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 [~ −9(6)%, ES −1.77] and contralateral limb [~ −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. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00421-022-04966-8.