Summary
Voluntary movement requires communication from cortex to the spinal cord, where a dedicated pool of motor units (MUs) activates each muscle. The canonical description of MU function, established decades ago, rests upon two foundational tenets. First, cortex cannot control MUs independently1 but supplies each pool with a common drive that specifies force amplitude2,3. Second, as force rises, MUs are recruited in a consistent order4–13 typically described by Henneman’s size principle14–19. While this paradigm has considerable empirical support, a direct test requires simultaneous observations of many MUs over a range of behaviors. We developed an isometric task that allowed stable MU recordings during rapidly changing force production. MU responses were surprisingly flexible and behavior-dependent. MU activity could not be accurately described as reflecting common drive, even when fit with highly expressive latent factor models. Neuropixels probe recordings revealed that, consistent with the requirements of fully flexible control, the cortical population response displays a surprisingly large number of degrees of freedom. Furthermore, MUs were differentially recruited by microstimulation at neighboring cortical sites. Thus, MU activities are flexibly controlled to meet task demands, and cortex has the capacity to contribute to that ability.
Competing Interest Statement
The authors have declared no competing interest.
