When it comes to adapting for racing in the heat, I feel like a stuck record. It’s low hanging fruit: at least a three percent improvement on race day if you can acclimate effectively in the two weeks leading into your event. The science has been published over and over. From the seminal work of Greenleaf et al. in the 70s to Armstrong and Maresh (1991), heat acclimatisation has been widely studied and accepted in the physiology world.
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Since we already know that heat acclimation improves performance (summarised well in Guy et al. 2014), there’s a great deal of interest in how to optimise benefit to time spent on acclimation – especially in a world where 140 characters is an essay! Short or Medium Term Heat Acclimation (STHA or MTHA) both work well, with the weight of literature backing MTHA as superior. So why aren’t athletes executing a simple task for this performance benefit?
I think the finger can be pointed three ways –
- Lack of plain language communication by scientists.
- Coaches are busily writing programs for too many athletes to keep up with current best practice.
- Athletes who too often focus on the latest and greatest equipment, rather than the critical skills associated with swim/bike/run.
Now that I have clipped the whole community (sorry!), let’s get to the nuts and bolts of heat acclimation.
To clarify, heat acclimation benefits racing in cool-temperate climates, as well as hot/humid conditions. But I really want athletes and coaches to focus on executing MTHA or STHA protocols effectively for racing in the heat (i.e. Ironman Cairns or Ironman 70.3 Cairns), rather than getting bogged down comparing the minutiae of performance benefits in different climates. Any specific questions, please shout at my handle (@drmitcha) or email me (Mitch@shinbonemedical.com).
There are physiological and psychological adaptations to heat. The physiological are well documented (see summary). The psychological benefits have been less extensively investigated, but still, point to a reduction in perceived exertion and improved thermal comfort (Maxwell et al. 2016). This adaptation should confer critical benefits on race day, allowing athletes to better make rational and optimal decisions about pacing and nutrition, in the absence of overwhelming heat stress.
Physiological Adaptations Summary:
- Increased plasma volume
(water in blood)
- Reduced heart rate/core temperature/skin temperature
- Reduced core temperature at onset
- Altered fuel utilisation
- Increased oxygen consumption/heat loss by radiation and convection
- Improved exercise economy in hot and cool environment (Armstrong LE, 1998).
A STHA protocol is considered seven days or less, while MTHA as eight to 14 days. Time spent exercising in the heat (35-40C) varies in these protocols from forty minutes to two hours, using walking/running/cycling at an intensity of 40-80% VO2max. Guy and colleagues (2016) synthesis of all these options suggests that MTHA is significantly more effective than STHA. For instance, plasma volume expansion in MTHA (7%) is double that of STHA (3.5%). So shooting for 45 minutes daily at 50% max for two weeks should be the target for all athletes trying to heat acclimate properly. It’s a big time commitment, but the performance gain is also large. The benefit starts to decay in days to weeks, so this is best done in close proximity to the event. I would suggest starting at 21 days before race day and completing with a week to taper fully acclimated.
Regarding acclimation mode, to my mind, cycling is the simplest to execute on a daily basis, and the easiest to control in terms of intensity (also minimal muscle damage). Keep the effort low. The heat is already stressful, and the muscles generate quite a lot of heat at low-moderate exercise intensity. Set up a cycling ergo in the laundry and turn on the clothes dryer with a wet towel inside to provide enough heat and humidity for the purpose. Trying to drag a treadmill into the laundry sounds like hard work! Avoid using a fan to allow more heat accumulation – the idea is to get the body temperature up and illicit heavy sweating. You will need towels, lots of towels.
Given that plasma volume expansion and increased sweat volume are the main effects of heat adaptation replace fluid and electrolytes as you go. Setting a set of scales next to the ergo is easy. You’ll also get a sense of sweat rate if you keep track of this each day- letting you know that adjusting to the heat is taking place. Additionally, this will allow you to approximate your sweat rate on race day and the fluid required to match sweat losses. Another easy measure is body temperature – pop a thermometer under your tongue while you are weighing in. A steady drop should occur over the two week period (anywhere from 0.5-1C).
As a practical note, athletes should be cognisant that plasma cortisol (stress hormone) is stimulated by exercise in the heat. Cortisol is a reliable indicator of strain on the body. Erring on mild doses of heat exposure are therefore recommended in the tapering athlete. The whole purpose of the taper is to reduce stress on the body and allow it to recover- blunting this recovery with high-intensity heat training would be counter-productive. Stick to the lower ends of the recommended intensity and use the daily dose as an adjunct to other sessions, rather than an ‘extra’ load. Concomitant reduction of the swim and run session volumes to match a ‘regular’ taper volumes should also alleviate some of the time and physical stress associated with the addition of heat acclimation.
The real world aspect of this process isn’t lost on me. It’s a big commitment – so don’t worry if you miss a day or two. Aspire to 14 sessions and consider 10 to 12 a job well done. The idea is to be acclimated before reaching your destination, so the final touches of the taper are effective. You may mix some passive heat adaptation with the active sessions, but keep in mind that this type of protocol remains unvalidated in the lab. Passive heat adaptation has been popularised through elite cycling and involves entering a sauna for 15-30 minutes at the end of a training session.
Engage with the process of heat adapting as you would any other part of your training. There is no sense arriving in Cairns, Hawaii or any other ‘hot’ race course in an ill-prepared manner. As they say, the proof is in the pudding. So get cooking.
Armstrong, L E and C M Maresh. The induction and decay of heat acclimatization in trained athletes. Sports Med. 12: 302-312, 1991.
Guy JH1, Deakin GB, Edwards AM, Miller CM, Pyne DB. Adaptation to hot environmental conditions: an exploration of the performance basis, procedures and future directions to optimise opportunities for elite athletes. Sports Med. 2015 Mar;45(3):303-11.
Santiago Lorenzo, John R. Halliwill, Michael N. Sawka, Christopher T. Minson Heat acclimation improves exercise performance Journal of Applied Physiology Published 1 October 2010 Vol. 109 no. 4, 1140-1147
Willmott AG, Hayes M, Waldock KA, Relf RL, Watkins ER, James CA, Gibson OR, Smeeton NJ, Richardson AJ, Watt PW, Maxwell NS. Short-term heat acclimation prior to a multi-day desert ultra-marathon improves physiological and psychological responses without compromising immune status. J Sports Sci. 2016 Dec 9:1-8.