Heat balance and cumulative heat storage during exercise performed in the heat in physically active younger and middle-aged men

On separate days, eight physically active younger (22 ± 2 years) and eight highly trained middle-aged (45 ± 4 years) men matched for physical fitness and body composition performed 90 min of semi-recumbent cycling at a constant rate of heat production (290 W) followed by 60 min of seated recovery in either a temperate (T, 30°C), warm (W, 35°C) or hot (H, 40°C) ambient condition. Rectal temperature (T _re) was measured continuously, while the rate of whole-body heat loss (H _L), as well as changes in body heat content (∆H _b) was measured simultaneously using direct whole-body and indirect calorimetry. No difference in H _L was observed between age groups for all ambient conditions. Accordingly, the average ∆H _b during the 90-min exercise was similar for the younger (+193 ± 52, 212 ± 82 and +211 ± 44 kJ for T, W and H, respectively) and middle-aged men (+192 ± 119, +225 ± 76 and +217 ± 130 kJ for T, W and H, respectively). This was paralleled by a similar increase in T _re of 0.40 ± 0.20, 0.36 ± 0.14 and 0.34 ± 0.23°C for T, W and H, respectively in the younger men and 0.37 ± 0.23, 0.32 ± 0.19 and 0.28 ± 0.14°C for T, W and H, respectively in the middle-aged men. After 60 min of recovery, ∆H _b was similar for the younger and the middle-aged men, respectively (−45 ± 52 and −38 ± 31 kJ for T; −57 ± 78 and −40 ± 25 kJ for W; and −32 ± 71 and 11 ± 96 kJ for H). End recovery T _re remained elevated to similar levels in both the younger and middle-aged men, respectively, for each of the ambient conditions (0.24 ± 019 and 0.18 ± 0.18°C for T; 0.25 ± 0.11 and 0.24 ± 0.14°C for W and 0.33 ± 0.21 and 0.33 ± 0.13°C for H). We conclude that highly trained middle-aged men demonstrate a similar capacity for heat dissipation when compared with physically active younger men.