Determining voluntary activation in synergistic muscles: a novel mechanomyographic approach

Purpose Drawing on correlations between the mechanomyographic (MMG) and the force signal, we devised a novel approach based on MMG signal analysis to detect voluntary activation (VA) of the synergistic superficial heads of the quadriceps muscle. We hypothesized that, after a fatiguing exercise, the changes in the evoked MMG signal of each quadriceps head would correlate with the changes in the level of VA in the whole quadriceps. Methods Twenty-five men underwent a unilateral single-leg quadriceps exercise to failure. Before and after exercise, VA was assessed by interpolated-twitch-technique via nerve stimulation during and after maximum voluntary contraction (MVC). The force and MMG signal were recorded from vastus lateralis, vastus medialis, and rectus femoris. The MMG peak-to-peak was calculated and the voluntary activation index (VA(MMG)), defined as the superimposed/potentiated MMG peak-to-peak ratio, was determined from the MMG signal for each head. Results VA(MMG) presented a very high intraclass correlation coefficient (0.981-0.998) and sensitivity (MDC 95% : 0.42-6.97%). MVC and VA were decreased after exercise in both the exercising [MVC:-17(5)%, ES -0.92; VA: -7(3)%, ES -1.90] and the contralateral limb [MVC: -9(4)%, ES -0.48; VA: -4(1)%, ES -1.51]. VA(MMG) was decreased in both the exercising [similar to-9(6)%, ES -1.77] and contralateral limb [similar to-3(2)%, ES -0.57], with a greater decrease in VA(MMG) noted only in the vastus medialis of the exercising limb. Moderate-to-very high correlations were found between VA(MMG) and VA (R-range: 0.503-0.886) before and after exercise. Conclusion VA(MMG) may be implemented to assess VA and provide further information when multiple synergistic muscle heads are involved in fatiguing exercises.