During limb movement, spinal circuits ensure the alternating activation of antagonistic flexor and extensor muscles. However, many movements require antagonist cocontraction, and the neural mechanisms that enable this remain unclear. Certain results suggest that during cocontraction, separate flexion and extension-related motor cortical output populations are coactivated, while other results suggest a distinct cocontraction-specific population is engaged. To test these hypotheses, I developed a novel, motor cortically dependent behavioral paradigm in which mice performed antagonist alternation or cocontraction during separate forelimb tasks. I recorded activity in motor cortical layer 5b during these tasks and found that population-level activity analyses failed to support either of these two hypotheses. Instead we found a task-specific mapping of neural activity to muscle output, and that neural activity correlations changed dramatically between alternation and cocontraction. We then performed calcium imaging of corticospinal apical dendrites during behavior and found that this task-dependent change in correlations was also found specifically in corticospinal neurons.

Overall, these findings indicate that motor cortex uses task-specific activity dynamics to mediate antagonist muscle cocontraction and alternation.