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

Abstract

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.

The Challenge of Performing Aerobic Exercise in Tropical Environments: Applied Knowledge and Perspectives

“…20 In fact, in hot/humid climates with heat dissipation mechanisms at their limits, runners with lower body mass have a distinct thermal advantage: 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. 21 …”

“…The advantages of smaller body mass when distance running in warm and humid conditions have been demonstrated; 20,21 see the "Running" subsection, above. The impact of the body surface area/mass ratio suggests that women are in a better situation to thermoregulate in hot/humid climate.…”

Sweat Rates, Sweat Sodium Concentrations, and Sodium Losses in 3 Groups of Professional Football Players

“…Because these direct comparisons between the LM and BK have always occurred under identical equipment and environmental conditions, the higher core temperatures in the LM are likely due to a larger body mass and, therefore, greater metabolic rate at a given exercise intensity combined with a lower body surface area (BSA) to mass ratio. 5,7,12 Similarly, the higher rates of sweating in the LM are likely caused by greater mass (higher metabolic rate) and larger BSA, as both are correlated with SwtR. 5,9,10 Adequate sweat production is critical for evaporative heat dissipation, particularly in large athletes exercising in protective equipment, but it may be challenging for heavy, salty sweaters to maintain electrolyte balance, and therefore fluid balance, when practicing twice per day on consecutive days, as is typical in both professional and high school football.…”

Sweat rate and fluid turnover in American football players compared with runners in a hot and humid environment

“…[1][2][3][4][5][6] Some of these studies have been experimental whereas others have reported field observations. 1 2 4-7 In these studies, runners, cyclists, and military personnel make up the largest subject pool.…”