Purpose: This study examined the association of muscle size and strength for the quadriceps and hamstrings, the relationship between the size of these muscles, and whether the H:Q size ratio influenced reciprocal strength balance—widely regarded as a risk factor for hamstrings injury. Methods: Knee extensor and flexor isometric, concentric and eccentric (50 and 350° s−1) strength were measured in 31 healthy, recreationally active young men. Muscle volume was measured with magnetic resonance imaging. Results: The knee flexors achieved higher concentric and eccentric torques (normalised to isometric values) than the extensors. Muscle volume explained a significant part of the inter-individual differences in strength in both extensors (isometric 71 %, concentric 30–31 %) and flexors (isometric 38 %, concentric 50–55 %). Notably, muscle size was related to knee flexor eccentric strength (r = 0.69–0.76; R2 = 48–58 %) but not extensor eccentric strength. Quadriceps and hamstrings volumes were moderately correlated (r = 0.64), with the majority of the variance in the size of one muscle (59 %) not explained by the size of the other muscle. The hamstrings-to-quadriceps (H:Q) volume ratio was correlated with the isometric (r = 0.45) and functional strength ratios (350° s−1, r = 0.56; 50° s−1, r = 0.34). Conclusions: Muscle size exhibited a differential influence on knee extensor and flexor eccentric strength. Quadriceps and hamstrings muscle size was related, and the H:Q size ratio contributed to their strength ratios. Muscle size imbalances contribute to functional imbalances and these findings support the use of hamstrings strength training with an emphasis on hypertrophic adaptations for reducing injury risk.
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