Femoroacetabular impingement syndrome (FAIS) is a painful condition of the hip resulting from a combination of asphericity of the femoral head-neck junction (cam morphology) and repeated hip flexion. In individuals with FAIS, hip pain is most commonly provoked during high impact sports such as football, ice hockey, and running. However, 50-80% of athletes with cam morphology remain asymptomatic. The overarching aim of this thesis was to determine how simulated sport participation, hip pain, and cam morphology influence biomechanics during running in individuals with FAIS.

Study 1 determined the effects of repeated sprint exercise on work done by lower limb joint moments during sprint acceleration in 18 healthy individuals. Repeated sprint exercise was chosen to simulate the demands of high-intensity sport participation in a laboratory setting. Results demonstrated that repeated sprint exercise decreased work done by the hip and knee moments during swing, but had minimal effects on work done by the ankle plantar flexors during stance. The repeated sprint exercise was highly demanding for the hip musculature and deemed likely to reveal biomechanical impairments when performed by individuals with FAIS.

Study 2 determined the reliability and validity of a custom rig used to measure isometric hip strength in principal (sagittal, frontal, and transverse) and intermediate planes of movement (combinations of hip extension, abduction, and external rotation). Due to the complex anatomy of the hip-spanning muscles, hip strength measured in the principal planes alone may not fully assess their strength capacity. On two occasions, 6.2±1.8 days apart, 20 healthy participants performed isometric hip strength tests using the custom rig and a motor driven dynamometer. Reliability was comparable for hip strength measures acquired using the custom rig and motor driven dynamometer. The custom rig was deemed appropriate for use in study 3 of this thesis.

Study 3 compared the effects of repeated sprint exercise on hip external biomechanics during sprinting and isometric hip strength between individuals with FAIS and asymptomatic individuals with and without cam morphology. Forty-nine recreationally active individuals with comparable levels of physical activity were divided into three groups (FAIS=15;​ asymptomatic cam morphology=15;​ control=18) based on their history of hip pain, results of a clinical impingement test, and presence of cam morphology assessed via the alpha angle on magnetic resonance imaging. Isometric hip strength and three-dimensional marker trajectories, ground reaction forces, and electromyograms from 12 lower limb muscles were recorded from participants while they performed 10 m overground sprinting trials performed both before and after repeated sprint exercise. Results showed ~27% of participants with FAIS reported increased hip pain following repeated sprint exercise. No between-group differences were observed in sprinting biomechanics or hip strength either before or after repeated sprint exercise. No associations were observed between self-reported pain and changes in hip moments during sprinting or hip strength in individuals with FAIS.

Study 4 used an electromyography-informed neuromusculoskeletal model to determine muscle and hip contact forces generated during sprinting in those with FAIS, as well as asymptomatic individuals with and without cam morphology. Analyses revealed no between-group differences in magnitude or direction of hip contact force during sprinting. Results demonstrated the largest anteriorly directed hip contact force occurred during the early swing phase of the sprinting cycle.

Taken together, studies from this thesis revealed that hip biomechanics during sprinting do not differentiate individuals with FAIS who report mild symptoms from asymptomatic individuals with and without cam morphology. Moreover, results highlighted a trade-off between improving sprint performance and reducing potential risk for developing hip pain. Maintaining high hip muscle forces during the early swing phase of sprinting was shown to be important for maintaining sprint performance across fatiguing exercise. However, hip loading during early swing generated the highest anteriorly direct hip contact force, potentially increasing risk for anterior acetabular cartilage damage. The cross-sectional nature of the studies included in this thesis prevented causal relationships from being inferred. Although specific mechanisms leading to FAIS-related hip pain remain unknown, this thesis puts forward hypotheses to explain novel results and suggests future research directions to better understand the short- and long-term consequences of hip pain and cam morphology