Influence of passive lower-body heating on muscle metabolic perturbation and high-intensity exercise tolerance in humans

The purpose of this investigation was to determine the influence of heat stress on the dynamics of muscle metabolic perturbation during high-intensity exercise. Seven healthy males completed single-legged knee-extensor exercise until the limit of tolerance on two separate occasions. In a randomized order the subjects underwent 40 min of lower-body immersion in warm water at 42°C prior to exercise (HOT) or received no prior thermal manipulation (CON). Following the intervention, muscle metabolism was measured at rest and throughout exercise using ³¹P-MRS. The tolerable duration of high-intensity exercise was reduced by 36% after passive heating (CON: 474 ± 146 vs. HOT: 303 ± 76 s; P = 0.005). Intramuscular pH was lower over the first 60 s of exercise (CON: 7.05 ± 0.02 vs. HOT: 7.00 ± 0.03; P = 0.019) in HOT compared to CON. The rate of muscle [PCr] degradation during exercise was greater in the HOT condition (CON: −0.17 ± 0.08 vs. HOT: −0.25 ± 0.10% s⁻¹; P = 0.006) and pH also tended to change more rapidly in HOT (P = 0.09). Muscle [PCr] (CON: 26 ± 14 vs. HOT: 29 ± 10%), [Pi] (CON: 504 ± 236 vs. HOT: 486 ± 186%) and pH (CON: 6.84 ± 0.13 vs. HOT: 6.80 ± 0.14; P > 0.05) were not statistically different at the limit of tolerance (P > 0.05 for all comparisons). These results suggest that the reduced time-to-exhaustion during high-intensity knee-extensor exercise following lower-body heating might be related, in part, to accelerated rates of change of intramuscular [PCr] and pH towards ‘critical’ values that limit muscle function.