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.