TOOLBOX: With summer in full swing, there’s no escaping riding in the heat, but hot temps inhibit the body’s ability to perform. Dr. Stephen Cheung is an expert on thermal stresses on human performance and offers up his favorite heat adaptation hacks for cyclists.
Since 2014, there has been a conference on the science of cycling held in the days before the Grand Depart of Le Tour. The two-day conference is held in the same city as the start, enabling the attendance of coaches and sport scientists from many of the teams. In addition, there are hard-core exercise physiologists like myself, coaches from around the world, performance staff from different national federations, etc.
For my part, I gave a talk on one of my main research areas, namely how heat stress affects cycling performance, along with the current state of knowledge concerning the process of heat adaptation.
What I will do in this article is to summarize the highlights, then link to two other venues for much further information – I call it my heat adapation hacks for cyclists.
Heat DOES Impair Exercise
First off, heat absolutely does impair exercise, and at much lower temperature than we may think. A nice analysis of multiple marathons across a range of location and years showed that, even a wet-bulb globe temperature (WBGT) of 15°C impaired marathon times compared to 10°C (Ely et al. 2007). This supported a 1997 study from my post-doc lab demonstrating that time to exhaustion was maximized at 10°C compared to 0 or 20°C, with a further impairment at 30°C (Galloway and Maughan 1997).
The 2007 study also showed that the less fit you were, the greater the impact of heat on performance capacity.
How big a decrement does this work out to in real-life cycling? A study in 2015 had Danish cyclists perform a 43.4 km outdoor TT in 5°C temperature, where they averaged ~300 W. These cyclists then went to a 2-week training/heat adaptation camp in Qatar. The day after arrival, they performed another 43.4 km outdoor TT and averaged a 15% decrease in average power (Karlsen et al. 2015).
Heat Adaptation DOES Work
If heat is indeed bad for performance, what can we do about it? While there are many day-of options such as pre-cooling, ice slurry ingestion, etc., the best way to prepare is by a gradual physiological adaptation to exercise in the heat. In 2016, my UK colleague Chris Tyler and I published a large meta-analysis quantifying the results of 92 studies on performance benefits from heat adaptation (Tyler et al. 2016). Some of the key findings:
• The classic physiological symptoms include a lower core temperature both at rest and during exercise. Resting and exercise heart rate is also decreased, likely as a result of greater plasma volume. Finally, sweating is improved, through both a lower core temperature for sweating to start along with a greater sweating sensitivity to increasing temperature.
• Heat adaptation protocols of as short as 4-5 days can be partially effective and better than none at all. However, the longer you can have for heat adaptation (2 weeks or even longer), the greater the physiological and performance benefits.
• There’s no real firm consensus on how hot an environment you need to train in, nor on how long each heat session should be. Most of the studies we looked at used 1-2 hours of moderate exercise, with ambient temperature usually 30°C or greater.
Heat Adaptation Hacks
From our analysis, the main stimulus for heat adaptation is not necessarily some magic formula or ambient temperature, exercise intensity, or duration. Rather, it seems that the main priority is to raise core temperature to a high (but not dangerously high) level, and to keep it there for at least an hour a day. This opens up the possibility of a number of hacks if you don’t have access to an environmental chamber like I have in my lab:
• Wear extra clothing even if it’s already warm out. A 2018 study (Ely et al. 2018) showed that piling on lots of extra cycling clothing in a temperate environment gets you most – but not all – of the same physiological stimulus as training in a hot environment.
• When training indoors, we all hear about the importance of having a big fan blowing at you to cool you off. One low-tech hack is to reduce or even eliminate the fan.
• After training, when your body temperature is already up, keep that thermal stimulus by having a hot bath or a sauna.
• One currently contentious idea is whether mild dehydration DURING exercise in the heat can accelerate heat adaptation (Garrett et al. 2014). You definitely want to ensure proper rehydration post-exercise though to ensure recovery.
A Podcast and a Video
For much more detailed information, you can check out both a podcast I gave on this topic, along with a full video of a keynote presentation I gave on this topic at a previous conference:
The podcast is available on www.scientifictriathlon.com, run by triathlete coach Mikael Eriksson. During my return from Nantes, my wife and I had a long layover in Lisbon where Mikael now lives. So in exchange for a walking tour around the ridiculously beautiful old city, I gave a “live” podcast while we were at lunch near the old castelo.
In 2017, I was an invited speaker at the Science + Triathlon conference, organized by the International Triathlon Union in Edmonton, Canada. The topic then was also on heat adaptation, specifically as it relates to triathlon. They had a professional video made of the talk, which is now available on YouTube.
The Edmonton organizers also had graphic illustrator Sam Hester prepare a graphic summary of my talk in real-time. She did an astounding job at capturing the information, and this cool summary is now hanging proudly outside my office door!
Ride fast and have fun!
Ely BR, Blanchard LA, Steele JR, et al (2018) Physiological Responses to Overdressing and Exercise-Heat Stress in Trained Runners. Med Sci Sports Exerc. doi: 10.1249/MSS.0000000000001550
Ely MR, Cheuvront SN, Roberts WO, Montain SJ (2007) Impact of Weather on Marathon-Running Performance: Med Sci Sports Exerc 39:487–493. doi: 10.1249/mss.0b013e31802d3aba
Galloway SD, Maughan RJ (1997) Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man. Med Sci Sports Exerc 29:1240–1249
Garrett AT, Goosens NG, Rehrer NJ, et al (2014) Short-term heat acclimation is effective and may be enhanced rather than impaired by dehydration. Am J Hum Biol 26:311–320. doi: 10.1002/ajhb.22509
Karlsen A, Nybo L, Nørgaard SJ, et al (2015) Time course of natural heat acclimatization in well-trained cyclists during a 2-week training camp in the heat. Scand J Med Sci Sports 25:240–249. doi: 10.1111/sms.12449
Tyler CJ, Reeve T, Hodges GJ, Cheung SS (2016) The Effects of Heat Adaptation on Physiology, Perception and Exercise Performance in the Heat: A Meta-Analysis. Sports Med 46:1699–1724. doi: 10.1007/s40279-016-0538-5