One of the hottest topics in endurance science during the last few years has been the idea that training in hot conditions can make you faster in “normal” conditions. Spending a week or two acclimating to heat triggers a series of adjustments in the body, including an increase in blood plasma volume (the “liquid” part of the blood, rather than the red cells that carry oxygen). That, in turn, seems to allow you to run a bit faster even in temperate weather.

The problem is that heat training is easier said than done. We don’t all have access to climate-controlled thermal chambers, so researchers have begun exploring other options. One is postrun saunas, which also boost plasma volume. Another option, presented in a newly published paper in the Scandinavian Journal of Medicine and Science in Sports by Michael Zurawlew and his colleagues at Bangor University (hat tip to Trent Stellingwerff for pointing it out), is even more surprising: hot baths.

The researchers put 17 subjects through a six day protocol that involved running for 40 minutes in temperate conditions (64°F), then sitting in a bath up to their necks for 40 minutes. The baths were hot (104°F) for 10 subjects, and neutral (93°F) for the other seven. Before and after this six-day protocol, the researchers took a bunch of performance and physiological measurements.

The main result? The hot baths were effective in producing heat acclimation: the subjects in that group had lower resting temperatures, and their core temperatures after exercising in both cool and hot conditions stayed lower. There was some evidence of an increase in plasma volume of about 3 percent after a few days, though this measurement wasn’t very accurate.

In terms of performance, the hot-bath group ran 4.9 percent faster in a 5K time trial at 91 degrees), but didn’t get significantly faster in a 5K time trial at 64 degrees. Getting a performance boost in cooler conditions is the holy grail of this kind of work; it’s possible, the researchers note, that they might have seen a performance boost in a longer time trial where overheating is more of a limiting factor.

So are hot baths the wave of the future? Given the entrenched culture of post-workout ice baths in endurance sports, I think the idea would face some resistance. There may also be some trade-offs between what a hot bath does to your muscles (bearing in mind that there are plenty of questions about whether ice baths really have any benefits) versus what it does for your thermoregulation. This may be another area where periodization comes into play: cold baths during heavy training, then a six-day bout of hot baths a week or two before your goal race?

Of course, it feels funny to be writing about heat acclimation right when winter is finally kicking into gear. But that may be the most important point. For those of us training in cold climates but prepping for races in warm places (or early spring races), postrun hot baths may offer the simplest solution yet for getting your body ready to race in hot conditions.

My final question, for the next study, is: does it really have to be 40 minutes in the tub? Because I already have enough wrinkles as it is.

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