Ice baths have been floating around for some time now as the coolest way to recover post-workout. But another way of enhancing your endurance performance that’s gaining traction – particularly in running circles – lies at the other temperature extreme. Heat training, or heat acclimation, involves exposing your body to warmer temperatures, either through exercise or more passive ways like saunas, to enhance fitness results.
It’s sometimes called a substitute for altitude training, as it mimics a few of your body’s adaptations experienced when at higher elevations with less oxygen available. This includes increased red blood cell production, which improves oxygen delivery to your muscles by enhancing the production of adenosine triphosphate (ATP), their main energy source.
But does it really work? While research has suggested that short-term heat training before endurance events in hotter climates helps athletes to regulate their body temperature and become more accustomed to the heat, it may not be suitable if you’re not aiming to exercise in higher temperatures in the near future. Here’s all you need to know.
Meet the experts: The information here has been checked and approved by: Lawrence Tackie, PT at Anytime Fitness with 15 years’ experience and a background in semi-pro sports like boxing and football; Joe Bingham, endurance competition, strength training and body transformation PT at Core Studio; Ellie Walting, PT, ambassador at realbuzz and founder of Rewild and Reset, wellness retreats encouraging reconnection with nature; Sarah Campus, a women’s and NASM-qualified PT and coach with over 10 years’ experience, and the founder of Soho House Chiswick Run Club.
What is heat training?
Heat training involves exercising in a warmer environment (active heat training), such as doing hot yoga, or exposing your body to high temperatures like saunas or hot baths (passive heat training) with the aim of improving your aerobic performance, cardiovascular efficiency and thermoregulation (the process by which you can maintain a stable body temperature).
‘The aim is to reach and maintain a core temp of 38-38.5 degrees,’ says sports-nutrition postgraduate student and runner, Sian.
How does heat training improve endurance?
Heat training causes a process in your body that leads to enhanced endurance capacity using the following steps:
1. Increased plasma volume
First, when you train in the heat or add heat after working out, your body makes more plasma, the liquid part of your blood, explains sports and exercise postgraduate sports and exercise student and runner, Charlie Norris. This is due to your body using more plasma to produce sweat, which triggers a hormonal response to expand plasma volume.
A review somewhat confirms this, highlighting that plasma volume usually expands after repeated heat exposure and another 2010 study showed that heat acclimation increased plasma volume. Increased plasma provides cardiovascular stability – your heart and blood vessels functioning consistently and effectively – and improves thermoregulatory function, or your body’s ability to maintain a stable internal core temperature, confirms research, allowing you to train more efficiently in the heat.
2. Increased red blood cell production
Increased plasma ‘will water down your red blood cells’, lowering hematocrit, the percentage of your blood made up of red blood cells, says Norris.
Your kidneys detect the relative drop in red blood cells. They release more erythropoietin (EPO), the hormone that stimulates red blood cell production, ‘to balance out’ the increased plasma, research and Norris explain.
Red blood cells – and in particular haemoglobin, a protein in these cells – carry oxygen from your lungs to the rest of your body. More red blood cells therefore mean more oxygen delivered to working muscles to increase endurance (for exactly how, keep scrolling!).
Research confirms this, with one study showing that five weeks of heat training increased the amount of haemoglobin and improved exercise performance in 23 (male) cyclists.
3. Better oxygen delivery
More oxygen in your muscles improves performance because it helps your body produce energy.
You makes adenosine triphosphate (ATP), your body’s energy molecule, using oxygen and food. Your muscles then use ATP to move and contract during aerobic activities like running and cycling – they can’t work without it. More ATP leads to being able to exercise for longer without getting as tired.
One study backing this up suggests that heat exposure leads to increased aerobic performance by proposing that heat exposure can encourage ATP production.
4. Increased sweating
Heat training improves your body’s capacity for thermoregulation, or maintaining a stable internal body temperature. Your body starts sweating sooner and produces more sweat overall, allowing your body to keep cool more efficiently by taking heat away through evaporation. A 2015 review found that exercising in heat improved sweating by making people sweat earlier and at a higher rate, ‘improving evaporative cooling, which reduces skin temperature and heat storage.’
Related StoriesIs heat training effective?
‘Heat training has its benefits – it’s great at boosting endurance, recovery and performance – but it needs to be done correctly and it’s certainly not a one-size-fits all,’ says women’s personal trainer with over 10 years’ experience, Sarah Campus. ‘Structure is important and you need to give time for the benefits to be seen.’
Some experts believe it is the most important thing an athlete can do when preparing to compete in the heat, like a marathon on a hot day, or in another country, with one review saying that heat acclimatisation was the ‘most important intervention one can adopt to reduce physiological strain and optimise performance’. Sian also notes that one study analysing heat training on eight athletes aged 38-46 before a multi-day desert marathon found that four days of 60 minutes at 45ºC and 30% relative humidity improved:
- Peak thermal comfort (where a person feels comfortable with the environment temperature and is neither too hot or cold
- Perceived exertion
- Feelings of fatigue
- Sweat rate by 14%
Another study pre-print released in May this year investigated the effects of post-exercise hot-water immersion on physical adaptations and performance in normal and lowered oxygen levels. 20 healthy volunteers (12 male, 8 female) completed cycling intervals (4×4 min at 90 ± 5% maximal heart rate, 3x per week) followed by hot water immersion at 42°C for 40-50 min, five times a week. After six weeks, they showed significant performance adaptations including:
- Lower resting heart rate
- Lower core temperature
- Increased haemoglobin concentration
- Greater time-to-exhaustion (under reduced oxygen conditions)
During the time-to-exhaustion test they also showed:
- Lower tidal volume (the amount of air inhaled or exhaled during a normal breath)
- Increased oxygen saturation (the percentage of haemoglobin carrying oxygen)
‘Beyond the physical gains, there’s a significant mental advantage that comes from committing to structured training,’ notes endurance competition and strength training PT, Joe Bingham. ‘It builds resilience and sharpens mental tolerance. If you can show up and push through in tough conditions, you’ll feel even more capable and composed when things are easier.’
How to practise heat training
Sian advises using heat exposure in the following ways:
- Hot baths (20-30 minutes). ‘Sit in a comfortably warm bath after exercise for 20 minutes, four or five times a week,’ suggests Norris – ‘That’s it. No extra training required.’
- Sauna
- Hot yoga
- Hot training camps
- Extra layers on easy runs or – as Norris advises – on the treadmill
To prepare for an event, ‘It should be done for a minimum of four days and it must be performed 10-14 days before the target event,’ she says.
Endurance optimisation coach Dr Phil Batterson notes that you need: ‘2–4 sessions to start adapting, 7–10 days for meaningful changes, 14+ days to see a full adaptation and 5+ weeks to increase your haemoglobin mass’. For this reason, before a competition, he recommends this pre-competition schedule:
- Start 3–6 weeks before your race
- Add 2–3 heat exposure sessions/week — run, bike, or train in the heat for 45–90 mins OR add post-workout passive heat (like a sauna or hot bath) 15–30 mins, 2–5x/week
- Practise proper hydration — don’t wait until race day to figure this out
Is heat training always suitable?
Sian notes that this style of training may only be applicable if you’re going to perform in a hot environment, and soon. Otherwise, ‘the adaptations may not help you at all,’ she reminds us. In fact, one 2015 literature review found that ‘heat acclimatisation does not transfer to improved performance in cool or moderate temperature conditions for competitive cyclists’ when matching exercise intensity based on heart rate in a hot versus indoor control environment.
Adaptations are lost as fast as they are gained, so you only need to start doing it 10-14 days before your event, she continues. Even then, it’s important to build up gradually.
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