Heat balance during exercise in the sun

Summary

10 male subjects, dressed only in white shorts, exercised for 120 min at 92 W on a bicycle ergometer suspended in a balance. For the first 60 min they were exposed to the sun, from 60.–90. min they were shaded, and from 90.–120. min again exposed to the sun. In 10 experiments they faced the sun, in 10 others their backs were exposed. The values (in W) in the heat balance equationM-W±C±R±E±L±S=0 were measured by partitional calorimetry:M metabolic rate,W external work rate,C convective heat loss,R short and long wave radiation exchange,E evaporative sweat loss,L pulmonary evaporative loss, andS rate of heat storage. Means of the measured values (W) are shown below.R in the heat balance equation equals the radiative short wave (R _gs ) and long wave (R _gl ) heat gains minus the radiative long wave heat loss (R _II ).

$$\begin{gathered} {\text{Period }} R_{gs} {\text{ }}R_{gl} {\text{ }} R_{II} {\text{ }} M{\text{ }} W{\text{ }} C {\text{ }}E {\text{ }}L {\text{ }}S \hfill \\ {\text{sun 40}}--{\text{60 min }} {\text{123}} {\text{ 599}} {\text{ 628}} {\text{ 592 }} {\text{88 }} {\text{77}} {\text{ 454}} {\text{ 10 }} {\text{37}} \hfill \\ {\text{shade 70}}--{\text{90 min }} {\text{22* }} {\text{599 }} {\text{618 }} {\text{544* }} {\text{88 }} {\text{55}} {\text{ 332* }} {\text{9}} --{\text{11}} \hfill \\ {\text{sun 100}}--{\text{120 min}} {\text{ 127}} {\text{ }} {\text{595}} {\text{626 }} {\text{564 }} {\text{87 }} {\text{68 }} {\text{429}} {\text{ 9}} {\text{ 37}} \hfill \\ {\text{*}} {\text{ significantly different from sun - period values (Willcoxon}} \hfill \\ {\text{test)}} \hfill \\ \end{gathered} $$

The direct gain from solar radiation is approximately 100 W. In the shade period the reduction in radiant heat gain is compensated for by the decreased evaporation of sweat.