The influence of fatigue-induced increase in relative work rate on temperature regulation during exercise

Abstract

Heat-loss responses during steady-load exercise are affected by an increase in relative work rate induced by muscle ischaemia or hypoxaemia. The present study investigated whether progressive increases in perception of exertion and relative oxygen uptake $(\% \ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{{2{\text{peak}}}} ),$ which occur during prolonged steady-load exercise as a result of progressively increased peripheral fatigue, might also affect the regulation of heat loss responses and hence the exercise-induced increase in mean body temperature. Ten male subjects first performed a ramp-test to exhaustion on a cycle ergometer to evaluate their initial peak oxygen uptake (Control $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{{2{\text{peak}}}}$). On a separate day, 120 min of cycling at constant power output corresponding to 60% of Control $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{{2{\text{peak}}}}$ was performed in thermoneutral environment (Ta = 23°C, RH = 50%, wind speed = 5 m s⁻¹). This was immediately followed by another maximal performance test (Fatigue $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{{2{\text{peak}}}}$). During prolonged exercise, median (range) rating of perceived exertion for whole-body (RPE_wb) increased (P < 0.01) from initial 3.5 (1–5) to 5.5 (5–9) at the end of exercise. Fatigue $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{{2{\text{peak}}}}$ and peak power output were 9 (5) and 10 (5)% lower (P < 0.01) when compared to control values. At the onset of exercise, heat production, mechanical efficiency, heat loss and mean body temperature increased towards asymptotic values, thereafter remained constant throughout the 120 min exercise, despite the concomitant progressive increase in relative work rate, as reflected in increased RPE_wb and relative oxygen uptake. It is thus concluded that the increase in relative work rate induced predominantly by peripheral muscle fatigue affects neither the level of increase in mean body temperature nor the regulation of heat loss responses during prolonged steady-load exercise.