In 1993, V. Mougios and A. Deligiannis wrote about the effect of water temperature on performance, lactate production and heart rate at swimming of maximal and submaximal intensity in the Journal of Sports Medicine and Physical Fitness.

The effects of water temperatures of 20°C (68°F), 26°C (78.8°F) and 32°C (89.6°F) were measured in submaximal and maximal swimming. At submaximal effort water temperature was related only to peak heart rate.

It has a direct effect on performance effort, heart rate, and lactate production at maximal effort.

1. At submaximal efforts, as water temperature rises so does heart rate, presumably because of increased circulation to remove heat (which is harder to do the hotter the water).

2. At maximum efforts, the hotter water increases all the physiological parameters of performance. A swimmer has to work harder the hotter the water to maintain a high level of effort (speed).

There is a level where the water temperature is dangerous because the body cannot dissipate the heat that is generated. That occurs at lower temperatures for maximum efforts and slightly higher temperatures for submaximum efforts. The main difference between the two is the amount of heat generated by the work intensity.

Implication. As the water temperature rises, swimming speed, maximum heart rate, and lactate production rise provided the effort level is maximal.

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