The swimming track start is a complex motor skill that utilizes asymmetric lower limb action. The purpose of this study was to explore whether it could be optimized by applying the commonly accepted view that there are asymmetries in the function and behaviors of the lower limbs. Initially, the study aimed to examine the relationship between various measures of lower limb asymmetry and the swimmers' preferences for forward foot placement in the swimming track start. Participants underwent a 7 week training period whereby both the left foot forward (LFF) and the right foot forward (RFF) track starts were practiced. The philosophy behind this training protocol was to ensure that participants received equal practice with the preferred and non-preferred stance so that a dominant stance, if it existed, could emerge. Consequently, the relationships between the dominant track start stance and the lower limb asymmetry measures could be determined more accurately. Participants were male (N=11) and female (N=11) swimmers, aged 12-16 years, from the UWA-Uniswim National Age Squad. Kinetic and kinematic data were collected for the track start prior to and following the 7 week training intervention. The intervention was finished when a participant had completed approximately 14 dive sessions where both the LFF and RFF track starts were practiced. The performance criterion measure was time to 5 m. Despite significant differences in vertical force and velocity contributions following the intervention, time to 5 m did not improve for either the LFF or the RFF track start. Four different measures of lower limb asymmetry were collected, including footedness, the preferred track start stance, and the dominant take-off limb for the unilateral and bilateral counter-movement jump (CMJ). Sixteen of 22 participants displayed changes in their dominant track start stance. Eleven participants showed biases for one stance (6 for the LFF & 5 for the RFF), and 11 participants remained or became more symmetrical. Results indicated that the preferred track start stance was the only measure of asymmetry that was significantly related to track start performance (x2[2]= 6.71, p=.04 for pre-intervention & x2[2]=7.77, p=.02 for post-intervention). All other measures of lower limb asymmetry were shown to be unrelated to track start preference and performance. It was suggested that the 7 week training intervention did not provide a sufficient amount of time to see conclusive effects on 5 m time or to make conclusive comparisons between the dominant track start stance and measures of asymmetry. Since the preferred track start corresponded with better performance less than 50% of the time, it was suggested that swimmers and coaches experiment with different dive techniques to find the start which is most effective for them and spend more time on them during training.

Non-contact injuries can occur when athletes use poor or inconsistent mechanics during typical sport-related movements like landing from a jump. Anterior cruciate ligament (ACL) injuries are especially devastating, and certain populations like female athletes and athletes with a previous ACL reconstruction (ACLR) are at greater risk of suffering an ACL injury, with altered biomechanical strategies being one proposed reason. Asymmetric landings where one limb experiences greater landing force can decrease joint stability and place the overloaded limb at greater risk for ACL injury. Additionally, a stiff landing, characterized by increased ground reaction force (GRF), extended joints at initial ground contact, and decreased joint flexion throughout the landing, has been proposed to increase ACL injury risk. While load distribution between limbs is a common landing assessment to determine injury risk, it is unclear what role limb stiffness plays in the likelihood of experiencing an ACL injury. Limb stiffness is simply the deformation of the limb in response to the downward force applied to the lower limb during ground contact, which can be approximated using GRF. Limb stiffness has been commonly used to assess performance in running, hopping, and jumping, however, its relationship with injury risk during landings is relatively unexplored. Past research has revealed that the ACL experiences peak strain prior to initial ground contact when the knee is at or near full extension. Additionally, expert video analyses have determined that ACL injuries most likely occur within 50 milliseconds of ground contact. It is possible that limb stiffness and limb stiffness asymmetry can be used during the early impact phase of landings to reveal ACLR- and sex-specific landing mechanics differences when the ACL appears to be most vulnerable. Moreover, game-like, sport-specific landing tasks with a greater horizontal component that load the ACL and those that divert attention away from landing strategies may uncover differences that do not appear in standard, controlled laboratory tasks. The overall goal of this project was to use limb stiffness, limb stiffness asymmetry, and related measures to analyze the early landing phase mechanics of groups at greater risk for ACL injury during game-like, sport-specific landings. First, in an ACLR cohort, greater knee power and knee work asymmetries were found when compared to healthy recreational athletes, supporting previous literature that found that athletes with an ACLR land unevenly by offloading their surgical limb. However, limb stiffness asymmetry was not different between groups, implying that the groups may have modulated limb stiffness differently between limbs. Second, minimal sex-by-task interactions were determined for landings that varied by horizontal approach prior to initial ground contact. Significant differences were found for most measures across tasks overall, however, male and female athletes displayed similar landing mechanics, indicating that expected sex-specific differences may not exist during the immediate landing phase when ACL injuries are thought to occur. Last a landing task that mimicked a ball in mid-air and diverted attention away from landing mechanics produced a sex-by-task interaction for peak impact force but no other measure. When comparing each sex-task pairing, a trend for greater peak impact force by female athletes during the distracted landing (p=0.098) was found which may indicate that future tasks with additional external focuses or another game-like component will reveal anticipated sex-specific differences. Increased time between limbs for initial ground contact for female athletes also revealed that a time-synchronized assessment of between-limb coordination may be beneficial for future research. / Doctor of Philosophy / Non-contact injuries can occur when athletes use poor or inconsistent mechanics during typical sport-related movements like landing from a jump. Anterior cruciate ligament (ACL) injuries are especially tough, and certain populations like female athletes and athletes with a previous ACL reconstruction surgery (ACLR) are at greater risk of suffering an ACL injury, with different movement techniques being one proposed reason. Uneven landings where one limb has greater landing forces can decrease joint posture and place the overloaded limb at greater risk for ACL injury. Additionally, a stiff landing, defined by larger ground reaction force (GRF), extended joints at initial ground contact, and decreased joint flexion throughout the landing, is thought to increase ACL injury risk. While landing force distribution between limbs is a common way of evaluating landings to determine injury risk, it is unclear what role limb stiffness plays in the likelihood of experiencing an ACL injury. Limb stiffness is simply the deformation of the limb in response to the downward force applied on the lower limb during ground contact, which can be estimated using GRF. Limb stiffness has been commonly used to assess performance in running, hopping, and jumping, however, its relationship with injury risk during landings is pretty limited. Past research has revealed that the ACL experiences maximum stretch prior to initial ground contact when the knee is or is almost completely straight. Additionally, expert video investigations have determined that ACL injuries most likely occur within 50 milliseconds of ground contact. It is possible that limb stiffness and limb stiffness asymmetry can be used during the early impact phase of landings to reveal sex- and ACLR-specific landing mechanics differences when the ACL appears to be most in danger. Additionally, game-like, sport-specific landing tasks with a greater horizontal element that load the ACL and those that redirect attention away from landing strategies may show differences that do not appear in basic laboratory tasks. The overall goal of this project was to use limb stiffness, limb stiffness asymmetry, and related measures to examine the early landing phase techniques of groups at greater risk for ACL injury during game-like, sport-specific landings. First, in a group of athletes with a previous ACLR, greater knee storage differences between limbs were found when compared to healthy recreational athletes, supporting previous research studies that found that athletes with an ACLR land unevenly by offloading their surgical limb. However, limb stiffness asymmetry was not different between groups, implying that the groups may have regulated limb stiffness differently between limbs. Second, only a couple measures were significantly affected by the combined effect of sex and task during landings that were different due to their horizontal element. Significant differences were found for most measures across tasks overall, however, male and female athletes had similar landing techniques, showing that the expected differences between sexes may not happen very early in the landing phase when ACL injuries are thought to happen. Last, a landing task that imitated a ball in mid-air and redirected attention away from landing mechanics produced a larger sex-specific difference for peak impact force compared to a basic landing task. When comparing each sex-task pairing, a trend for greater peak impact force by female athletes during the distracted landing (p=0.098) was found which may show that future tasks with additional distractions or another game-like element will reveal expected differences between sexes. Increased time between limbs for initial ground contact for female athletes also revealed that looking at the coordination of both limbs on the same timescale may be useful for future research.