If you’ve ever done an ice bath, you will certainly love hearing there may be more benefits to sitting in a hot tub as opposed to a freezing cold one. I’ve used ice baths after some long runs and have felt the immediate benefits of decreased soreness. This new studied principal of using hot baths can help increase your endurance by increasing your blood plasma.
One of the hottest topics in endurance science during the last few years has been the idea that training in hot conditions can make you faster in “normal” conditions. Spending a week or two acclimating to heat triggers a series of adjustments in the body, including an increase in blood plasma volume (the “liquid” part of the blood, rather than the red cells that carry oxygen). That, in turn, seems to allow you to run a bit faster even in temperate weather.
Source: Can Hot Baths Make You Faster?