Propagation of beta bursts from the motor cortex to the motor units of multiple upper-limb muscles

Beta band (13-30 Hz) oscillations have been closely associated with motor control, yet their precise functional significance remains a subject of debate. Recent research suggests that beta activity occurs in transient bursts, which may better capture its role in movement regulation than sustained oscillations. Although cortical and subcortical beta bursts have been extensively studied, their transmission to muscles-particularly in the upper limb-remains poorly understood and has been limited by traditional bipolar EMG techniques. In this study, we used high-density surface electromyography (HDsEMG) and electroencephalography (EEG) to investigate the cortico-peripheral dynamics of beta bursts in forearm extensor muscles during isometric contractions at the motor unit (MU) level. We show that MU activity in the upper limb exhibits discrete beta bursts that are temporally aligned with cortical beta activity. Notably, beta bursts in the periphery were time-locked to cortical bursts, suggesting strong coordination and synchronization of bursting across the corticospinal tract. We also found stronger beta synchronization in the extensor's ulnar regions compared with the radial regions, indicating muscle-specific differences in beta projections to the motor neuron pools. These findings provide the first demonstration of beta burst propagation from cortex to upper-limb MUs and show that HDsEMG can reliability detect such events in the upper limb. This work supports the cortical origin and structure of peripheral beta activity and demonstrates its potential as a neurophysiological biomarker for targeting corticospinal dynamics in motor disorders such as Parkinson's disease.<b>NEW & NOTEWORTHY</b> We provide the first evidence of cortical beta bursting in motor unit (MU) activity of forearm extensors across force levels. MU beta bursts are time-locked to cortical beta bursts, supporting direct corticospinal transmission in the upper limb. Our findings indicate that burst timing is robust to changes in contraction levels and demonstrate a robust cortical-peripheral beta relationship during isometric contractions. They also reinforce the utility of HDsEMG for investigating burst dynamics and refining neurophysiological biomarkers.