The main aim of this thesis was to examine hamstrings anatomy and its influence on knee flexor muscle function in healthy young men. A secondary aim was to better understand the implications of hamstrings anatomy and function, and their variability, in relation to the risk of strain injury. The functional and conventional H:Q ratios (examined up to high angular velocities) as well as the knee joint angle-specific isometric H:Q ratio exhibited good test-retest reliability at joint positions that closely replicated the conditions of high injury risk. Football players did not exhibit any differences in angle-specific or peak torque H:Q ratios compared to recreationally active controls. Knee extensor and flexor strength, relative to body mass, of footballers and controls was similar for all velocities, except concentric knee flexor strength at 400° s-1 (footballers +40%; P < 0.01). Muscle volume explained 30-71% and 38-58% of the differences between individuals in knee extensors and flexors torque respectively across a range of velocities. A moderate correlation was also found between the volume of these antagonistic muscle groups (R2= 0.41). The relative volume of the knee extensors and flexors explained ~20% of the variance in the isometric H:Q ratio and ~31% in the high velocity functional H:Q ratio. Biceps femoris long head exhibited a balanced myosin heavy chain isoform distribution (47.1% type I and 52.9 % total type II) in young healthy men, while BFlh muscle composition was not related to any measure of knee flexor maximal or explosive strength. Biceps femoris long head proximal aponeurosis area varied considerably between participants (>4-fold) and was not related to biceps femoris long head maximal anatomical cross-sectional area (r= 0.04, P= 0.83). Consequently, the aponeurosis:muscle area ratio exhibited 6-fold variability (range, 0.53 to 3.09; CV= 32.5%). Aponeurosis size was not related to isometric or eccentric knee flexion strength. The findings of this thesis suggest that the main anatomical factor that contributes to knee flexors function in vivo is hamstrings muscle size, while muscle composition and aponeurosis size do not seem to have a significant influence. The high inter-individual variability of the biceps femoris long head proximal aponeurosis size suggests that a disproportionately small aponeurosis may be a risk factor for strain injury. In contrast, biceps femoris long head muscle composition does not seem to explain the high incidence of strain injuries in this muscle. Quadriceps and hamstrings muscle size imbalances contribute to functional imbalances that may predispose to strain injury and correction of any size imbalance may be a useful injury prevention tool. Finally, regular exposure to football training and match-play does not seem to influence the balance of muscle strength around the knee joint.

The broad aim of this thesis was to progress understanding of the neuromechanics of joint stability and injury mechanisms by investigating the interactions between neuromuscular function and balance perturbations as well as the influence of sex and fatigue on these variables. Knee extensor (KE) and plantar flexor (PF) isometric strength parameters (maximum voluntary torque (MVT), explosive voluntary torque (EVT)) were related in young healthy adults. EVT of KE and PF were correlated at 4/5 time points during the rising torque-time curve for all absolute (r = 0.488-0.755) and relative (to body mass (BM) (r = 0.517-0.669) and MVT (r = 0.353-0.480)) expressions of EVT. These results suggest that KE and PF function is related for both maximum and explosive torque. Males were stronger for KE (+89%) and PF (+55%) than females. Males also displayed greater EVT at all time points in KE (+57-109%) and at 50-150 ms in PF (+33-52%). When MVT and EVT were normalised to BM, males continued to be stronger at all time points in KE (+23-60%) and from 100-150 ms (18-20%) in PF. No sex differences were found when EVT was normalised to MVT. Furthermore, sex differences were discovered in muscle morphology. Females had a smaller knee flexor (KF):KE size ratio, a proportionately small sartorius (SA) and gracilis (GR) and a proportionately larger vastus lateralis (VL), potentially predisposing females to greater risk of ACL injury. Females had a larger biceps femoris long head (BFlh) as a proportion of the KF than males, which may contribute to the higher risk of hamstring strain injury (HSI) in males. Regarding explosive performance and perturbation response, explosive PF torque had a weak to moderate correlation with COM displacement (COMD) from 400-500 ms (r = -0.346 to -0.508) and COM velocity (COMV) from 300-500 ms (r = -0.349 to -0.416), with weaker correlations between explosive KE torque and COMV at 400 ms (r = -0.381 to -0.411) but not with COMD. These findings suggest that greater explosive torque results in better control of the COM in response to unexpected perturbations. The effects of football simulated fatigue on these factors resulted in reduced maximal KF and KE torque. However, football simulated fatigue was not found to reduce EVT of either muscle group, or explosive H/Q ratio. Football simulated fatigue resulted in impaired balance response to unexpected perturbation in the posterior but not the anterior direction.