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 (31)P-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.