Characteristics of motor unit recruitment in boys and men at maximal and submaximal force levels

The aim of this study was to compare voluntary activation (VA) and motor units (MU) recruitment patterns between boys and men at different contraction levels of the knee extensor muscles. We hypothesized that boys and men would display similar VA and MU recruitment patterns at low submaximal force levels, but that boys would display a lower utilization of their higher-threshold MU and a lower VA at near-maximal and maximal force levels than men. 11 prepubertal boys and 13 men were tested at the optimal knee angle. Next, VA was assessed using the twitch interpolation technique during maximal (MVC) and submaximal isometric voluntary contractions. Mean firing rate (MFR), recruitment threshold (RT) and motor unit action potential size (MUAP(SIZE)) were extracted to characterize neural strategies. No significant difference between groups was found for VA at every contraction level. Similarly, no significant difference was found for the MFR vs. RT relationship parameters between groups. For the vastus lateralis (VL) muscle, the MUAP(SIZE) vs. RT relationship differed between boys and men independent of the contraction level (p<0.05). Boys also displayed a different MFR vs. MUAP(SIZE) relationship on the VL muscle independent of the contraction level (p<0.05). To conclude, no difference between boys and men was found for VA regardless of the contraction level investigated. Differences in motor unit recruitment parameters between boys and men seem to be explained by different muscle dimensions between groups.