Exercising in Warm Environment is More Stressing As Measured by Cortisol And TNFα

As Hosick et.al. found (Hosick, 2010) the exaggerated core temperature response to exercise in warm water environment triggers a significant increase in cortisol and TNFα, both parameters of increased metabolic stress in the 8 male subjects (men age 24y) who participated in "two 40 min trials of cycle ergometry at 65% of VO₂peak immersed to chest level in cool (25 °C) and warm (38.5 °C) water".

Cortisol & TNF-alpha values for subjects before and after 40min exercise immersed in cool or warm water (Hosick, 2010)

With a score of 16 (warm ) vs. 16 (cool) the rating of perceived exertion (Borg scale 6-20) was also significantly elevated and correlated with the heart rate (180BPM, warm vs. 144BPM, cool) of the subjects. The researchers conclude that:

"[...] this finding is of importance to individuals, coaches, and fitness professionals attempting to
understand ideal training load and design in warm or hot environments." (Hosick, 2010)

Just another reason not to train in a gym without proper air-condition in the summer time ;-)