Contralateral Selectivity of Upper-Limb Motor Pools via Targeted Stimulation of the Cervical Spinal Cord.

Abstract

Fourteen participants with intact nervous systems completed two laboratory visits, during which posterior root-muscle reflexes (PRMRs) were evoked via a 3 × 3 cathode matrix applied over the cervical spine. An incremental recruitment curve at the C7 vertebral level was initially performed to attain resting motor threshold (RMT) in each muscle. Paired pulses (1 ms square monophasic with inter-pulse interval of 50 ms) were subsequently delivered at a frequency of 0.25 Hz at two intensities (RMT and RMT + 20%) across all nine cathode positions. Evoked responses to the 1st (PRMR1) and 2nd (PRMR2) stimuli were recorded in four upper-limb muscles. A significant effect of the spinal level was observed in all muscles for PRMR1, with greater responses being recorded caudally. Contralateral stimulation significantly increased PRMR1 in Biceps Brachii (p < 0.05, F = 4.9, η2 = 0.29), Flexor Carpi Radialis (p < 0.05, F = 4.9, η2 = 0.28) and Abductor Pollicis Brevis (p < 0.01, F = 8.9, η2 = 0.89). Post-activation depression (PAD) was also significantly increased with contralateral stimulation in Biceps Brachii (p = 0.001, F = 9.3, η2 = 0.44), Triceps Brachii (p < 0.05, F = 5.4, η2 = 0.31) and Flexor Carpi Radialis (p < 0.001, F = 17.4, η2 = 0.59). A level of unilateral motor pool selectivity may be attained by altering stimulus intensity and location during cervical tSCS. Optimising these parameters may improve the efficacy of this neuromodulation method in clinical cohorts.