Advantages of smaller body mass during distance running in warm, humid environments

The purpose of this study was to examine the to which lighter runners might be more advantaged than larger, heavier runners during prolonged running i n warm humid conditions. Sixteen highly trained runners with a range of bod y masses (55-90 kg) ran on a motorised treadmill on three separate occasion s at 15, 25 or 35 degreesC, 60% relative humidity and 15 km.h(-1) wind spee d. The protocol consisted of a 30-min run at 70% peak treadmill running spe ed (sub-max) followed by a self-paced 8-km performance run. At the end of t he sub-max and 8-km run, rectal temperature was higher at 35 degreesC (39.5 +/-0.3 degreesC, P<0.05) compared with 15<degrees>C (38.6+/-0.4 degreesC) a nd 25 degreesC (39.1+/-0.4 degreesC) conditions. Time to complete the X-km run at 35 degreesC 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 degreesC. 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 degreesC (r=0.50, P<0.04), whereas no correlation was evident at 15<degr ees>C. potential evaporation estimated from sweat rates was positively asso ciated with body mass (r=0.71, P<0.002) at 35<degrees>C. In addition, the d ecreased rate of heat production and mean running speed during the 8-km per formance 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 hea vier runners, those with a lower body mass have a distinct thermal advantag e when running in conditions in which heat-dissipation mechanisms are at th eir limit. Lighter runners produce and store less heat at the same running speed; hence they can run faster or further before reaching a limiting rect al temperature.