P02 Group III/IV muscle afferent feedback does not modulate intracortical excitability and inhibition following fatiguing exercise.
Latella C[1], Taylor JL[1, 2]
- Centre for Exercise and Sports Science Research (CESSR), School of Medical and Health Sciences, Edith Cowan University, Perth, Australia
- Neuroscience Research Australia, Randwick, Australia
Objective: Feedback from group III/IV muscle afferents is known to cause a reduction in motor drive and voluntary activation of the muscle during intense exercise (1,2). However, the effect of group III/IV muscle afferent feedback on intracortical excitatory and inhibitory networks is not well understood. Methods: On two separate days, 16 participants (11M,5F) completed a sustained maximal isometric index finger abduction until force decreased by 40%. On one day, post-exercise blood flow occlusion of the hand maintained group III/IV afferent firing whereas recovery was allowed on the other day (control). Pain was assessed using a 0-10 scale. Single- and paired-pulse transcranial magnetic stimulation (TMS) assessed motor evoked potentials (MEP), intracortical facilitation (ICF), and inhibition at 2 and 3 ms (SICI2, SICI3, respectively) in the first dorsal interosseous muscle. Maximal M waves (MMAX) were also elicited pre- and post-exercise. Results: Pain was significantly greater for the occlusion condition (p<0.001). MMAX (-8.7% and -5.4%, P<0.001) and the MEP/MMAX ratio (-46.5% and -49.8%, P=0.007) were reduced post-exercise for occlusion and control, respectively, but were not different between conditions (MMAX: P=0.559, MEP/MMAX: P=0.415). No differences were observed for SICI2 (P=0.084), SICI3 (P=0.342) or ICF (P=0.087) between conditions and across time. Conclusion: Consistent with previous findings (3), the reduction of MEPs after exercise is not influenced by the maintained firing of fatigue-sensitive muscle afferents. Additionally, intracortical networks do not appear to be modulated. This suggests that the failure of voluntary activation with group III/IV muscle afferent firing occurs via mechanisms outside of the primary motor cortex.
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