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Those who are active year-round know that summer workouts are more tiring than those done in cooler weather. The good news is that it’s not a sign that you’re suddenly out of shape. Exercising in warm temperatures is not the same as exercising in cooler temperatures and the body’s physiology has to adjust. How does the body adapt and can these changes translate to performance gains in cooler temperatures?

The body takes about 10 days to acclimate to exercising in heat. The most noticeable signs that it has adapted to warmer weather are sweating more easily and a lower exercising heart rate. Less perceptible physiological changes include greater volume of plasma—the liquid portion of the blood in which the red blood cells are suspended—less salt released through sweat and more efficient heart and muscle function.

Because these physiological adaptations improve exercise performance in heat, scientists and athletes have wondered if these changes also mean enhanced performance in cool conditions. The jury, though, is still out. A study in 2010 in the Journal of Applied Physiology reported that exercising in heat did improve exercise performance in cooler weather. A new study published last month in the American Journal of Physiology—Heart and Circulatory Physiology concluded the opposite: heat training only improved performance in hot conditions, but not temperate ones. Nonetheless, both studies show that the body can adapt to new conditions relatively quickly. So, when you find it hard to catch up when it’s hot, be patient. You’re not out of shape, it’s just your body is catching up.

Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.