The purpose of this study was to examine the extent to which lighter runners might be more advantaged than larger, heavier runners during prolonged running in warm humid conditions. Sixteen highly trained runners with a range of body masses (55-90 kg) ran on a motorised treadmill on three separate occasions at 15, 25 or 35 degrees C, 60% relative humidity and 15 km x h(-1) wind speed. The protocol consisted of a 30-min run at 70% peak treadmill running speed (sub-max) followed by a self-paced 8-km performance run. At the end of the submax and 8-km run, rectal temperature was higher at 35 degrees C (39.5+/-0.4 degrees C, P<0.05) compared with 15 degrees C (38.6+/-0.4 degrees C) and 25 degrees C (39.1+/-0.4 degrees C) conditions. Time to complete the 8-km run at 35 degrees C was 30.4+/-2.9 min (P<0.05) compared with 27.0+/-1.5 min at 15 degrees C and 27.4+/-1.5 min at 25 degrees C. Heat storage determined from rectal and mean skin temperatures was positively correlated with body mass (r=0.74, P<0.0008) at 35 degrees C but only moderately correlated at 25 degrees C (r=0.50, P<0.04), whereas no correlation was evident at 15 degrees C. Potential evaporation estimated from sweat rates was positively associated with body mass (r=0.71, P<0.002) at 35 degrees C. In addition, the decreased rate of heat production and mean running speed during the 8-km performance run were significantly correlated with body mass (r=-0.61, P<0.02 and r=-0.77, P<0.0004, respectively). It is concluded that, compared to heavier runners, those with a lower body mass have a distinct thermal advantage when running in conditions in which heat-dissipation mechanisms are at their limit. Lighter runners produce and store less heat at the same running speed; hence they can run faster or further before reaching a limiting rectal temperature.
This study examined the effects of heat stress on the accumulation of plasma ammonia, lactate, and urate during prolonged running. Nine highly trained endurance runners completed two running trials in a counterbalanced fashion in cool (15 degrees C) and in hot (35 degrees C) humid (60% relative humidity) conditions. Subjects ran on a motorised treadmill at 70% of peak treadmill running speed for 30 min (submaximal) followed by a self-paced 8-km performance run. Blood was drawn at pre-exercise, end-submaximal and end-performance run and analysed for plasma ammonia, lactate, and urate. Four subjects failed to complete the performance run in the heat and the performance times for the rest of the subjects was increased from 27.3 (0.6) min in cool conditions to 31.3 (1.2) min in hot conditions (P < 0.05). The end-performance rectal temperature was 38.6 (0.1) and 39.2 (0.1) degrees C (P<0.05) in cool and hot conditions, respectively. Differences in plasma lactate at the end of submaximal running were not significant. However, at the end of performance runs lactate was 6.0 (0.9) m mol x l(-1) in cool and 3.1 (0.5) mmol x l(-1) in hot conditions, values that were significantly different (P<0.05). Plasma ammonia increased from pre-exercise to approximately equal to 59 micromol x l(-1) at the end-submaximal runs for both coditions and further at the end of performance runs to 108.5 (11) micromol x l(-1) (P<0.05) in hot but not in cool conditions. Plasma urate increased from pre-exercise to 311.2 (25.9) micromol x l(-1) at end-submaximal runs and to approximately equal to 320.4 micromol x l(-1) at end-performance runs in hot and cool environments. The findings that plasma urate accumulation was similar at the completion of running in both conditions, while ammonia was significantly augmented in hot conditions compared with cool, suggest that ammonia accumulation during heat stress exercise might be derived from sources other than purine catabolism.
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