Repetitive Activation Compromises Motoneuron Excitability During Fatiguing Exercise

Abstract

Although fatiguing exercise is known to decrease motoneuronal excitability, the mechanisms underlying this impact remain unclear. PURPOSE: To investigate the role of repetitive motoneuron activation in determining the decrease of motoneuronal excitability during fatiguing exercise. METHODS: On 2 separate days, healthy young subjects (26 ± 6 yrs) performed intermittent isometric knee extensions (at 20% of maximal voluntary quadriceps torque): 1) voluntarily (VOL; i.e. requiring repetitive motoneuron activation), and 2) electrically-evoked (EVO, femoral motor nerve stimulation at 20 Hz; no motoneuron activation). The exercise consisted of 50 s contractions followed by 10 s breaks during which potentiated twitches (Qtw) were assessed to monitor the development of peripheral fatigue during each trial. Exercise continued in each trial until the goal of achieving a similar ~40% reduction in Qtw (ΔQtw) was reached. Before and immediately after exercise, cervicomedullary stimulations (CMS) were used to elicit unconditioned (CMS only) and conditioned (transcranial magnetic stimulation followed by CMS, 100 ms interval) cervicomedullary motor-evoked potentials (CMEPs). All CMEPs were normalized to M-waves and evoked during a constant electromyographic (EMG) activity corresponding to 20% of the EMG obtained during pre-exercise MVCs. RESULTS: In both trials, ΔQtw was, per design, comparable (~40%; P = 0.9) and unconditioned CMEPs were similar before and after exercise (P = 0.23). Conditioned CMEPs were decreased following VOL (-79%, P < 0.05). While conditioned CMEPs were also significantly decreased following EVO (-62%), the exercise-induced reduction was substantially smaller compared to VOL. CONCLUSION: Repetitive activation of the motoneurons contributes to the decrease in motoneuronal excitability during voluntary exercise. It remains unclear whether motoneuronal responsiveness to synaptic input and/or the efficacy of the corticomotoneuronal synapse account for the repetitive activation-related depression. However, as motoneuron excitability still falls during evoked exercise, other mechanisms, potentially related to neural feedback, also contribute to the exercise-induced fall in motoneuronal excitability.