Passive Heat Training

Is it worth doing?

Exercise in the heat is the method with the strongest evidence. Across 211 papers, an average of 8 sessions of about 90 minutes at about 39°C lowered heart rate at the end of exercise by 17 beats a minute and core temperature by 0.43°C, expanded plasma volume by 5.6% and improved time trial performance by 3.1%¹. Passive heat is the easier option, and its results are less certain. A 2025 analysis pooling 10 studies (199 people) found the effect on performance in hot conditions trivial and imprecise, and rated the certainty of the evidence low to very low². A second 2025 review of 14 trials found post-exercise heating may improve running performance, at least in hot conditions, with no effect on cycling performance, and rated its evidence low to moderate³.

Coaches of runners who race in heat find that most of the benefit of exercise in the heat comes in the first nine to eleven exposures and that it fades quickly, so the sessions keep going until race week.

So take it if you have a sauna or a hot tub, you are racing in heat or at altitude, and the sessions cost you none of your training. If you have no access, skip it. Nothing about your race depends on it.

Where it helps

A hot race is where it is most likely to help. In a 2016 trial of 17 men, 6 days of 40 minutes running followed by 40 minutes in 40°C (104°F) water improved a 5 km time trial in 33°C heat by 4.9% and lowered resting core temperature, and a 5 km time trial in 18°C did not improve⁴. A cool race day is therefore no reason to do it: the 2025 pooled analysis found trivial effects on performance in cool, thermoneutral conditions².

For a race at altitude, treat it as a small bonus. In a 2024 analysis of 9 studies (79 recreationally trained men) of heat acclimation, mostly exercise in the heat, heat adaptation had moderate benefits on resting and submaximal measures in low oxygen and small effects on performance⁵. That is a reason to keep the sessions you are already doing, and not a reason to add more.

The research is mostly in men. Across 73 passive heat studies, 427 of 1,392 participants were women, and none of the studies was designed to compare men and women⁶. The evidence is less certain for women than for men.

How it fits your build

Your coach decides whether heat sessions belong in your build and how many, and writes each one into your plan as an optional session.

Sessions go on rest days or after an easy run, not on a hard day, a long-run day or a day with two runs, and not on race day, the day before it or after the race. A newer runner usually does without, because heat is an extra stress and a newer runner has less room for it.

What a session looks like

Runsense's session is up to 20 minutes in a dry sauna at 160 to 180°F (about 71 to 82°C) or in a hot tub or hot bath at 100 to 104°F (about 38 to 40°C). Build up gradually, starting with fewer minutes. Drink before and after, and see the guide on hydration and electrolytes. To pace a hot race, see race-day pacing.

The studies used longer and hotter sessions, so Runsense's is the cautious end. A 2021 trial in runners used about 28 minutes at 101 to 108°C after training, three times a week for three weeks. Heart rate in a 30-minute run in 40°C heat was 11 beats a minute lower than in runners who trained as usual⁷.