Training of one limb improves performance of the contralateral, untrained limb, a phenomenon known as cross transfer. It has been used for rehabilitation interventions, i.e. mirror therapy, in people with neurologic disorders. However, it remains unknown whether training of the upper limb can induce the cross-transfer effect to the trunk muscles. Using transcranial magnetic stimulation over the primary motor cortex (M1) we examined motor evoked potentials (MEPs) in the contralateral erector spinae (ES) muscle before and after 30 min of unilateral arm cycling in healthy volunteers. ES MEPs were increased after the arm cycling. To understand the origin of this facilitatory effect, we examined short-interval intracrotical inhibition (SICI) and cervicomedullary MEPs (CMEPs) in neural populations controlling in the ES muscle. Notably, SICI reduced after the arm cycling, while CMEPs remained the same. Using bilateral transcranial direct current stimulation (tDCS) in conjunction with 20 min of the arm cycling, the amplitude of ES MEPs increased to a similar extent as with 30 min of the arm cycling alone. These findings demonstrate that a single session of unilateral arm cycling induces short-term plasticity in corticospinal projections to the trunk muscle in healthy humans. The changes are likely driven by cortical mechanisms.

Background: A substantial proportion of individuals with incomplete spinal cord injury (SCI) exhibit reduced volitional control of trunk muscles due to damage of the corticospinal tract, resulting in impaired postural control and function. Studies using conventional bipolar electromyography (EMG) showed altered activity of the trunk muscles in people with SCI. However, the spatial distribution of trunk muscle activity remains unknown. Therefore, this study aimed to investigate the spatial distribution of the erector spinae (ES) below the level of injury in individuals with incomplete SCI during postural tasks and multidirectional reaching tasks using high-density EMG. Methods: Twenty-one individuals with incomplete SCI and 21 age-matched healthy controls performed two postural tasks (rapid shoulder flexion and external perturbation tasks) and multidirectional reaching tasks. EMG amplitude of the thoracic ES and displacement of the arm, trunk, and centre of pressure were recorded during the tasks. Results: Overall, the EMG amplitude of the ES was lower, and activation was distributed more in the caudal part of the ES in individuals with SCI compared with the controls during the postural and reaching tasks. EMG amplitude was greater during reaching forward than returning to the upright posture in the controls; however, this phase-dependent difference in the EMG amplitude was not present in individuals with SCI. Conclusions: Our findings demonstrate alterations in regional activation of the thoracic ES during postural and reaching tasks after SCI, suggesting a loss of selective control from the central nervous system to the trunk muscles.

Arm cycling is used for cardiorespiratory rehabilitation but its therapeutic effects on the neural control of the trunk after spinal cord injury (SCI) remain unclear. We investigated the effects of single session of arm cycling on corticospinal excitability, and the feasibility of home-based arm cycling exercise training on volitional control of the erector spinae (ES) in individuals with incomplete SCI. Using transcranial magnetic stimulation, we assessed motor evoked potentials (MEPs) in the ES before and after 30 minutes of arm cycling in 15 individuals with SCI and 15 able-bodied controls (Experiment 1). Both groups showed increased ES MEP size after the arm cycling. The participants with SCI subsequently underwent a six-week home-based arm cycling exercise training (Experiment 2). MEP amplitudes and activity of the ES, and movements of the trunk during reaching, self-initiated rapid shoulder flexion, and predicted external perturbation tasks were measured. After the training, individuals with SCI reached further and improved trajectory of the trunk during the rapid shoulder flexion task, accompanied by increased ES activity and MEP amplitudes. Exercise adherence was excellent. We demonstrate preserved corticospinal drive after single arm cycling session and feasible home-based arm cycling exercise training for individuals with SCI for trunk rehabilitation.