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.