Explosive hamstrings-to-quadriceps force ratio of males versus females

Females are known to exhibit a greater risk of ACL injury compared to males. Lower explosive hamstrings-to-quadriceps (H/Q) force ratio in the first 150 ms from activation onset could reflect an impaired capacity for knee joint stabilisation and increased risk of ACL injury. However, the explosive H/Q force ratio has not been compared between the sexes. The neuromuscular performance of untrained males and females (20 of each) was assessed during a series of isometric knee flexor and extensor contractions, specifically explosive and maximum voluntary contractions of each muscle group. Force, in absolute terms and normalised to body mass, and surface EMG of the hamstrings and quadriceps were recorded. Hamstrings force was expressed relative to quadriceps force to produce ratios of explosive H/Q force and H/Q maximum voluntary force (MVF). For the explosive contractions, agonist electromechanical delay (EMD) and agonist neural activation were also assessed. The H/Q MVF ratio was greater in males (56 %) than females (50 %; P < 0.001). However, the explosive H/Q force ratio was similar between the sexes at each time point (25-150 ms) from activation onset. Explosive hamstrings, but not quadriceps, force relative to body mass was greater for males compared to females. There were no sex differences in EMD or agonist activation for either of the muscle groups. The lack of a sex difference in early phase isometric explosive H/Q force ratio suggests other factors might be more important in determining the substantially higher knee injury rates of females.