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 G...