Intermittent running: muscle metabolism in the heat and effect of hypohydration

Purpose: This study reports two studies that investigated the reason for a poorer intermittent supramaximal running performance previously found in th e hat (Maxwell et al., The effect of climatic heat stress on intermittent s upramaximal running performance in humans. Exp. Physiol. 81:833-845, 1996). The first study tested the hypothesis that it was due to different rates o f substrate metabolism. The second study tested whether a greater level of hypohydration led to an earlier exhaustion time. Methods: A maximal anaerob ic running test (MART) was the exercise model used. This involved repeated 20-s runs, each at increasing intensities, with 100 s of passive recovery b etween runs. Results: In study 1, eight male subjects performed the MART on two occasions at either 32.8 +/- 0.3 degrees C, 80.5 +/- 1.6% relative hum idity (RH), or 21.3 +/- 0.4 degrees C, 48.8 +/- 2.2% RH. Needle biopsy samp les were taken from the vastus lateralis muscle before and immediately afte r the MART. In study 2, 11 male subjects performed the MART in a moderately hypohydrated (HYPO) and euhydrated (EUH) state while in a cool environment . In study 1, performance was significantly worst in the hot compared with the cool environment (138 +/- 7 vs 150 +/- 6 s, respectively, P < 0.05). No differences were observed in the change in muscle glycogen (100.3 +/- 15.1 vs 107.0 +/-: 15.6 mmol glucosyl units kg dry muscle(-1)) or muscle lactat e (102.9 +/- 18.2 vs 100.5 +/- 16.6 mmol.kg dry muscle(-1)) between the hot and cool environments, respectively. In study 2, performance was worse in the HYPO (148 +/- 9 s) compared with the EUH (154 +/- 9 s) trial (P < 0.05) . Conclusions: These results indicate that a reduced intermittent supramaxi mal running performance in the heat is not caused by greater muscle glycoge nolysis or lactate accumulation. Further, a poorer intermittent sprinting p erformance is experienced in a hypohydrated compared with a euhydrated stat e.