Non-local muscle fatigue is mediated at spinal and supraspinal levels

The objective was to measure the corticospinal excitability and motoneuron responsiveness of the right and left Biceps Brachii (BB), and left Abductor Digiti Minimi (ADM) muscles in response to submaximal isotonic fatiguing contractions performed by the right BB muscle. With the familiarization session, ten young moderately active male subjects came to the lab on seven occasions. Three sets of 3 min seated elbow curls at 25% of one-repetition maximum (1RM) separated by a 1-min rest performed by the right BB muscle were used as the fatiguing protocol. The motor evoked potential (MEP), cervicomedullary motor evoked potential (CMEP), and compound muscle action potential (Mmax) of the right BB muscle (baseline and after each set of the fatiguing task), the left BB and ADM muscles (baseline, post-fatigue, post-10, and post-20 min) were measured. MEP and CMEP were then normalized to Mmax for statistical analysis. The results showed that in the right BB muscle, there was a significant reduction in the MEP after performing the fatiguing task (p= 0.03), while no significant effect of time was seen in the CMEP (p= 0.07). In the left BB muscle, the MEP significantly decreased from pre-fatigue to post-fatigue (p= 0.01) and post-10 (p= 0.001), while there was a significant decline in the CMEP post-fatigue (p= 0.03). In the left ADM muscle, MEP significantly decreased post-fatigue (p= 0.03) and no changes were seen in the CMEP (p= 0.12). These results not only confirm the incidence of non-local muscle fatigue (NLMF) in response to performing submaximal isotonic fatiguing contractions but also as a new finding, imply that both spinal and supraspinal modulations account for the NLMF response.