Spinal cord stimulation involves the application of electrical currents to alleviate chronic neuropathic pain and most recently, activity-dependent spinal neuromodulation has been used for motor recovery of the lower limbs in spinal cord injuries. Despite its extensive use over decades, the precise mechanisms underlying the impact of burst spinal cord stimulation on motor neurons and muscle recruitment remains unclear. To investigate this, we employed different stimulation frequencies to explore its effects on motor neuron recruitment on individuals with chronic neuropathic pain.

Participants performed isometric ankle-dorsal flexions at 15% and 30% of their maximum voluntary contraction force along with four isometric ramp contractions for each SCS waveform: off, tonic (130Hz), sham and burst (40Hz). We recorded muscle activity using high-density surface electromyography (HDs-EMG) with a 64-electrode grid placed on the tibial anterior muscle (TA). By decomposing the HDs-EMG signals into single motor unit action potentials, we evaluated motor unit recruitment and de-recruitment thresholds, discharge rate, inter-spike interval and common synaptic input to motor neurons (coherence). We analyzed the activity of the motor units tracked across all four stimulation types and compared their properties using a Friedman test (Fig.1).

In this ongoing study, we included nine patients (5 females; 4 males; mean age 59 years) with chronic neuropathic pain (leg, lower back, or phantom limb pain), all with thoracic epidural stimulation at Th7 - Th8 spinal level. While a few subject-specific variations were observed, our study suggests that SCS across different waveforms (tonic versus burst versus sham) usually applied to treat chronic pain disorders does not significantly influence motor neuron recruitment in the TA muscle among the participants, considering short-term changes in the stimulation type.

Contrary to reported activity-dependent, targeted spinal cord stimulation for lower limbs motor recovery, SCS using tonic and burst waveforms was found not to impact motor neuron activity. The data provided is of preliminary nature, however enlargement of the study cohort and an extended long-term follow up are on the way.