The Effect of a High Ambient Temperature on Gross Efficiency in Cycling at Sub-Maximal Intensities

Abstract

INTRODUCTION: The occurring decrement in time trial performance in heat might be caused by a temperature-induced change in gross efficiency, attributable to a metabolic disruption that has been found to occur at elevated muscle temperatures (∼ 40°C). PURPOSE: To determine the effect of a high ambient temperature on gross efficiency during cycling. Due to methodological limitations, efficiency can presently be determined only at sub-maximal intensities. METHODS: Ten male cyclists performed randomly ordered bouts of ergometer cycling exercise at three constant intensities (40%, 60% and 80% maximal power output) in a hot and dry climate (H) (temperature: 35.5 ± 0.5°C, relative humidity (RH) 15.5 ± 3.2%) and in a thermo-neutral climate (N) (15.6 ± 0.3°C and 20.0 ± 10.3% RH). Each bout consisted of 20 minutes of cycling at 100 rpm. VO, RER and mechanical power output were measured to calculate gross efficiency. Skin temperature and rectal temperature were measured. Muscle temperature was measured only in the H-climate. RESULTS: Gross efficiency (GE) and VO2 (l/min) did not differ between climates (2-way ANOVA, p<0.05). Skin and core temperature were higher in heat, but maximal muscle temperature (Tmmax) did not exceed 40°C.TableDISCUSSION: There was no effect of a high ambient temperature on gross efficiency during cycling. Since no differences in VO2were observed between H and N and since core, skin and muscle temperature did not exceed 40°C, no significant metabolic disruption occurred. Based on the presented data at sub-maximal intensities, we do not expect changes in gross efficiency during time trial exercise of maximal effort. Anticipatory changes in pacing strategy to avoid hyperthermia or metabolic disruption might explain the occurring deterioration in time trial performance in heat. Another possibility is a reduction in peak power output. CONCLUSIONS: Since performance decrements in heat do not seem to be related to changes in gross efficiency, either loss of peak power output and/or anticipatory changes in pacing strategy may be better candidates to explain this reduced performance.