Background: Corticospinal excitability projecting to the trunk muscles increases during tonic muscle contractions of the upper extremities – a phenomenon known as ‘cross facilitation’. However, the modulation of rhythmic bilateral dynamic muscle contractions of upper limbs on the corticospinal excitability of the trunk muscles in healthy adults remains unknown.
Objectives: This study aimed to investigate the presence of cross facilitation in the erector spinae (ES) during rhythmic bilateral arm cycling and whether postural position influenced the degree of facilitation.
Methods: Twenty healthy adults underwent the study. Transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) was used to elicit motor evoked potentials (MEP) in the ES during bilateral arm cycling and during static sitting with and without trunk support. Peak-to-peak amplitudes of MEPs in the ES where measures during arm cycling and static sitting to determine the corticospinal excitability of the ES. To examine the cortical and spinal excitability underpinning the modulation of arm cycling on amplitudes of ES MEPs, both short-interval intracortical inhibition (SICI) and cervicomedullary MEPs (CMEPS) were obtained during rhythmic arm cycling and static sitting with trunk support in a subset of ten participants.
Results: MEP amplitudes of the ES were greater during rhythmic arm cycling than during static sitting when the participant was seated with trunk support (174.79±18.01% of static sitting ES MEP) and without trunk support (120.42±10.03% of static sitting ES MEP). SICI was decreased during rhythmic arm cycling (51.42±3.31%) compared to static sitting (44.66±3.90%), whereas the amplitudes of CMEPs were increased during static sitting (0.09±0.05) compared to rhythmic arm cycling (0.07±0.03) in most participants.
Conclusion: Cross facilitation between the arm and trunk muscles was present during rhythmic bilateral arm cycling, shown by an increase in the amplitudes of ES MEPs. This facilitatory effect experienced was similar when the trunk was both supported and unsupported. SICI was reduced during arm cycling, while the amplitudes of CMEPs were increased during static sitting, suggesting differential modulation of cortical and spinal circuits on corticospinal excitability of the ES between the movements, with the facilitatory effect during arm cycling likely mediated by the motor cortical circuits. Our findings suggest that performing bilateral arm cycling in both supported and unsupported seated posture may induce short-term neuroplasticity in the corticospinal pathways projecting to the ES, leading to changes in trunk motor function in the long term. This highlights the potential for the use of simple arm cycling exercise for trunk rehabilitation in individuals with impaired trunk control.
The study protocol was approved by the School of Sport, Exercise, and Rehabilitation Sciences Research Ethics Committee at University of Birmingham (MRC2022-03) and performed in accordance with the Declaration of Helsinki.