Valdunciel, A. P. | bioRxiv Science

bioRxiv2025

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

Beta band (13-30 Hz) oscillations are crucial for motor control, though their functional significance remains debated. Recent research suggests that beta activity occurs in transient bursts, which may better capture its role in movement regulation than sustained oscillations. While cortical and sub- cortical 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 synchronisation of bursting across the corticospinal tract. We also found stronger beta synchronisation in the extensor carpi ulnaris compared to the extensor carpi radialis, indicating muscle-specific differences in shared neural drive. 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 Parkinsons disease. Significance StatementWe provide the first evidence of cortical beta bursting in motor unit (MU) activity of forearm extensors across varying force levels. Our results show that MU beta bursting is time-locked to cortical beta bursts, supporting the idea of direct corticospinal transmission in the upper limb. Our findings highlight the value of MU level analysis in understanding beta burst transmission and indicate that burst timing is robust to changes in contraction levels. Unlike traditional bipolar EMG, MU-level analysis offers higher temporal precision and source specificity, enabling the reliable detection of beta bursts in individual muscles. These findings demonstrate a robust cortical-peripheral beta relationship during isometric contractions and reinforces the utility of HDsEMG for investigating burst dynamics and refining neurophysiological biomarkers.

neuroscience↗

bioRxiv2025

Projection of Cortical Beta Band Oscillations to a Motor Neuron Pool Across the Full Range of Recruitment

Cortical beta band oscillations (13-30 Hz) are associated with sensorimotor control, but their precise role remains unclear. Evidence suggests that for low-threshold motor neurons, these oscillations are conveyed to muscles via the fastest corticospinal fibers. However, their transmission to motor neurons of different sizes may vary due to differences in the relative strength of corticospinal and reticulospinal projections across the motor neuron pool. Consequently, it remains uncertain whether corticospinal beta transmission follows similar pathways and maintains consistent strength across the entire motor neuron pool. To investigate this, we examined beta activity in motor neurons innervating the tibialis anterior muscle across the full range of recruitment thresholds in a study involving 12 participants of both sexes. We characterized beta activity at both the cortical and motor unit levels while participants performed contractions from mild to submaximal levels. Corticomuscular coherence remained unchanged across contraction forces after normalizing for the net motor unit spike rate, suggesting that beta oscillations are transmitted with uniform strength to motor neurons, regardless of size. To further explore beta transmission, we estimated corticospinal delays using the cumulant density function, identifying peak correlations between cortical and muscular activity. Once compensated for variable peripheral axonal propagation delay across motor neurons, the corticospinal delay remained stable, and its value (approximately 14 ms) indicated projections through the fastest corticospinal fibers for all motor neurons. These findings demonstrate that corticospinal beta band transmission is determined by the fastest pathway connecting in the corticospinal tract, projecting uniformly across the entire motor neuron pool.

bioengineering↗