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How does hot weather affect your cycling performance?

James Witts
21 Jul 2021

Your body will do all it can to stay at a stable temperature when it's hot or cold, but cycling has other ideas

This recent good weather is enough to tempt even the most indifferent cyclists out on their bikes. However there will come a point in which the temperature becomes too much and your performance starts to be negatively affected, just like when riding on the very coldest of days. 

The big issue is your body’s homeostasis – the combination of mental and physical trickery that conspires to keep your core as close as possible to around 37°C, regardless of the outside temperature or your work rate.

Tales may abound of pros producing extraordinary feats of endurance at both ends of the temperature scale, but we wanted to know how the average cyclist is affected by the external temperature, both hot and cold, and whether you can train to maximise your performance. 

Feeling the heat

‘Heat is more problematic than the cold,’ says Simon Hodder, professor of ergonomics at Loughbourgh University – even though it might not feel like it.

‘In the cold you have a strong natural heating mechanism – your exercising metabolism – but it’s much harder for your body to cool down than it is to heat up.’

Sweating goes some way to keep you cool, but its effect is limited.

Some experts believe therefore that the body has an in-built pace regulating system that prevents you going too hard and overheating, although the mechanism is not fully understood.

Professor Tim Noakes of Cape Town University suggests it ties in with his central governor model of fatigue, where a subconscious mechanism in the brain draws on factors such as experience, duration of exercise and environment to set a sustainable pace.

‘In fact, that’s where my model of fatigue derived from,’ Noakes says. ‘I realised there must be a regulator that slows people down in the heat to ensure they avoid heatstroke.’

Spain climbing

Noakes suggests this psychological limiter is why athletes rarely experience heatstroke even in extreme heat.

Core temperature does rise, however, and during exercise in the heat we settle on a figure of around 39°C.

If it creeps over 40°C, that’s when issues with heat exhaustion (feeling faint, dizzy or sick, cramping) can strike or lead to heatstroke, which is more dangerous.

That said, many of the issues around cycling in the heat stem from dehydration, which has the following effects: your blood thickens, meaning the heart has to work harder; your ability to process glucose and create energy drops because energy production requires water; the amount of blood and oxygen supplied to your leg muscles falls because blood is being channeled to the surface to cool the body.

A study from Dr Dan Judelson of California State University showed that a sustained state of dehydration impaired strength, power and high-intensity muscular endurance by 2%, 3% and 10% respectively.  

Keeping things fluid

But what level of dehydration begins to affect core temperature and impede performance?

Historically, 2% was deemed as the tipping point, but recent research from Brock University scientist and cyclist Stephen Cheung suggests this figure isn’t set in stone.

 ‘My studies showed that a loss of 3% wouldn’t impact as much as you’re told it would,’ says Cheung.

‘It might increase your heart rate a little and increase your core temperature a bit but none of our subjects reached any critical levels.’

Cheung’s research is supported by a paper in the British Journal Of Sports Medicine entitled ‘Current hydration guidelines are erroneous: dehydration does not impair performance in the heat’.

The researchers showed that when well-trained cyclists performed a 25km time-trial in the heat, their body temperature was higher beyond 17km of the time-trial but no other differences were observed.

For longer rides, a well-designed hydration plan is a must, and measuring your sweat rate is a useful starting point.

Cycle for an hour in hot conditions, drinking nothing, and weigh yourself before and after to see how much weight you’ve lost.

As a rough measure, every 1kg should be replaced with a litre of fluid, including electrolytes to replace those lost in sweat. 

cycling cold

A higher level of fitness will also help you maintain a stable core temperature. As your fitness grows, you experience a host of adaptations that include an improved sweat response to dissipate heat quickly. 

‘Improved aerobic capacity also leads to elevated plasma volume and cardiac output,’ says Cheung. ‘This minimises the competition for blood distribution between skeletal muscle and skin.’

In short, as Froome and Valverde and their colleagues rack up the miles, their bodies develop a greater capacity for, and a slower rate of, heat storage – and the same goes for you.

Familiarity breeds content

Acclimatising to hot conditions will also help, although it’s not necessarily realistic for most recreational riders.

Research has shown that healthy adults exposed to conditions that elevate their core temperature by 1°C to 2°C for 60 to 90 minutes over a period of four to 10 days will afterwards elicit a lower resting core temperature, greater blood plasma volume and an increased sweat rate.

So the advantages that British riders have in cold conditions are reversed when it comes to heat.

Froome, for instance, grew up in Africa and has a superb thermoregulatory system that means he can disperse heat and maintain his optimum core better than many of his northern-European rivals. 

Ultimately, the best practical advice for maintaining performance in both hot and cold conditions is to wear the right gear and simply to get out there and ride.

The fitter you are, the better you’ll maintain a steady core temperature and the more you’ll adapt to the extreme conditions. 

The chill wind

Windchill is something every cyclist is uncomfortably aware of, and there are various calculations to ascertain the cooling effect dependent on bike speed.

As an example, if you’re doing 25kmh in an ambient temperature of 12°C, it will feel a whisker over 8°C. In other words the 25kmh breeze has a windchill effect of 4°C.

If it’s 2°C, the windchill moves up a notch and makes it feel nearer -3°C. Since cyclists are always creating our own fast-moving draught, this presents us with a problem.

‘Your body aims to retain a core temperature of around 37°C,’ says Nadia Gaoua, senior lecturer at the school of applied science at London’s South Bank University.

‘This keeps the brain and heart performing efficiently. If your core drops by just 2°C, you begin to experience the symptoms of hypothermia.’

Cycling body temperature

Even before that, if your core temperature drops below 37°C performance will decrease for three key reasons.

First, maximum heart rate falls because your body restricts bloodflow to your peripheries in an attempt to maintain core temperature.

This results in a lowering of cardiac output – the amount of blood pumped each minute – which hampers your ability to deliver oxygen to the working muscles.

That is, as soon as you feel your fingers or toes going numb on that chilly Sunday morning, your aerobic output is already heading south. 

Also, haemoglobin’s molecular structure binds to oxygen molecules more tightly when cold.

That reduces oxygen delivery further, raising the body’s reliance on energy from anaerobic means, which means you’ll have less in the tank for that sprint to the next coffee stop.

Thankfully, countering this problem is your metabolism. Studies have shown that for every calorie of energy your muscles burn, just 25% is translated into movement.

The other 75% is converted to heat, and how much heat you produce is linked to your maximum oxygen uptake capacity (VO2 max). The higher your VO2 max, the more heat you produce. 

This internal heat production means we’re unlikely to suffer any severe effects from cold when we’re on the bike, which unfortunately gives us no easy excuse to stay at home in the warm.

‘From our research, the temperatures in the UK rarely reach such levels that cause severe physiological problems,’ says Gaoua,

‘It’s more a matter of bike control. Shivering lowers motor control, which is more likely to affect performance than a drop in core temperature.’

Hodder confirms that it’s the restricted bloodflow to the extremities to prevent further heat loss from the skin that presents the biggest problems for cyclists.

This is a phenomenon known rather unpleasantly as ‘physiological amputation’. 

‘Cooling of exposed skin happens quickly but it’s more the perception that’s uncomfortable than it is a dangerous physiological issue,’ says Hodder.

‘This is felt in the toes and hands, and also the face. You have a large surface area with little insulation so lose heat pretty quickly.’

Preventing as much heat loss as possible with gloves and appropriate layers is sensible from a comfort, control and also a performance point of view because a 1°C drop in muscle temperature (for instance, in the quads) can result in a 10% drop in performance. 

A neck warmer should complete the look. As well as filling the gap between the jacket’s neckline and your chin, you can pull it up to cover your mouth – useful for many cyclists who have a history of upper respiratory tract infections and who blame the cold on their condition.

In fact, there are tales of cross-country skiers swallowing Vasoline with the aim of coating their airways as a protective measure against cold air.

That’s not advised, but the condition does affect at least 4% of the population. Yet studies show it’s the dryness of the air and not the temperature that triggers the response.

Hence, wearing a snood or balaclava can help because it moistens the air as it’s inhaled rather than keeping out the cold.

UK cyclists will also be pleased to know you’ll perform better in the cold than Colombia’s Nairo Quintana and Eritrea’s Daniel Teklehaimanot – relatively speaking, of course.

‘We have studies that show cyclists who are used to the cold don’t show decreases in physical and cognitive performance to the same level as riders from hot countries,’ says Gaoua.

‘So someone from Britain will cope with cold better than someone from Africa, although it’s more habituation than acclimation; it’s more behavioural than physiological.’ 

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