BACKGROUND: Hamstrings-to-quadriceps ratio (HTQ) can provide unique information regarding muscle development, training habits, and injury risk, particularly of the knee. As females are typically more prone to knee injuries than males, examining sex-based differences in HTQ may provide insight to the mechanisms causing these differences. The purpose of this study was to compare HTQ during leg extension and flexion contractions in females versus males. METHODS: Twenty-seven females and males (mean±95% confidence interval, n=14 females, age=24±4yrs; n=13 males, age=25±6yrs) participated. Ultrasound images quantified quadriceps, hamstrings, and thigh muscle cross-sectional area (CSA). Peak torque (PT) was taken from maximal voluntary isometric contractions (MVICs) and isokinetic leg extension and flexion contractions from 60-300°·s-1. HTQ was calculated by dividing hamstrings CSA by quadriceps CSA and PT from leg flexion by PT from leg extension for all contractions. Independent samples t-tests examined differences in CSA and HTQ from CSA. Mixed-factorial ANOVAs examined differences in PT and HTQ from PT. RESULTS: Males had larger quadriceps, hamstrings, and thigh CSA, and isometric and isokinetic PT than females for leg extension and flexion across velocity (p<0.001). There were no sex-based differences in the HTQ from CSA or PT across velocity (p>0.109). For both groups, leg extension PT decreased from MVIC to 300°·s-1 (p<0.035). For the females, leg flexion PT was the same from MVIC to 60°·s-1 (p=1.000), decreased from 60 to 180°·s-1 (p<0.002), and plateaued from 180 to 300°·s-1 (p>0.071). For the males, leg flexion PT decreased from MVIC to 300°·s-1 (p<0.025). For both groups, HTQ increased from MVIC to 60°·s-1 (p<0.001), then plateaued from 60 to 300°·s-1 (p=1.000). CONCLUSIONS: Both groups had similar patterns of response for leg extension PT and HTQ across velocity, though leg flexion PT for females remained constant from moderate to fast velocities. Thus, factors other than quadriceps and hamstrings muscle size and strength may be responsible for the potential sex-based difference in knee injury risk. Worth noting, among these recreationally-trained participants, HTQ across velocity was <0.59, below the generally recommended ratio of 0.67. It may be beneficial for both females and males to prioritize hamstrings strengthening exercises, as HTQ of <0.67 tend to be associated with greater knee injury risk.

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