Our long-term goal is to better understand how the nervous system controls muscles to generate movement. Our overall hypothesis is that the nervous system coordinates muscles by flexibly recruiting muscle synergies, defined here as groups of muscles simultaneously activated in fixed ratios, in order to map high-level task goals into motor actions. Here we studied muscle coordination in the context of balance control – a task that requires multisensory integration and coordination of multiple muscles, yet has a clear goal of controlling the center of mass (CoM), which can be achieved by using different strategies. If muscle synergies are a common mechanism used by the nervous system for balance control, we would expect to see the same muscle synergies used in a variety of strategies. Therefore we investigated the robustness of the muscle synergies in a variety of human postural strategies, such as standing, stepping and walking, to determine whether muscle synergies are a consistent underlying mechanism used by the nervous system. We hypothesized that muscle synergies are recruited to control a task-level variable (e.g. CoM direction) that is not specific to a particular postural strategy.

We demonstrated that similar muscle synergies are used in reactive responses to standing balance perturbations, in reactive stepping responses, in walking, and in reactive postural responses during walking, suggesting a common neural mechanism not only for balance control in various contexts, but for movement in general. The differences in the timing and spatial organization of muscle activity in standing, stepping, and walking postural responses were largely explained by altering the recruitment of a common set of muscle synergies, with the addition of only a single muscle synergy specific to each behavior. We demonstrated the functionality of muscle synergies by showing that each muscle synergy was correlated with a particular force produced at the ground and component of CoM acceleration both in stepping and in non-stepping postural responses. These results suggest that muscle synergies reflect the neural organization of the motor system, representing motor modules recruited to achieve a common biomechanical function across different postural behaviors. Additionally, muscle synergies used during walking were recruited during atypical phases of the gait cycle in response to an unexpected perturbation, in order to maintain balance and continue walking, suggesting a common neural mechanism for different balance requirements during walking. The compositions of muscle synergies used during walking were similar to those used during walking perturbations as well as standing balance perturbations, suggesting that muscle synergies represent common neural mechanisms for CoM movement control under different dynamic conditions. These results are of interest to a variety of fields such as rehabilitation science, prosthetics, and robotics.