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Transcutaneous spinal cord stimulation (tSCS) at the cervical level may facilitate improved upper-limb function in those with incomplete tetraplegia. While clinical trials are ongoing, there is still much debate regarding the transmission pathway as well as appropriate stimulation parameters. This study aimed to explore the extent to which cervical tSCS can induce mono-synaptic reflexes in discrete upper-limb motor pools and examine the effects of altering stimulus location and intensity. Methods: 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. Results: 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). Conclusions: 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.

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Spinal cord injuries cause loss of muscle function and subsequently reduce independence. Therapeutic interventions such as transcutaneous spinal cord stimulation are increasingly being used to help improve motor functioning however, a comprehensive understanding of the biomechanical elements of movement may help optimize stimulation protocols. Twenty healthy participants completed five sit-to-stand (STS) transitions while initial knee angle and arm facilitation were altered. Electromyography (EMG) activation of four lower limb muscles and centre of pressure dynamics were recorded. Acute initial knee angles resulted in a change in duration of phases within the STS, and restrictive arm positioning caused the time to completion to increase (p=0.04). Muscle activation patterns across phases were compared and showed significant differences between phases in both the Tibialis Anterior and Rectus Femoris (p<0.006). Acute initial knee angles were also found to significantly increase Biceps Femoris activation across multiple phases (p=0.034). Altering the starting position and limb movement result in vastly different temporal and muscular strategies to complete the STS. Thus, joint angle and upper limb facilitation should be considered when designing rehabilitative interventions for clinical cohorts. Clinical Relevance-This study quantifies the contribution of musculature and joint kinematics on maintaining balance during the sit to stand transition. Such results may be used to inform targeted rehabilitative training protocols and improve the efficacy of therapeutic interventions such as transcutaneous spinal stimulation.