The effect of exercise induced hyperthermia on muscle fibre conduction velocity during sustained isometric contraction.

This study investigated the effect of dynamic exercise in a hot environment on muscle fibre conduction velocity (MFCV) of the knee extensors during a sustained isometric contraction. Seven trained male cyclists (mean [±SD], age, and V˙O(2max) were 35±9.9 and 57.4±6.6 ml kg(-1) min(-1)) cycled for 50 min at 60% of peak power output in either: (1) 40°C (HOT); or (2) 19°C (NEUTRO); and (3) remained passive in 40°C (PASS). Post-intervention a 100 s maximal sustained isometric contraction (SMC) of the knee extensors was performed. Rectal temperature increased (p<0.01) for both HOT and NEUTRO with PASS unchanged and with HOT rising higher (p<0.01) than NEUTRO (38.6±0.4 vs. 37.6±0.4°C). Muscle temperature increased (p<0.01) for all three conditions with HOT rising the highest (p<0.01) (40.3±0.5 vs. 38.3±0.3 and 37.6±1.3°C for NEUTRO and PASS, respectively). Lactate showed higher accumulation (p<0.01) for HOT than NEUTRO (6.9±2.3 vs. 4.2±2.1 mmol l(-1)). During SMC the torque, electromyography root mean squared (RMS) and MFCV all significantly (p<0.01) declined. Only in HOT did MFCV decline significantly (p<0.01) less than torque and RMS (9.9±6.2% vs. 37.5±17.8% and 37.6±21.4%, respectively). In conclusion, during exercise induced hyperthermia, reduced motor unit recruitment as opposed to slower conducting properties of the muscle fibre appears to be responsible for the greater reduction in torque output.