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 (VAMMG) as the superimposed/potentiated MMG peak-to-peak ratio, was determined from the MMG signal for each head. 

Results: VAMMG presented very-high reliability (ICC:0.975-0.998) and sensitivity (MDC95%: 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]. VAMMG decreased in both the exercised [~-9(6)%, ES:-1.00] and the contralateral limb [~-5(2)%, ES:-1.12], with larger VAMMG decrements occurring in vastus medialis in the exercised limb only. Moderate to very high correlations were found between VAMMGand VA (R= 0.503-0.886) before and after fatigue. 

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

研究目的：本研究提出一种基于肌机械信号（mechanomyographic, MMG）的新方法，用于检测股四头肌协同浅表头的自愿激活（voluntary activation, VA）水平。我们提出假设：在疲劳任务完成后，股四头肌各头的MMG信号变化与整体股四头肌的VA水平具有相关性。

研究方法：招募25名男性受试者[年龄：25(1)岁；体重：77(2)kg；身高：1.81(0.02)m]，使其完成单侧单腿股四头肌力竭运动。在干预前后，通过最大自主收缩（maximum voluntary contraction, MVC）期间及之后的神经刺激插值抽搐技术评估VA水平。同步采集肌力及股外侧肌、股内侧肌、股直肌的MMG信号。计算MMG信号的峰峰值，并基于各肌肉头的MMG信号计算自愿激活指数（voluntary activation index, VAMMG），即叠加/强化MMG峰峰值的比值。

研究结果：VAMMG具有极高的信度（组内相关系数ICC：0.975~0.998）与敏感度（95%置信度最小检测变化MDC95%：0.4%~4.26%）。疲劳可使受累侧与对侧肢体的MVC及VA水平均出现下降：受累侧MVC下降17(5)%，效应量ES=-0.92；VA下降7(3)%，ES=-1.90；对侧肢体MVC下降15(7)%，ES=-0.87；VA下降7(3)%，ES=-2.32。受累侧与对侧肢体的VAMMG均出现降低：受累侧VAMMG下降约9(6)%，ES=-1.00；对侧肢体下降约5(2)%，ES=-1.12，且仅在受累侧肢体中，股内侧肌的VAMMG下降幅度更大。疲劳前后，VAMMG与VA均呈现中等至极强的相关性（相关系数R=0.503~0.886）。

研究结论：VAMMG可用于评估VA水平，且当疲劳任务涉及不同协同肌肉头时，可提供更为丰富的相关信息。