The recent blog by Prof Neil Walsh on Train Cool - Bathe Hot covered the scientific aspects of a publication on "heat acclimation through hot baths" (first author Mike Zurawlew). The findings of the study were presented in a very clear way, but often a research paper leaves the reader with a number of practical questions. Research is conducted in a very controlled way and it is sometimes difficult to extrapolate findings to different situations, populations or events. Therefore I caught up with Neil Walsh and Mike Zurawlew and asked them about "the practical stuff that you don't read about in research papers"....

Neil and Mike, the conditions in the study you published were fairly extreme 33°C. How does this compare to conditions athletes may experience? (how extreme or how common is this?)

Yes, the conditions were challenging in the hot trial (33°C, 40% RH) in our study. Nevertheless, we present strong evidence that the intervention also reduces thermal strain in cooler conditions. We observed lower core body temperature, lower core temperature at sweating onset and lower perceived exertion when the participants exercised in ‘temperate’ 18°C conditions after the intervention. So if for example a non-heat-acclimated endurance athlete was performing in Rio (likely 24-28°C) we believe that taking a hot bath after exercise on 6 days (see “How to” guidelines below) would provide heat acclimation – this would in-turn reduce thermal strain during exercise and potentially improve performance, as I discuss below.

The run was approximately 1 hour. Do you expect to find similar benefits for longer events (ironman distance in hot conditions such as Hawaii?)

I see no reason why not. When heat acclimated one would expect self-selection of a higher pace during endurance exercise in the heat (as we have seen, albeit for a short 5 km run). Future studies need to confirm this. We speculated in the paper that had we used a longer running test we may have even seen an improvement in performance in ‘temperate’ 18°C conditions after the intervention: the rise in core temperature during a longer test may become a limiting factor. However, whether heat acclimation improves performance in temperate conditions is controversial, as we discuss in our paper.

Was your study performed in the Welsh winter or summer? What difference would this make?

In North Wales, with average winter temperatures of 3-9°C and summer temperatures of 13-20°C, the season makes little difference (as anyone who lives here will testify!). Although the data collection was performed year round, key is that the number of hot bath and control bath participants was the same in each season; and, none of our participants were heat acclimated; none had been exposed to hot conditions in the preceding 3 months.

How well trained were your runners? They were around 19 min for 5k? Do you expect to see benefits for athletes who are somewhat acclimated? And would these effects be seen in elite runners? Would it be seen in slower runners?

These are excellent questions. Although comparing the level of heat acclimation gained in highly trained vs. recreationally active was not our principal aim we believe our data move us some way towards an answer.

One might expect that endurance trained individuals would, by nature of having some heat acclimation at the outset, benefit less than recreationally active individuals from our hot bath intervention; viz., part of getting fit is getting hot!

It’s worth remembering that most traditional exercise-heat-acclimation studies have individuals perform daily exercise-heat-stress for a ‘fixed duration’ (e.g. 90 min) at a ‘fixed relative intensity’ in the heat (e.g. 50% of maximal oxygen uptake). In contrast, our hot bath exposure was to a large extent ‘self-regulated’ i.e. the individual removed themselves at their volition if they could not complete the 40 min.

In the published study we had 5 recreationally active individuals (2-4 h endurance exercise per week, average 5 km TT, 22 min 22 s) and 5 trained endurance individuals (>6 h endurance exercise per week, average 5 km TT, 18 min 11 s). The endurance trained demonstrated reductions in resting core temperature and exercising core temperature in the heat that were comparable to the recreationally active. Also, the endurance trained improved their 5km TT performance in the heat by 4%, on average.

The endurance trained improved their 5km TT performance in the heat by 4%, on average.

That the endurance trained also benefited from the hot bath intervention is good news but of no surprise for the following reason – the endurance trained completed a longer duration in the hot bath on most days (of the max 40 min). For example, 3 of the 5 endurance trained completed the full 40 min hot bath on day 1 but only 1 of the 5 recreationally active. As such, the endurance trained actually experienced a greater stimulus for heat acclimation.

An important caveat is that we did not have a sufficient sample size to statistically compare training status in our paper; nevertheless, the preliminary results for this comparison are encouraging. We hope to have a paper with a larger sample size for the training status comparison very soon.

Are there any other potential benefits of a hot bath besides running performance?

I think the Romans can stake a claim on being the first to extol the health benefits of a hot bath! Anyone with aching bones and muscles who has bathed at a Roman spa will testify to this.

In the Bell Jar, Sylvia Plath wrote: “I am sure there are things that can’t be cured by a good bath but I can’t think of one.”

This is clearly an exciting area, ripe for investigation. Beyond the purported benefits of a hot soak to relieve aching muscles and limbs, research endeavours are focusing right now on the benefits of a hot bath for cardiovascular function, glucose regulation and immunity. As we mention in the blog, there is potential for a hot bath to benefit the injured athlete and those who struggle to perform sufficient exercise e.g. elderly or sick.

How does the magnitude of heat acclimation in your study compare with traditional heat acclimation?

The available evidence suggests very well indeed. Although we haven’t directly compared our short-term heat acclimation method with current practice; whereby, daily exercise is performed in the heat with core temperature clamped at 38.5°C, there are reasons to believe that a hot bath is a good alternative.

There are clear practical advantages i.e. no need for an environmental chamber and clamping of core temperature; a hot bath after exercise does not interfere with training or the taper ahead of competition in the heat; and, a hot bath may be incorporated into the post-exercise washing routine.

Beyond the practical advantages, we see a reduction in resting core temperature in almost all the participants we have tested with our method to date; this can be considered a ‘pre-cooling effect’. Short term (5-6 day) and even long term (10-14 day) exercise-heat-acclimation studies do not always show a reduction in resting core temperature. Clearly studies are required to compare this new method with current practice. As we discuss in the paper, and below, there is sound physiological reasoning for why our hot bath method might be favourable – combination of raised core and skin temperature.

Would sauna have a similar or better effect?

One study we discuss in our paper (by Scoon et al.) demonstrates the benefit of regular sauna bathing after exercise on exercise performance in the heat. However, that study is somewhat limited in terms of practicality and no measures of thermoregulation were made.

Are there any risks associated with the hot bath approach?

Yes, it is absolutely imperative that the individual follows the “How to” guidelines in the attached infographic.

Just like when an individual exercises in the heat it is important to listen to the cues to stop!

We know that very highly motivated individuals can ignore the cues to stop during exercise in the heat – high motivation is a risk factor for heat exhaustion, which can progress to the potentially fatal condition, heat stroke.

It is neither safe nor necessary for everyone to complete the full 40 min hot bath on each of the 6 days. In fact, as shown in Table 1 in our paper, only 4 individuals completed a 40 min hot bath on day 1.

Our guidelines allow for progression each day as the individual adapts.

Careful consideration should be given to the thermal strain in the exercise before the hot bath. Our participants only performed 40 min of moderate intensity exercise before the hot bath. They were “warm but fairly comfortable” at the end of the exercise in temperate conditions.

We recommend that the individual sits for a few minutes after getting out of the hot bath – this allows blood pressure to return to normal and limits the risk of feeling light headed.

What can you tell us about the mechanisms?

The explanation for the benefits we see of taking a hot bath after exercise require investigation but likely involve the combined elevation of core body temperature and skin temperature. As we discuss in the paper, there are studies showing that to achieve full heat acclimation it is important to have elevations in both core body temperature and skin temperature.

Reference

Zurawlew, Walsh, Fortes, Potter. Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scand J Med Sci Sports DOI: 10.1111/sms.12638

Full paper can be found here on Researchgate.