As we all know, the heat is a bit of a hot topic in long-distance triathlon. Our World Championships take place in the hot and humid conditions of Kona, Hawaii, and many of us compete in hot conditions in other events around the world. We, therefore, know that we need to acclimate to heat before participating in these hot-weather events (8). A lot of athletes are also starting to use heat as a means of up-regulating regular endurance training adaptations, a bit like how we have used exposure to high altitudes to trigger positive changes (1). In this blog, we are going to discuss whether one quite practical strategy – post-exercise sauna bathing – is appropriate for use in these contexts; that is, inducing heat acclimation adaptations to improve physiological responses to exercise in the heat, and inducing endurance adaptations to improve performance in cooler conditions. Before we start it is worth acknowledging that the potential for sauna bathing to induce benefits outside of the typical parameters we focus on in long-distance triathlon. For example, a recent meta-analysis reported preliminary evidence that repeated passive heat exposure may stimulate hypertrophy of skeletal muscle, as well as strength (9), and a review in Mayo Clinic Proceedings published in 2018 concluded that there is evidence that sauna bathing reduces the risk of hypertension, cardiovascular disease, as well as nastiness like arthritis and flu (4).
Post-exercise sauna bathing and heat acclimation
With heat acclimation, we are seeking physiological adaptations that will improve our thermoregulatory capabilities during exercise performed in hot environments (we discuss this in detail in LDT 103). These adaptations include an enhanced sweat response for heat dissipation and control of core temperature, expanded plasma volume, and reduced heart rate during exercise in the heat (2). A variety of heat acclimation strategies to achieve these adaptations have been studied, with the most obvious being training in hot conditions, either outside in hot weather or in the temperature-controlled environmental chambers available in some exercise physiology laboratories. However, these approaches are not feasible for many athletes, who either live in temperate or cold climates or do not have access to an environmental chamber.
One strategy that has been studied that is more widely-available is post-exercise sauna bathing. Post-exercise sauna bathing involves first doing your regular training session, and then getting into the hot dry heat of a sauna (upwards of 80°C with only 5-10% relative humidity) for 30-40 minutes. The rationale behind this approach is that you have initially disturbed and stressed your body through exercise, and then get into a very hot environment that, even at rest, will push your core temperature and sweat rate up even further. As increases in core and skin temperature are likely at least part of the mechanism behind the heat acclimation responses we see with training in hot conditions (2), we might hope that we’ll get some of these adaptations with post-exercise sauna bathing.
It is fair to say that there aren’t an abundance of studies exploring the potential role of post-exercise sauna bathing in heat acclimation, although it’s likely that more research will emerge in the next few years. The studies that have been published are however promising, with expansions in plasma volume (11, 12), reduced heart rate during exercise in the heat (3), and reduced core temperature during exercise in the heat (3) all observed with regular post-exercise sauna bathing. These studies have involved ~30 min exposures to ~80-100°C three or four times per week, which, although it doesn’t sound like much, is actually pretty demanding when you’re in there!
Post-exercise sauna bathing and temperate training adaptation
With regards to using heat as a means of up-regulating regular adaptations to training, in order to improve performance in temperate conditions, the research is in its relative infancy here. There have been a few studies reporting positive haematological (blood), and in some cases performance, adaptations (5–7, 10). However, and again, many athletes do not have access to hot environments for training. Accordingly, a recent study assessed whether post-exercise sauna bathing improved markers of endurance performance in temperate conditions.
Kirby et al. (3) recruited a mixed-sex cohort of middle-distance runners to a control or post-exercise sauna intervention group. The control group performed their regular training for three weeks, whilst the sauna group did their regular training but bathed in a sauna for ~30 min three times per week for the three-week intervention period. The sauna in this study was very hot – 101-108°C – with only 5-10% relative humidity, and thus would have provided a considerable thermoregulatory stimulus for the participants. An incremental test was performed in temperate conditions (18°C) before and after the intervention period to assess markers of temperate performance. Promisingly, in the post-exercise sauna group, VO2max and the running speed at 4 mmol.L-1 blood lactate concentration (an indicator of the maximum metabolic steady-state) improved, whereas these changes were not observed in the control group. We eagerly await studies with performance tests to follow-up on these exciting results!
Therefore, whilst there are not a vast number of studies out there, post-exercise sauna bathing does seem to offer some benefits for heat acclimation as well as temperate performance. It appears that as little as 30-40 minutes in the sauna after training, 3-4 times per week, is sufficient to induce a positive effect; although I would be sure to note that 30-40 min in the sauna immediately after a training session is no walk in the park!
Another practical consideration could be Sauna exposure during recovery weeks or de-loading periods. The sauna can be a great way to maintain some of the key cardiovascular adaptations whilst still reducing the normal muscular load associated with swimming, cycling and running. For example, we’ve found this to be particularly useful with athletes who do endurance-based sports, but still want to achieve some strength or hypertrophy via gym work (not often the case in the LDT athlete!). Generally, to get substantial strength gains endurance training has to be reduced; however, the Sauna can help maintain the cardiovascular system during such periods of reduced endurance training thereby aiding the main focus of training.
If you are considering trying this strategy, it is really important to be careful. When these strategies are investigated, participants are invariably carefully monitored by the researchers, typically with continuous measurement of core temperature and heart rate and monitoring of heat strain symptoms. It would be very easy to get too hot in a sauna and find yourself in a bit of trouble. It is, therefore, necessary to be quite conservative in how you go about a post-exercise sauna; it is probably worth building up from shorter exposures to start, lengthening these as you become acclimated, and only sauna bathing after easy training sessions (i.e. not high-intensity workouts). You will also want to make sure there is someone keeping an eye on you, and you have access to a drink something cold close to the outside of the sauna (e.g. a shower) in case you do get into trouble.
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