Short read
Guy Scoon, Will Hopkins, Simon Mayhew and James Cotter of the University of Otago and AUT University in New Zealand published this crossover study in the Journal of Science and Medicine in Sport in 2007. Six male middle and long distance runners completed a three-week sauna phase, a three-week control phase and a three-week washout. In the sauna phase they averaged 12.7 sessions, each about 31 minutes at 89.9 degrees and each directly after training. Performance was tested as a run to exhaustion of about 15 minutes at current 5-kilometre best pace.
Compared with the control phase of the same runners, time to exhaustion rose by 32 %, which the authors equate with roughly 1.9 % improvement in a real time trial. Plasma volume rose by 7.1 % and red cell volume by 3.5 %. The change in performance correlated very closely with the change in plasma volume, which points to an expanded blood volume as the main mechanism.
The sample is six people, all men and all highly trained runners. The 32 % refers to an exhaustion test, not to a race time; this is where the most common citation error arises.
The study is the most quoted work on heat and endurance and one of the smallest. Its crossover design with a washout period is a strength, its sample size a clear limit. It shows a response in six trained men under these conditions, not a performance gain that others can expect.