Cortical Activity Associated with Motor Unit Action Potentials: Motor Cortex Control of the Recruitment of Motor Units

This paper examines cortical involvement in the recruitment of motor units, rejecting a previous common drive theory, and supporting the theory that explains cortex as having a more specific role in the particular motor units that are recruited, showing a command rather than simply a drive. Results from this study demonstrate a relationship between cortical activity and individual motor unit firing in humans non-invasively by recording EEG and high-density surface EMG in parallel. EMG data, from 2 subjects, were decomposed into individual motor units and ERPs time-locked to the specific motor units were obtained. Inter-trial coherence was computed to further measure a direct relationship between cortical and motor unit activity. There was coherence at low frequency EEG oscillations around the time of the extracted motor unit spikes. Frequencies were also similar between subjects. Additionally, cross-validation analysis was done to demonstrate an ability to predict motor unit recruitment from EEG recordings. Results suggest cortical activity can be consistently associated with motor units and that there is a more precise relationship than just a readiness potential from movement onset. With this fine-grained control in the recruitment of specific motor units and the flexibility of recruitment, motor unit recruitment strategies are potentially trainable. This is important because of its potential for use in rehabilitation after injury or motor skill acquisition. This study sets forth foundational knowledge that allows for further research to better understand neuromuscular control.