Purpose: Contributions of neural and muscular factors to muscle strength change with growth, but such changes remain unclear in young populations. This study aimed to clarify the association between muscle strength and neural and muscular factors in youth athletes. Methods: Maximal voluntary contraction (MVC) during isometric knee extension, the motor unit firing rate (MUFR), and muscle thickness (MT) of the vastus lateralis were measured in 70 youth male soccer players (mean [SD]; chronological age = 16.3 [0.6] y, peak height velocity age = 13.1 [1.0] y). MUFR and MT were measured with high-density surface electromyography and ultrasonography, respectively. Results: For MUFR and MT, correlations with MVC were calculated and the values of different MVC groups were compared. A significant correlation between MVC and MT (r = .49, P < .01) was noted, but not MUFR (r = .03, P > .05). There was also no significant correlation between MT and MUFR (r = −.33, P > .05). In addition, comparison among groups (higher-/middle-/lower-strength groups) revealed that MT in the lower-strength group was significantly lower than in middle-and higher-strength groups (P < .01). Conclusion: In youth athletes, muscle strength is associated with muscular factors, rather than neural factors, and muscular and neural factors may independently contribute to muscle strength.

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