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 the heat (Maxwell et al., The effect of climatic heat stress on intermittent supramaximal running performance in humans. Exp. Physiol. 81:833-845, 1996). The first study tested the hypothesis that it was due to different rates of substrate metabolism. The second study tested whether a greater level of hypohydration led to an earlier exhaustion time.

METHODS: A maximal anaerobic running test (MART) was the exercise model used. This involved repeated 20-s runs, each at increasing intensities, with 100 s of passive recovery between 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 humidity (RH), or 21.3+/-0.4 degrees C, 48.8+/-2.2% RH. Needle biopsy samples were taken from the vastus lateralis muscle before and immediately after 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 worse 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 x kg dry muscle(-1)) or muscle lactate (102.9+/-18.2 vs. 100.5+/-16.6 mmol x 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 supramaximal running performance in the heat is not caused by greater muscle glycogenolysis or lactate accumulation. Further, a poorer intermittent sprinting performance is experienced in a hypohydrated compared with a euhydrated state.