Determining voluntary activation in synergistic muscles: new insights from a mechanomyographic approach

Aim: We propose a new approach based on the mechanomyographic (MMG) signal to detect the voluntary activation (VA) of the synergistic superficial heads of quadriceps muscle. We  hypothesised that, after a fatiguing task, the changes in MMG signal of each quadriceps’ head would correlate with the level of VA of the whole quadriceps.

Methods: Twenty-five men [age:25(1) yrs; body mass:77(2) kg; stature:1.81(0.02) m] underwent a unilateral single-leg quadriceps exercise up to failure. Before and after the intervention, VA was assessed by the interpolated twitch technique via nerve stimulation during and after a maximum voluntary contraction (MVC). Along with force, the MMG signal was recorded from vastus lateralis, vastus medialis, and rectus femoris muscles. The MMG peak-to-peak was calculated and the voluntary activation index (VA_MMG) as the superimposed/potentiated MMG peak-to-peak ratio, was determined from the MMG signal for each head. 

Results: VA_MMG presented very-high reliability (ICC:0.975-0.998) and sensitivity (MDC_95%: 0.4%-4.26%). Fatigue reduced MVC and VA in both the exercised [-17(5)%, ES:-0.92 and -7(3)%, ES:-1.90 for MVC and VA, respectively] and the contralateral limb [-15(7)%, ES:-0.87 and -7(3)%, ES:-2.32 for MVC and VA, respectively]. VA_MMG decreased in both the exercised [~-9(6)%, ES:-1.00] and the contralateral limb [~-5(2)%, ES:-1.12], with larger VA_MMG decrements occurring in vastus medialis in the exercised limb only. Moderate to very high correlations were found between VA_MMGand VA (R= 0.503-0.886) before and after fatigue. 

Conclusions: VA_MMG may be implemented to assess VA and could provide further information when different synergistic muscle heads are involved in a fatiguing task.