Ten well-trained runners spent five weeks lying in 40°C water for 45 minutes, five times a week, on top of their normal training. Their VO2max rose by 2.7 mL/kg/min, roughly 4.4%, and their treadmill speed at VO2max increased by 0.8 km/h. Their training was otherwise unchanged.
The study, led by Elliott Jenkins at Cardiff Metropolitan University and published in The Journal of Physiology in November 2025, has since travelled through running and cycling circles as a cheap alternative to an altitude camp. Rowers have hot-weather regattas, high training volumes and coaches who guard session quality closely. On paper the idea fits. The evidence needs a closer look.
The Cardiff Protocol
For a study of its size, the design was controlled. Each runner, nine men and one woman with a mean baseline VO2max of 64.5 mL/kg/min, completed two five-week blocks in a crossover: one with hot-water immersion added to habitual training, one without. Training volume and intensity were matched between blocks, so the researchers could attribute any difference to the water.
Participants immersed to the neck in water of at least 40°C, dropping to chest depth only if they could not otherwise finish the session. Up to 500 mL of drinking water was allowed. Most sessions took place straight after a run, when core temperature was already elevated.
Haemoglobin mass was assessed twice through carbon-monoxide rebreathing, with blood variables tracked weekly. Cardiac structure came from four-dimensional speckle-tracking echocardiography before and after each block.
More Blood, a Fuller Heart
Following the hot-water block, haemoglobin mass rose by 33 g and total blood volume increased by 284 mL. Left-ventricular end-diastolic volume, the amount of blood in the heart's main pumping chamber before it contracts, increased by 10 mL. Measures of myocardial strain and diastolic filling rate did not change; the result reflected greater filling rather than increased contractility or wall thickness.
Regression analysis identified haemoglobin mass as the strongest single predictor of the VO2max change, with the cardiac filling volume adding further explanatory power. Together the two accounted for more than 80% of the variance.
The proposed sequence runs like this: heat expands plasma volume in the first fortnight, diluting the blood; the kidneys detect reduced oxygen transport and increase red-cell production; by weeks four and five, red-cell volume climbs while total volume stays high. The Cardiff team did not measure erythropoietin directly, so the kidney signal remains a plausible explanation rather than a demonstrated one.
The Meta-Analysis Says Slow Down
A single well-run study of 10 athletes is a strong prompt, not a settled answer. A 2025 systematic review and meta-analysis pooled 10 studies of post-exercise sauna or hot-water immersion covering 199 adults, and reached a cooler verdict. The effect on performance in hot conditions was trivial and not statistically significant, with a ratio of means of 1.04 and a confidence interval spanning 0.94 to 1.15. Results for performance in thermoneutral settings and lactate-threshold speed were also trivial. VO2max showed a small pooled effect. The authors rated the certainty of evidence as low or very low across every outcome, citing small samples, publication bias and low statistical power.
Earlier bath studies help explain the gap. A 2019 six-day protocol in recreationally active men, using 40 minutes at 40°C after a 40-minute run, produced classic heat-acclimation changes: lower resting and exercising core temperature, lower end-exercise heart rate, earlier sweating. It produced no significant increase in haemoglobin mass or blood volume. A comparable six-day study produced a 4.9% improvement in 5 km treadmill time at 33°C but none at 18°C.
Short blocks make athletes better in the heat. The Cardiff study is the first passive-heat trial to show blood-building over a longer block, and it has not yet been replicated.
Where It Fits a Rowing Week
The strongest argument for post-session heat over training in the heat is that the erg or water session stays intact. Anyone who has tried to hold a split in a 30°C boathouse knows the heat becomes the limiter before the legs do. A bath afterwards adds thermal load without touching stroke quality, boat feel or the power profile of the piece.
It still imposes a substantial load: the Jenkins protocol demanded 225 minutes a week in hot water. Swedish runner and coach Göran Winblad tested it on himself and described the experience in his video: "After 10 minutes, you're thinking, 'Yeah, this is sort of nice. This is fine.' But after 40 minutes, you're absolutely cooked." He said his heart rate sat where it would on an easy jog. His verdict was that the same 225 minutes spent running would probably deliver more for most athletes, because the bathtub produces none of the tendon, economy or neuromuscular adaptations that the sport itself does. The same logic applies to a rower, with the added point that no bath teaches a crew to move together.
The case is clearer when the target is a hot regatta rather than a VO2max number. At the 2025 World Championships in Shanghai, temperatures exceeded 30°C with humidity routinely above 80%, and World Rowing was ready to suspend racing based on wet-bulb globe temperature readings. Eva Szanto, chair of the Events Commission, said the reading "might change very quickly, as soon as the wind picks up or the cloud comes in". Teams travelled early to acclimatise and used ice vests and ice baths on race day.
World Rowing's 2026 Heat Safety Guidelines recommend exposing yourself to heat for 60 to 90 minutes, at least four times a week, over roughly two weeks, followed by maintenance exposures. British Rowing's sports science guidance puts the threshold for performance effects at 25°C, notes that sweat rates in heat can pass 1 to 2 litres an hour, and warns that losing 2% of body weight to fluid impairs cooling. A post-session bath is one way of meeting those exposure targets without wrecking the week's key work.
The Cardiologist's Caveat
Forty-five minutes at 40°C or above is a real cardiovascular stress. In the 2019 six-day study, only three of 13 participants finished the full 40-minute immersion on day one; all 13 managed it by day six. Cleveland Clinic cardiologist Curtis Rimmerman said that hot water raises body temperature while lowering blood pressure, prevents sweat from evaporating, and makes "hot tubs and saunas […] potentially dangerous for patients with known or suspected heart disease". Anyone with a cardiac history, heat intolerance or a current illness should not be copying a laboratory protocol at home.
So should a rower take the bath? In the six weeks before a hot regatta, with training volume already maxed out and session quality worth protecting, the evidence for heat tolerance is solid and the Cardiff study offers a possible bonus on top. As a year-round shortcut to a bigger engine, the pooled data does not yet support the hype. The minimum effective dose is unknown, the only positive VO2max trial involved 10 runners, and nobody has put a rower in the tub and then on the erg.
