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It happens at the start of every summer — you head out for a ride in conditions much hotter than you’re used to and you suffer more than you thought was possible. Your body isn’t used to the conditions and even a gentle ride can feel like an ordeal.
Thankfully it’s more than possible to acclimate to hot weather. In fact, we all do it every summer even if we’re not aware of it. But as exercise physiology student James Gibbon writes, taking a proactive approach to heat acclimation can serve your cycling well.
What is heat acclimation and why should I care?
Heat acclimation is a term that describes a series of biological adaptations in response to repeated exposure to heat. The adaptations produced allow the acclimated individual to better cope with exercise in hot conditions.
It has been well proven that endurance exercise performance becomes significantly impaired in hot environmental conditions. This occurs because exercising in the heat forces the body to make a compromise between maintaining exercise intensity and maintaining a safe body temperature. This is where heat acclimation (HA) can help.
Originally HA was designed to prevent heat-related illness in military personnel and workers in challenging occupations such as mining and field work. In more recent years research has shown that, with some modifications, HA protocols can be incredibly effective when applied to athletes preparing to compete in hot environments.
One study found cyclists were 11 minutes slower over a 43km TT in the heat (37°C) compared to cool conditions (8°C), however after just one week of acclimation this deficit was reduced to just three minutes. After two weeks of acclimation, their TT time in the heat was identical to that in the more favourable cool condition.
Heat acclimation in cycling
Heat acclimation is a highly relevant matter in a number of sports, and certainly in cycling. Travelling to a region with a drastically hotter climate can prove difficult for athletes who live and train in cooler climates. This issue is even more significant with cross-hemisphere travel where the seasons are opposing, especially as competitors of the hosting region may be benefiting from natural acclimatisation.
An excellent example of this is the Tour Down Under, held every January at the height of the Australian summer, where temperatures regularly exceed 40°C. The pro peloton is dominated by European riders who travel to the race from the middle of the European winter. It’s no coincidence that only two northern hemisphere riders have managed to win the Tour Down Under in the last 10 years!
While other factors are of course at play — such as the tendency for Australian riders to be in form early in the year — it’s no coincidence the naturally acclimated Aussie riders excel in extremely hot conditions, while the lesser acclimatised northern hemisphere riders suffer. It’s no surprise that, in recent years, athletes have been employing HA strategies in preparation for such events to minimise the disadvantage they face.
Not just for pros
You may be reading this and thinking “that’s great and all, but I’m not a globetrotting WorldTour cyclist”. Fortunately that’s not the end of matters. Recent evidence has emerged showing HA can benefit exercise performance in cool climates, not just hot. This has huge implications — with the correct application any cyclist may be able to benefit from HA, under any set of conditions. In fact it’s likely that fit, moderately trained cyclists have more to gain from HA than elite cyclists whose blood has little room left for further enhancement.
A key study comes from Lorenzo et al. who heat acclimated 12 sub-elite cyclists using 90 minutes of moderate cycling per day for 10 consecutive days. Cycling was low/moderate intensity and was conducted in conditions of 40°C and 30% humidity. Following acclimation, performance improved significantly in the heat, but crucially also in cool conditions (13°C). VO2max and lactate threshold both increased by 5%, which was linked to a 6% improvement in TT performance. Other studies have reported similar results, with improvements in VO2max and lactate threshold common.
In many ways the principles of HA are very similar to those of altitude training, even if the specific physiology is somewhat different. Heat and altitude are challenging environmental conditions, both placing stress on the body’s physiological systems. Over time the stress becomes a stimulus for adaptation to the environment, and the adaptations produced can benefit performance on return to normal conditions; not just in the heat or at altitude.
What makes HA exciting from an applied perspective is that it is much more convenient to conduct than altitude training, making it a realistic training tool for the masses, not just a committed minority.
So, how can I heat acclimate?
There are two main ways of acclimatising to the heat: active and passive protocols.
These protocols involve exercising in the heat. They create optimal acclimation as they better replicate real-world conditions than do passive protocols. The downside is active protocols can be inconvenient and may detract from other training.
1. Environmental chamber: An environmental chamber is a room where conditions can be strictly controlled (temperature, humidity, oxygen and carbon dioxide content). Chambers represent a ‘gold standard’ and are commonly used for high-quality research.
These facilities, though excellent, are hard to gain access to and are very expensive to use, making them an unrealistic option for most. For those who can access such facilities a typical protocol will involve one to two hours of sub-maximal cycling for 7-10 consecutive days. The benefit of a chamber is that you can set exact conditions, with optimal conditions being those that replicate the anticipated temperature and humidity of the competition/event you are acclimating for.
2. ‘DIY’ chamber: This method won’t be found in the scientific literature but is known to be used by even by pro cyclists as a cheaper, more convenient alternative to an environmental chamber. This method involves cycling on a turbo trainer at home under conditions that will elevate body temperature. This can involve turning up the central heating to increase room temperature and using no fan.
Though somewhat rudimentary, the combination of heat production from exercise and hot environmental conditions will raise core temperature and induce sweating, which are the fundamental stimuli for heat acclimation.
These protocols may not be quite as effective as active ones, however they are less time-intensive, easier to conduct and won’t detract from your regular training routine.
3. Hot-water immersion: This method involves hot-water bathing immediately after your ride or cardiovascular-based workout, whilst your core temperature is still elevated from exercise. A study by Zurawlew et al. found that 40 minutes of bathing in 40°C water for six consecutive days improved the 5km running time of participants by 5%.
Unfortunately this improvement was only observed under hot running conditions, meaning it may only beneficial to those acclimating for a hot environment; and it has not yet been proven effective in cyclists.
4. Post-exercise sauna: This method is identical in principle to hot water immersion, but employs the use of a sauna rather than a bath. This method would be a great option following a spin class or other cardiovascular-based workout at a gym/leisure centre. One running-based study applied 12 post-exercise sauna sessions spread over three weeks. HA produced a 32% increase in time-to-exhaustion in cool conditions, equating to a 2% improvement in TT performance.
Another study currently being conducted is showing significant improvements in VO2max and lactate threshold in cool conditions, following three sauna exposures a week for three weeks.
How will I know when I’m acclimated?
Whilst some markers of acclimation require technical equipment, others are easy to measure. Acclimation will increase blood volume and improve circulation, reducing strain on the heart. This can be observed through a reduced heart rate when working at the same absolute intensity as in the pre-acclimation state, and this should be more noticeable under hot conditions.
Another easy-to-measure marker of acclimation is sweating. Acclimation increases sweat losses; as a result you should find greater weight loss pre vs post workout in the acclimated state versus the non-acclimated state. It is essential to compare sweat loss following an identical workout, and weigh yourself nude (as clothes can retain sweat and affect the reading). Though harder to quantify once acclimated you should also experience an increased thirst drive, reduced perceptions of effort and improved comfort in the heat.
So how does it work?
Improvements in thermoregulation and exercise performance are due to a multitude of adaptations which are all inter-connected. Arguably the single most important adaptation is plasma volume expansion; more simply put, an increase in the clear fluid component of the blood. This plasma expansion causes a slight reduction in haematocrit (dilution of the blood).
For many cyclists this may sound odd as most riders seek increases in haematocrit not reductions, with some riders even resorting to the use of EPO doping to achieve this. Plasma volume expansion is beneficial though as it improves the efficiency of the circulation, aiding the supply of nutrients to the muscle and better removing waste products. In hot conditions the increased plasma volume is even more useful as there is essentially more blood available to supply both the muscle for exercise and the skin for cooling.
Other important adaptations include improved sweating dynamics, improved skin blood flow, improved cardiac efficiency, improved thermal tolerance, lowered metabolism and glycogen sparing.
Things to keep in mind
Heat acclimation can be a great way to boost your fitness, but don’t overdo it! Know your limits and stop yourself if you feel unwell. Heat acclimation will require you to be uncomfortably hot, but it is essential to listen to your body and not ignore any symptoms of exertional heat illness.
Symptoms typically start with severe cramps, but this can progress to heat exhaustion, and progress further to heat stroke. More details regarding heat illness can be found here.
People typically underestimate how much they sweat during exercise and fail to rehydrate properly. If conducting a HA programme you should ideally track sweat losses and rehydrate appropriately following acclimation sessions. A good recommendation is to drink water with added sodium at 1.5x the volume of sweat loss.
Exercising in a dehydrated state will offset the benefits of heat acclimation, so it is essential for both health and performance to stay hydrated.
Much like altitude training, the beneficial effects of HA are only temporary, and so the timing of a HA protocol must be considered. A recent review concluded that key adaptations are lost at a rate of roughly 2.5% per day, so the closer HA is placed to competition the better. For athletes who don’t want to conduct HA during their tapering period an alternative option is to conduct HA 2/3 weeks before competition and conduct occasional ‘top-up’ acclimation sessions during the taper to competition.
Heat acclimation certainly doesn’t have the same popularity as altitude training, but that may soon change. HA has the potential to vastly improve performance in hot conditions, and can also be of significant benefit in cool conditions. HA can be made convenient and affordable, and could be a great way to take your cycling fitness up a level.
About the author
James Gibbon is a postgraduate research student at The University of Birmingham, England. Studying in the School of Sport and Exercise Sciences, James’ current research is focused on sauna-based heat acclimation, which is producing very promising results. James works within a larger group which focuses primarily on tackling research in environmental physiology.
James has worked first-hand with world-class athletes in extreme environments, and knows just how important acclimation can prove. He is also a passionate recreational cyclist who combines his love of cycling with science, and intends to pursue a career as an exercise physiologist in the near future. You can find James on LinkedIn.