Falls are the leading cause of morbidity and mortality among the elderly and result from a failure of the nervous system to appropriately coordinate muscles to maintain balance. Because muscle activity represents the output of the nervous system, examining muscle activation may reveal differences in neural mechanisms underlying falls;​ however, there is enormous spatial and temporal variability in muscle activation patterns, making them difficult to functionally interpret. Recent work has demonstrated that the spatial and temporal features of muscle activity can be functionally yet separately explained by muscle synergies and task-level feedback, respectively. The spatial coordination of muscles has been functionally characterized in a variety of motor tasks using muscle synergies, or groups of muscles with fixed ratios of coactivation. However, the temporal recruitments of such muscle synergies as well as the underlying neural mechanisms have largely been uninvestigated. Conversely, temporal activation of individual muscles throughout postural responses has been functionally characterized using task-level feedback of center of mass (CoM). However, CoM feedback has only been applied to perturbations where the body starts from rest and CoM kinematics (displacement, velocity, acceleration) are highly correlated.

I hypothesize that the nervous system continuously uses task-level feedback of CoM to recruit muscle synergies throughout standing balance tasks. Here, I unified the muscle synergy hypothesis with task-level feedback to functionally explain the spatiotemporal features of muscle activity throughout human postural responses. I first demonstrated that the temporal recruitment of muscle synergies throughout discrete sagittal perturbations could be well-reconstructed using delayed feedback of CoM. I then developed complex perturbations to test the robustness of delayed CoM feedback on muscle activity and muscle synergy recruitment. Delayed feedback of CoM was shown to robustly modulate muscle activity throughout continuous sagittal perturbations that decouple CoM kinematics in magnitude. Moreover, delayed feedback of CoM was shown to robustly modulate muscle synergy recruitment throughout multidirectional discrete and biphasic perturbations that decouple CoM kinematics from each other in direction. These results suggest that a consistent spatial and temporal structure of motor outputs exists across static and dynamic states. Such an organization may aid in functionally identifying pathologic strategies for maintaining balance.