Are warm baths the secret to getting faster?

As temperatures drop and Canadian runners settle into winter, you can’t beat the appeal of a long, hot bath after a cold run. Beyond easing sore muscles, new research suggests it may actually make you faster.

A recent study from Cardiff Metropolitan University in the Journal of Physiology analyzed whether passive heat exposure, specifically hot-water immersion, could boost aerobic performance in well-trained runners. The small study followed 10 athletes with VO₂ max levels around 60 over five weeks. Each runner completed five 45-minute hot-water immersion sessions per week while maintaining their usual training volume.

Trained runners who used a hot bath for 45 minutes per day (40C/104F; 5x/week; 5 weeks) increased their VO2 max by ~4%, plus:

– ⬆️ Hb mass by 33g
– ⬆️ blood volume & cardiac function
– ⬆️ speed at VO2 max by 0.8 km/h (0.5 mph) pic.twitter.com/SsxHFcPJff

— Brady Holmer (@Brady_H) November 24, 2025

The findings were surprising. After five weeks, runners saw an average increase of four per cent in their VO₂ max. According to Jack Daniels’ running formula, that increase is potentially enough to shave around 40 seconds off a 5K time, simply by sitting in hot water for 30 to 40 minutes, five times a week.

Researchers found that passive heat immersion increases hemoglobin and total blood volume, which are both key components of delivering more oxygen to our muscles. By week four, the hemoglobin mass had risen by three per cent from the starting baseline measurement. These metrics were comparable to those seen in elite athletes training at altitude, suggesting that meaningful gains can occur even without the cost or logistics of heat or altitude training.

Runners have long used saunas, heat chambers and humid environments in their training, and heat exposure has often been referred to as the “poor man’s altitude training.” Although passive heat immersion does not directly increase red blood cell count the way altitude training does, it offers similar physiological benefits without the price tag of an altitude tent or a flight to Flagstaff, Ariz., or St. Moritz, Switzerland. Until now, it has not been clear whether passive heat training, without exercising in the heat, could produce long-term adaptations comparable to those from traditional heat or altitude training.

Still, researchers noted one limitation: most participants completed their immersion sessions immediately after workouts, when their core temperatures were already elevated. This makes it unclear whether the adaptations resulted from the immersion itself or from prolonged exercise-induced hyperthermia. Either way, with a consistent routine, hot-water immersion remains a simple home method that could aid recovery and potentially help you get faster.