all males during bilateral landing. However, during a unilateral landing, females had a less forgiving landing compared to males, which could also indicate an increased injury risk. Future work should investigate reinjury rates to determine whether variability impacts injury risk and if differences in injury risk between males and females are associated with differences in variability.

ACL injuries are severe injuries and also the most common diagnosis among alpine skiers. The purpose of this study was to evaluate alpine skiers isokinetic strength and the risk of ACL injury. 31 alpine skiers were included in this study, 17 men and 14 women, who had conducted physiological assessments between 1999 and 2007 during their active careers. The participants were sent an online questionnaire in 2018 regarding ACL injuries during their active alpine careers. The risk of injury was then evaluated by comparing injured athletes concentric isokinetic strength, hamstring to quadriceps ratio and limb symmetry index with non-injured athletes. Results showed a significant likelihood of injury for having a lower hamstring to quadriceps ratio in the left limb but not in the right limb. There was no significant likelihood of injury and having a greater limb asymmetry. Because of the small number of participants and few ACL injured there is difficult to draw any conclusions. However, there seems to be a relationship of having a lower H/Q ratio on the left limb and the risk of ACL injury. Further research is recommended.

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