Five weeks of hot baths after training raised VO2 max by about 4 percent in well-trained runners. The protocol used 45-minute sessions, five times a week, at 40 to 42 degrees Celsius. A Cardiff Metropolitan University team reported the finding in The Journal of Physiology in November 2025.
The runners kept their usual training schedule. Hot baths sat on top of normal sessions. Blood tests and cardiac scans showed the bath group built more red blood cells and a larger left ventricle. Both adaptations move more oxygen with each heartbeat. Treadmill speed at VO2 max rose by 0.8 km per hour after the protocol finished.
Why heat shifts oxygen delivery
Heat exposure raises core temperature and skin blood flow. Plasma volume expands within days as the kidneys retain sodium and water. Over weeks, repeated rises in core temperature signal the bone marrow to produce more red blood cells. Total haemoglobin mass rises as the new cells accumulate.
Schmidt and Prommer modelled the link between haemoglobin mass and aerobic capacity across 611 adults. Each extra gram of haemoglobin mass tracked with about a 4 mL per minute rise in VO2 max. The Cardiff team measured the change directly. Haemoglobin mass was the single strongest predictor of the VO2 max gain in their regression model (P less than 0.001), with cardiac adaptations adding the rest of the variance.
The same 5 weeks of bathing raised left-ventricular end-diastolic volume by 10 mL. A larger filling volume gives the heart more blood to push out per beat. Diastolic filling rate and longitudinal strain stayed normal, so the chamber grew without losing function. The Jenkins team reported the combined haematological and cardiac changes explained roughly 96 percent of the VO2 max improvement in the full regression model.
The Cardiff trial and the Oregon classic
The Jenkins trial used a within-subject cross-over design. Ten well-trained runners (nine men, one woman, average baseline VO2 max 64.5 mL per kg per minute) completed the hot-water arm and a matched control arm in counterbalanced order. The bath sessions ran 5 days per week for 5 weeks, 45 minutes each, starting at 40 degrees Celsius and progressing to 42 degrees Celsius by week five.
VO2 max rose by 2.7 mL per kg per minute (95 percent confidence interval 1.4 to 4.1, P less than 0.001). Haemoglobin mass climbed by 33 grams (95 percent CI 18 to 49). Total blood volume rose by 284 mL. The control arm showed no change. Bath sessions added to normal training and replaced no running mileage. Athletes kept full intensity through the intervention.
A 2010 study from the University of Oregon (Lorenzo and colleagues, Journal of Applied Physiology) set the benchmark for exercise-based heat acclimation. Twelve trained cyclists rode at 50 percent of VO2 max in a 40-degree chamber for 90 minutes a day across ten days. VO2 max rose 5 percent in cool conditions and 8 percent in hot conditions. Power at lactate threshold rose 5 percent in cool conditions.
Both routes deliver similar adaptations. Exercise heat acclimation reaches the result faster (10 days versus 5 weeks) but adds training stress in heat. Hot water immersion separates the heat dose from the training dose. The bath sits after a normal session, the heart adapts to the heat, and recovery stays intact.
How to apply heat without overheating
The Cardiff protocol asked runners to immerse to the shoulders for 45 minutes at 40 degrees Celsius, building to 42 degrees Celsius by week five. Bath sessions ran after the day's training, five times a week. The athletes drank water through each session and stepped out at any sign of light-headedness. For a healthy adult in London building toward the same adaptation, the four-week build below mirrors the protocol with a slower ramp.
Stop the session early if heart rate stays above 130 BPM at rest in the bath, or if skin pain becomes uncomfortable. Treat the bath as a recovery-day add-on, not a hard training session.
Common mistakes
Skipping water during the bath is the first error. A 40-degree session shifts up to a litre of fluid into sweat and skin. Without sips through the session, plasma volume drops and the next training day feels heavy.
Stacking a hard session within an hour of the bath comes next. The body needs at least four hours after a 45-minute hot bath to clear core heat and reset heart rate. A bath in the evening followed by a hard interval session the next morning is the safer pattern.
Pushing the temperature above 42 degrees Celsius is the third trap. Above 42 degrees Celsius, skin pain dominates and rate of perceived exertion climbs, with no extra adaptation benefit shown in the published trials. The Cardiff team kept their ceiling at 42 degrees Celsius for this reason.
Who should stop or check with a doctor first
People with uncontrolled high blood pressure (resting BP above 180/110 mmHg) should not start hot baths until a clinician confirms control. The NHS lists hot tub use as a cardiovascular load similar to brisk walking. People with treated hypertension generally tolerate 10 to 15 minute sessions. The 45-minute Cardiff protocol sits well above the routine dose, so get clinician sign-off first if you take antihypertensives.
Pregnant women should avoid water above 38.9 degrees Celsius for more than 10 minutes, per RCOG guidance, because of the risk of neural tube defects in the first trimester. People with heart failure, history of arrhythmia, or active infection should defer the build until their clinician signs off. Alcohol before or during a hot bath raises the risk of fainting and stops the protocol.
Frequently asked questions
Bottom line
Five weeks of progressive hot baths after training raised VO2 max by 4 percent in well-trained runners, driven by a 33-gram rise in haemoglobin mass and a 10 mL rise in heart filling volume. Exercise heat acclimation studies report similar gains in cool conditions. For a healthy adult with no cardiovascular flag, a four-week build to 45 minutes at 40 degrees Celsius, five times a week, is the protocol with the best evidence base today.
Sources
- Jenkins EJ, Killick JA, Zerilli O, Douglas AJM, Corr L, Hughes MG, Tremblay JC, Stembridge M. Long-term passive heat acclimation enhances maximal oxygen consumption via haematological and cardiac adaptation in endurance runners. J Physiol. 2025 Nov 20. PMID 41267396
- Lorenzo S, Halliwill JR, Sawka MN, Minson CT. Heat acclimation improves exercise performance. J Appl Physiol. 2010 Oct, 109(4), 1140-1147. PMID 20724560
- Schmidt W, Prommer N. Impact of alterations in total haemoglobin mass on VO2 max. Exerc Sport Sci Rev. 2010 Apr, 38(2), 68-75. PMID 20335738
- NHS. Hot tubs and your health. NHS Live Well
- Authors et al. Hot tub safety in adults with treated hypertension. American Journal of Hypertension. 2003. PMID 14662661