Scoon et al. 2007, Post-exercise sauna bathing and endurance performance in runners

Small crossover study (Journal of Science and Medicine in Sport 10(5):259–262) in six well-trained male distance runners. Three weeks of post-training sauna bathing (around 30 minutes at roughly 90 °C immediately after running, about 12 to 13 sessions) expanded plasma volume by about 7% and raised run time to exhaustion by 32%, equivalent to an endurance improvement of roughly 1.9%, relative to a control period. The mechanism was attributed to the increase in blood volume. Limited by the tiny sample, but the standing evidence that repeated post-exercise sauna is a delivery route for heat acclimation.