Sex differences in muscle morphology of the knee flexors and knee extensors

IntroductionFemales experience higher risk of anterior cruciate ligament (ACL) injuries; males experience higher risk of hamstring strain injuries. Differences in injury may be partially due to sex differences in knee flexor (KF) to knee extensor (KE) muscle size ratio and the proportional size of constituent muscles.PurposeTo compare the absolute and proportional size, and mass distribution, of individual KE and KF muscles, as well as overall size and balance (size ratio) of these muscle groups between the sexes.MethodsT1-weighted axial plane MR images (1.5T) of healthy untrained young males and females (32 vs 34) were acquired to determine thigh muscle anatomical cross-sectional area (ACSA). Maximal ACSA (ACSAmax) of constituent muscles, summated for KF and KE muscle groups, and the KF:KE ratio were calculated.ResultsFemales had 25.3% smaller KE ACSAmax (70.9±12.1 vs 93.6±10.3 cm2; P2 vs 55.1±7.3cm2; PConclusionSex differences in KF:KE ACSAmax ratio may contribute to increased risk of ACL injury in females. Sex discrepancies in absolute and proportional size of SA, GR, VL and BFlh may contribute further anatomical explanations for sex differences in injury incidence.