Brain Oscillations Extend Beyond Task-Relevant Motor Neuron Pools and Contribute to Shaping the Functional State of the Motor System

It remains unknown whether oscillatory brain activity that shapes sensorimotor state is routed selectively to task-relevant muscles or broadcast across the motor system. Here we combined electroencephalography with large-scale recordings of spinal motor neurons innervating the tibialis anterior. Participants maintained a submaximal dorsiflexion while performing a Go/No-Go task in which the instructed response was either a ballistic dorsiflexion or a ballistic handgrip, making the tibialis anterior task-relevant or task-irrelevant, respectively. Alpha- and beta-band modulations observed at the cortical level were largely expressed in motor neuron output, including in the task-irrelevant motor neuron pool, indicating broad propagation of cortical dynamics to spinal motor neurons. The peripheral expression of these modulations differed across frequency bands: alpha was partly effector-dependent, consistent with more selective transmission to the task-relevant pool, whereas beta was largely effector-independent, consistent with broader expression across motor neuron pools. Using simulation-based inference, we found that task-related changes in motor output were best explained by modulations in net excitatory drive, whereas alpha- and beta-band inputs contributed primarily to motor neuron synchronization. A complementary simulation showed that this synchronization may facilitate the rapid build-up of motor output following a sudden increase in excitatory drive. These results support a parallel control architecture in which low-frequency drive determines motor output, whereas higher-frequency oscillatory inputs are broadly distributed and shape the functional state of motor neuron pools in preparation for action.