Heat acclimatisation
The human body is extremely good at doing the bare minimum to preserve resources. This is why doing something once, or sporadically, has little impact. Lift weights at the gym 6 times over a year and you’re looking at no change. Lift weights 6 times in 2 weeks and your body will respond by making you better at lifting weights. Once it’s convinced that it’s worth expending resources it will make the necessary changes. It makes very specific changes to make the specific activity easier. As anyone who has had gaps in their training will know only too well, when you stop doing an activity the body will gradually stop providing the resources that keep it in the best shape for that activity.
We know that consistency matters, and specificity too. But specificity is actually not so black and white; take the example of Jim Walmsley running insanely fast 100 milers off of bike training, or a season of cross country skiing.
So when does specificity matter, and why does it sometimes not seem to, and what has this got to do with heat acclimatisation?
To prepare the body for a task - a trail race or ultra marathon for example - we must get it to make all the changes that we need for the race. In some ways it doesn’t matter how we get it to make those changes, so long as it does make them.
When we look at acclimatisation we can think about what physiological changes we need and work backwards.
For heat we need:
to lower the point at which the body starts to sweat and cool itself;
to increase sweat rate (with lowered sodium in sweat);
to increase blood plasma volume and red blood cell volume;
to change perceived effort in heat.
Previous interventions attempted to simulate heat whilst running. This often meant wearing more clothing, even bin bags. But this might actually be counterproductive if it reduces the quality of the run session. More recent research has looked at supplementing running with indoor biking in a hot room and most interestingly hot baths or saunas post-exercise.
Just get in a hot bath after running…
We can achieve all of the necessary adaptations with post-exercise heat stress (bath / sauna). This is highly desirable as it requires no special equipment (beyond a bath) and allows the athlete to continue quality training.
One research study found that 6 days of post-exercise hot water immersion (40 minutes at 40°C) is effective (in men). As the body tends to respond to anything if we are persistent (consistency trumps all). However, heat training increases plasma volume, which temporaily dilutes the blood so, where possible, heat exposure should begin at least 6 weeks before your target race (4-5 weeks heat training then a gradual taper into the race)
Increased plasma volume happens quickly but it’s not the whole story
Imagine it like a cup of orange juice with 30ml of juice. You add 10ml of water and now have 40ml of liquid. You have the same amount of juice but it is less concentrated. When you stimulate blood plasma volume increase through heat training it triggers production of erythropoetin and consequently more red blood cells: creating 40ml of orange juice without the dilution. But that second part takes time and is especially important for mountain athletes competing at altitude.
Best protocol
Since research studies involving human participants become more costly the longer they take, they tend towards shorter interventions. But heat stress is an additional stressor so just throwing it on top of training and hoping for the best is not necessarily the best option. In my coaching practice (and personal experience) I’ve had a lot of success with 12 sessions (hot baths or saunas, or hot indoor bike) spread out over at least 4 weeks (then tapering into the race). Blood tests before, during and after the intervention show the following methods have worked for individuals in my team:
12 saunas building up from 5 mins to 30 mins spread over 4 weeks
6 weeks of 3 weekly hot bike rides indoors, over dressed with no fan, followed by short hot shower and an electroltye drink
12 hot baths building from 5 to 30 mins spread over 4 weeks
How to measure what works for you
Blood testing before, during and after can show how you respond to different interventions and is something I use with athletes preparing for races in very hot climates. I like athletes to also get an iron panel done if they can as it may be helpful to support the increased red cell production which follows the initial increase in plasma volume (and dilution of the blood). I never suggest supplementing iron without testing first as excess iron is dangerous.
Finally…
Remember that the body will revert back without continued stimulus but you can reduce the stimulus exponentially from 10 days before your race (heat stress is stress just like training).
Of course, be mindful of not doing anything that makes you feel unwell, or dizzy. You can always start at lower temperatures and for shorter time periods.
References
Zurawlew, M.J., Walsh, N.P., Fortes, M.B. and Potter, C., Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scand J Med Sci Sports, 2016, 26: 745-754. https://doi.org/10.1111/sms.12638
Oberholzer Laura, Siebenmann Christoph, Mikkelsen C. Jacob, Junge Nicklas, Piil Jacob F., Morris Nathan B., Goetze Jens P., Meinild Lundby Anne-Kristine, Nybo Lars, Lundby Carsten,Hematological Adaptations to Prolonged Heat Acclimation in Endurance-Trained Males, Frontiers in Physiology, 2019, 10. doi: https://doi.org/10.3389/fphys.2019.01379
Saunders, Philo U., Garvican-Lewis, Laura A., Chapman, Robert F., Périard , Julien D., Special Environments: Altitude and Heat International Journal of Sport Nutrition and Exercise Metabolism, 2019, 29, 210-219. https://doi.org/10.1123/ijsnem.2018-0256
Guy S.M. Scoon, William G. Hopkins, Simon Mayhew, James D. Cotter, Effect of post-exercise sauna bathing on the endurance performance of competitive male runners, Journal of Science and Medicine in Sport, 2007, 10 https://doi.org/10.1016/j.jsams.2006.06.009