A-15O Free Communication/Poster Heat Stress and Dehydration
Most research examining thermal stress during exercise relates to low-intensity, endurance efforts (< 70% VO2peak, > 2hr) with little study of high-intensity, moderate duration (> 80% VO2peak, −60 min) exercise.
To determine the effect of different ambient temperatures (Ta) on laboratory 1-h cycling time trial (TT) performance.
16 male, trained cyclists (VO2peak 63.4 ±6.4 ml/kg/min, peak power output 386 ±49 W) each undertook between 3 and 6 self-paced TT during which they were instructed to cover as much distance as possible, in an environmental chamber at a range of different Ta, (between 12 and 32 °C) with constant relative humidity (63 ±6%). Elapsed time, distance covered, power output, heart rate, skin and core temperature were recorded throughout a TT. Prior to all rides, subjects received 6 ml/kg BM of a 6% CHO beverage, and during TT efforts, water ad-libitum. A polynomial regression with a different intercept was generated for each participant and used to analyze the relationship between distance covered and Ta.
Using elementary differential calculus, the curvilinear relationship between Ta and performance predicted that the greatest distance was covered at 18.6 °C, with a predicted drop of 509m and 2155m at 12 and 32 °C respectively. The highest power outputs were predicted to be achieved at Ta of 21.3, 18.9, 18.0 and 17.5 °C for the 1 st to 4 th quarters of the TT respectively. A linear increase in HR, core temperature, mean body temperature, sweat loss and self-selected water intake was found with increasing Ta.
These findings are the first to show that optimal 1-h TT performance in the laboratory setting occurs at an Ta of approximately 19 °C. Whether similar performance and physiological responses would occur in a ‘field’ setting with higher convective heat losses has yet to be examined.