Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB291

Poster Communications

Effects of dynamic arm cycling on motor cortical excitability in humans

E. Fujitake1, S. Chiou1,2, P. Strutton1

1. The Nick Davey Laboratory, Department of Surgery and Cancer, Faculty of Medicine, Imperial College London, London, United Kingdom. 2. School of Sport, Exercise, and Rehabilitation Sciences, University of Birmingham, Birmingham, United Kingdom.

Isometric contractions of upper limb muscles have been shown to increase corticospinal excitability of trunk muscles, known as crossed-corticospinal facilitation. Neural mechanisms underlying this effect are likely to be cortical in origin as work has demonstrated a reduction in intracortical inhibition but no change in spinal excitability during the contractions. Further, we have previously shown that excitability of corticospinal pathways controlling contralateral erector spinae (ES) muscles was facilitated by unilateral arm cycling, a dynamic contraction of arm muscles, in healthy subjects. However, the neural mechanisms underlying the increased corticospinal excitability of the trunk muscles during dynamic arm contractions, a rhythmic movement, remains unclear. Hence, the aim of this study was to investigate activity of intracortical neural circuits within the motor cortex during static and dynamic arm muscle contractions in healthy subjects using paired-pulse transcranial magnetic stimulation (TMS) paradigms. EMG activity of 9 subjects were recorded at the contralateral erector spinae (ES) muscle to the cycling arm (T12 vertebral level), and at the biceps brachii (BB) of the cycling arm. Three tasks were conducted; a control task (arms by the side), dynamic task (arm cycling at 60 revolutions per minute), and a static task (exerting a level of EMG matched to the dynamic task). TMS was applied to the hotspot for ES over the primary motor cortex and short-interval intracortical inhibition (SICI) and short-interval intracortical facilitation (SICF) were examined at the pedal position corresponding to maximal BB EMG (6 o'clock position). Prestimulus EMG in ES was matched across the tasks. Our results showed that ES MEPs were greater during the dynamic and static tasks than the control task (dynamic: 172.348 [7.94]% of MEP during the control task; static: 143.53 [29.86]%). SICI was reduced and SICF was increased during the dynamic (SICI: 106.85[18.77]%; SICF: 228.17 [93.32]%) and static tasks (SICI: 109.41[23.57]% test MEP; SICF: 212.93 [38.09]%) compared to the control task (SICI: 68.76 [15.38]%; SICF: 152.59 [73.17%]) when the test stimulus intensity was adjusted for the dynamic and static tasks. Our findings show that both dynamic and static contractions of upper limb muscles can increase corticospinal excitability of the contralateral trunk muscles. We further demonstrate that the increased corticospinal excitability of the trunk muscles likely involves cortical mechanisms. These findings improve our understanding to cortical control of trunk muscles during voluntary contractions of upper limb muscles and could have implications for development of rehabilitation programmes in subjects with impaired function of the trunk, such as those with spinal cord injury and back pain.

Where applicable, experiments conform with Society ethical requirements