Independent directional tuning of the human triceps surae muscles during standing postural control

The triceps surae, composed of the soleus (SOL) and medial (MG) and lateral (LG) gastrocnemii, are synergists that act as a functional unit to plantarflex the ankle. However, anatomical differences suggest that each muscle is capable of generating distinct torques at the ankle, raising the possibility that each can be independently controlled to suit the needs of a given task. This possibility was explored by investigating the activation patterns of the triceps surae during two balance tasks that use different neuromechanical control strategies to maintain equilibrium. High-density surface EMG was recorded from the triceps surae of 14 healthy young adults during multiple trials of dual- and single-legged standing. Newly developed analyses examined how each muscle tuned its activity with center of pressure (COP) movement throughout 2-D space. During dual-legged standing, only the SOL and MG were active, and both tuned their activity uniformly with anteroposterior COP movement. By contrast, during single-legged standing, each muscle showed robust activation and significantly different directional tuning, with the LG most active before medial COP movement, whereas SOL and MG were most active before lateral COP movement. Further analyses demonstrated the LG could be activated entirely independent of the SOL and MG, and vice versa, with independent activation of each muscle causing different angular deflections of the COP during single-, but not dual-legged standing. These observations reveal a sophisticated level of neural control, whereby the nervous system exploits subtle differences between highly similar muscles to tune stabilizing torques in a task-dependent manner.<b>NEW & NOTEWORTHY</b> By applying new analytical techniques to high-density surface EMG and kinetic data, we show that each triceps surae muscle produces directionally tuned torques that the nervous system exploits to regulate equilibrium in a task-dependent manner. No intramuscular differences were observed, suggesting that each triceps surae muscle is treated as homogeneous, but functionally distinct, actuators. This fractionated control of synergists may be critical to the maintenance of equilibrium and impaired by musculoskeletal and neurological disorders.