Data from: The nervous system does not compensate for an acute change in the balance of passive force between synergist muscles

It is unclear how muscle activation strategies adapt to differential acute changes in the biomechanical characteristics between synergist muscles. This issue is fundamental to understanding the control of almost every joint in the body. The aim of this human experiment was to determine whether the relative activation of the heads of the triceps surae (Gastrocnemius medialis [GM], Gastrocnemius lateralis [GL] and Soleus [SOL]) compensates for differential changes in passive force between these muscles. Twenty-four participants performed isometric ankle plantarflexion at 20N.m and 20% of the active torque measured during a maximal contraction, at three ankle angles (30&[deg] of plantarflexion, 0&[deg] and 25&[deg] of dorsiflexion; knee fully extended). Myoelectric activity (electromyography) provided an index of neural drive. Muscle shear modulus (elastography) provided an index of muscle force. Passive dorsiflexion induced a much larger increase in passive shear modulus for GM (+657.6&[plusmn]257.7%) than GL (+488.7&[plusmn]257.9%) and SOL (+106.6&[plusmn]93.0%). However, the neural drive during submaximal tasks did not compensate for this change in the balance of the passive force. Instead, when considering the contraction at 20% MVC, GL RMS EMG was reduced at both 0&[deg] (-39.4&[plusmn]34.5%) and DF 25&[deg] (-20.6&[plusmn]58.6%) compared to PF 30&[deg], while GM and SOL RMS EMG did not change. As a result, the GM/GL ratio of shear modulus was higher at 0&[deg] and DF 25&[deg] than PF 30&[deg], indicating that the greater the dorsiflexion angle, the stronger the bias of force to GM compared to GL. The magnitude of this change in force balance varied greatly between participants.

目前尚不明确肌肉激活策略如何适应协同肌之间生物力学特性的差异性急性变化。这一问题对于理解人体几乎所有关节的运动控制均具有基础性意义。
本项人体实验旨在探究小腿三头肌（腓肠肌内侧头（Gastrocnemius medialis, GM）、腓肠肌外侧头（Gastrocnemius lateralis, GL）与比目鱼肌（Soleus, SOL））的相对激活是否可代偿这些肌肉间被动肌力的差异性变化。24名受试者在膝关节完全伸直的状态下，于3个踝关节角度（跖屈30°、中立位0°、背屈25°）分别完成两项等长踝关节跖屈任务：一是20N·m的固定扭矩任务，二是基于最大自主收缩（Maximal Voluntary Contraction, MVC）扭矩20%强度的亚最大任务。
本研究采用肌电活动（electromyography, EMG）作为神经驱动的评价指标，以肌肉剪切模量（通过弹性成像（elastography）获取）作为肌肉肌力的评价指标。
实验结果显示：被动背屈可使腓肠肌内侧头的被动剪切模量出现显著升高（+657.6%±257.7%），其增幅远高于腓肠肌外侧头（+488.7%±257.9%）与比目鱼肌（+106.6%±93.0%）。然而，亚最大强度收缩任务中的神经驱动并未代偿这种被动肌力平衡的变化。
进一步针对20% MVC强度的收缩任务分析发现，与跖屈30°工况相比，腓肠肌外侧头的肌电均方根值（root mean square EMG, RMS EMG）在中立位0°（-39.4%±34.5%）与背屈25°工况下均有所降低（-20.6%±58.6%），而腓肠肌内侧头与比目鱼肌的肌电均方根值未发生显著变化。
由此可见，剪切模量的GM/GL比值在中立位0°与背屈25°工况下高于跖屈30°工况，表明背屈角度越大，相较于腓肠肌外侧头，肌力向腓肠肌内侧头的偏向性越强。这种肌力平衡变化的幅度在受试者个体间存在显著差异。