Characteristics of Motor Unit Recruitment in Boys and Men at Maximal and Submaximal Force Levels

Background: It has been proposed that motor unit recruitment may differ between boys and men. A lower voluntary activation level (VA) has been frequently reported in prepubertal children than adults during maximal efforts, although it does not seem to be the case at the muscle length maximizing force production, i.e optimal muscle length. Conversely, no study has compared prepubertal children and men at submaximal force levels, where the low musculotendinous stiffness and the potential resulting tendon slack may affect the neural strategies in children. Specifically, VA could be higher in children at low force levels to take up the tendon slack.

Aims: The aim of the present study was to compare VA difference between boys and men at different contraction levels of the knee extensor muscles. We hypothesized that (i) prepubertal boys would display a higher VA than men at low force levels because of their lower musculotendinous stiffness and (ii) Boys and men VA differences may result in differences of motor unit recruitment.

Methods: Fourteen boys (9-11 years) and 15 men (18-30 years) were tested at the optimal knee angle (i.e. maximal torque development). For both groups, VA was assessed using the twitch interpolation technique during maximal (MVC) and submaximal (20, 30, 50, 70, 80, 90% of MVC) isometric voluntary contractions. VL aponeurosis stiffness was calculated from ultrasonography data. Motor unit (MU) mean firing rate (MFR) and recruitment threshold were estimated using a validated electromyography decomposition algorithm during submaximal trapezoidal contractions (20, 30, 50, 70, 80% of MVC). The MFR vs. RT relationship was computed to characterize neural strategies.

Results: No significant difference between groups was found for VA at every contraction level. Boys displayed a lower VL aponeurosis stiffness than men (46.9 ± 14.8 and 65.4 ± 13.7 N.mm^-1; p < 0.01). In addition, no significant difference was found for the MFR vs. RT relationship parameters between groups.

Discussion and Conclusion: Interestingly, no difference between boys and men was found for VA or motor unit recruitment strategies, whatever the contraction level investigated. It is suggested that at the optimal muscle length, the mechanical properties of the musculotendinous system do not affect the neural recruitment strategies, possibly because the mechanical state of the muscle is optimized. Additional investigations at non-optimal muscle lengths are required to test this assumption.