Skeletal muscle twitch relaxation kinetics following heat stress endurance: carbohydrate versus placebo.

PURPOSE: To compare post-exercise twitch contractile properties and M-wave characteristics after 60-km self-paced cycling in the heat following placebo (PLA) or 6% carbohydrate (CHO) ingestion as beverages or capsules.

METHODS

Ten well-trained male cyclists completed five randomised Latin-square trials: water (H₂O), CHO beverage (CHOb), PLA beverage (PLAb), CHO capsules (CHOc), and PLA capsules (PLAc). Neuromuscular function was assessed before and after the time trial, and after a sustained knee-extensor contraction at 20% MVC to task failure. Measures included MVC, voluntary activation (VA), M-wave amplitude, evoked twitch properties, and integrated electromyography (iEMG).

RESULTS

Performance time (p = 0.336) and time to task failure (p = 0.394) did not differ between conditions. iEMG increased during the 20% MVC in all trials (all p < 0.0001). There were no condition × time interactions for MVC, VA, M-wave amplitude (all p ≥ 0.335), or twitch contractile variables (all p ≥ 0.115). Time effects were observed for rate of relaxation, which decreased in all trials (p = 0.0302 to p = 0.000239), contraction duration, which increased in all trials (p = 0.0412 to p = 0.000091), and half-relaxation time, which increased in H₂O, PLAc and CHOc (p ≤ 0.00322).

CONCLUSIONS

Prolonged self-paced cycling in the heat produced a reproducible neuromuscular response dominated by slowed twitch relaxation. However, 6% CHO ingestion did not clearly attenuate these changes compared with PLA, suggesting that the principal impairment reflected exercise and heat load rather than CHO availability or delivery mode.