Exercise-induced fatigue influences the excitability of the motor pathway during single-joint isometric contractions. This study sought to investigate the influence of fatigue on corticospinal excitability during cycling-exercise. Eight males performed fatiguing constant-load (80% Wpeak, 241±13W) cycling to exhaustion during which the percent increase in quadriceps electromyography [∆EMG; vastus-lateralis (VL) and rectus-femoris (RF)] was quantified. During a separate trial, subjects performed two brief (~45-s) non-fatiguing cycling bouts (244±15W and 331±23W) individually chosen to match the ∆EMG across bouts to that observed during fatiguing cycling. Corticospinal excitability during exercise was quantified by transcranial magnetic, electric transmastoid, and femoral nerve stimulation to elicit motor-evoked potentials (MEP), cervicomedullary-evoked potentials (CMEP), and M-waves in the quadriceps. Peripheral and central fatigue were expressed as pre- to post-exercise reductions in quadriceps twitch-force (∆Qtw) and voluntary quadriceps activation (∆VA). While non-fatiguing cycling caused no measureable fatigue, fatiguing-cycling resulted in significant peripheral (ΔQtw: 42±6%) and central (ΔVA: 4±1%) fatigue. During non-fatiguing cycling, the area of MEPs and CMEPs, normalized to M-waves, similarly increased in the quadriceps (~40%, P<0.05). In contrast, there was no change in normalized MEPs or CMEPs during fatiguing-cycling. As a consequence, the MEP/CMEP ratio was unchanged during both trials (P>0.5). Therefore, although increases in muscle activation promote corticospinal excitability via motoneuronal facilitation during non-fatiguing cycling, this effect is abolished during fatigue. We conclude that the unaltered excitability of the corticospinal pathway from start of intense cycling exercise to exhaustion is, in part, determined by inhibitory influences on spinal motoneurons obscuring the facilitating effects of muscle activation.
- Motor cortex
- Corticospinal pathway
- Motor neuron
- Copyright © 2016, Journal of Neurophysiology