Fatigue diminishes motoneuronal excitability during cycling exercise

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 men performed fatiguing constant-load (80% W-peak; 241 +/- 13 W) cycling to exhaustion during which the percent increase in quadriceps electromyography (Delta EMG; vastus lateralis and rectus femoris) was quantified. During a separate trial, subjects performed two brief (similar to 45 s) nonfatiguing cycling bouts (244+/-15 and 331+/-23W) individually chosen to match the Delta 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 postexercise reductions in quadriceps twitch force (Delta Q(tw)) and voluntary quadriceps activation (Delta VA). Whereas nonfatiguing cycling caused no measureable fatigue, fatiguing cycling resulted in significant peripheral (Delta Q(tw) : 42+/-6%) and central (Delta VA: 4 +/- 1%) fatigue. During nonfatiguing cycling, the area of MEPs and CMEPs, normalized to M waves, similarly increased in the quadriceps (similar to 40%; P < 0.05). In contrast, there was no change in normalized MEPs or CMEPs during fatiguing cycling. As a consequence, the ratio of MEP to CMEP was unchanged during both trials (P > 0.5). Therefore, although increases in muscle activation promote corticospinal excitability via motoneuronal facilitation during nonfatiguing 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.