The association between motor unit behavior and joint power at early and late phases of concentric ankle dorsiflexions.

Humans optimize muscle activation strategies to minimize energy costs at faster movement velocities. However, empirical evidence on the relationship between joint power and motor unit control strategies is lacking. Therefore, we investigated whether the range of motion and movement velocity influence the association between concentric ankle joint power and motor unit discharge rates in the tibialis anterior muscle. Eleven males performed submaximal concentric ankle dorsiflexion at 5°/s and 20°/s. High-density surface electromyography was recorded from the tibialis anterior muscle and decomposed at the two movement velocities to extract motor unit behavioral data. Ankle power and HD-sEMG from each movement velocity were separated into the first (INI) and second half (END) of the concentric motion. The results showed that the ankle power during INI at 20°/s (3.1 ± 0.7 W/kg) was greater when compared to END at 20°/s (1.8 ± 0.5 W/kg) and to both INI and END at 5°/s (0.5 ± 0.1 W/kg, p < 0.001). The relative increase in discharge rate slope from 5°/s to 20°/s was greater during INI (5.4 ± 1.0 a.u.) when compared to END (2.1 ± 1.0 a.u., p < 0.0001). Moreover, we found significant associations between ankle power and average discharge rate (r = 0.53, p < 0.005) and discharge rate slopes (r=-0.41, p < 0.005) during INI at 20°/s, but not at 5°/s. Our results demonstrate that neural drive modulation in dynamic contractions is determined by the increase in power required to execute movements, demanding specific motor unit control strategies at the start of concentric actions. Moreover, only movements performed at 20°/s demonstrated significant relation between concentric power and motor unit rate coding.