Anyone who has really pushed their limits on a bike during an event will have experienced burning thighs, screaming lungs, a pounding heart and oceans of sweat.

But have you ever wondered what is happening to your body when you pedal? And how you can energise it to ensure you arrive at the finish line in the most efficient way?

We lift the lid on the physiology and psychology involved so you can leave the competition wheezing in your wake.

• Read more: How to get fit fast

Heart of the matter

You’re lined up on the start line. The nerves are cranked up, meaning your kidneys’ adrenal medulla is stimulated into action and secreting adrenaline.

It’s the hormonal cause of that ‘flight or flight’ mechanism and results in a cascade of cardio repercussions before you’ve even set off.

“These include an increase in heart rate and cardiac output,” says cardiologist Andre La Gerche. “It also leads to a rise in blood pressure and metabolism.”

Further adrenaline-fuelled physiological changes include the relaxation of the smooth muscle in the wall of the bronchioles, which leads to a better supply of air to the alveoli; an increase in respiration rate; and dilation of coronary blood vessels and those of skeletal muscle, thus providing increased blood supply to those areas.

So, your body’s started to prepare for the physical challenge ahead. But it’s still a shock to the system when you pedal, especially if your ride opens with a high mountain pass.

This is where ‘oxygen uptake kinetics’ comes into play. Essentially, this is the speed at which your body can deliver oxygen to working muscles from a state of rest and so click swiftly into a steady state.

Not surprisingly, the fitter you are, the more proficient your system is at flicking the switch from one to the other, with distance runner Paula Radcliffe reportedly having the fastest VO2 kinetics of all time at 8-9 seconds.

However, one study showed that recreational endurance athletes improved theirs by 30% after only six weeks of training. For this brief period, you’re leaning heavily on the anaerobic system to generate energy without oxygen.

When the oxygen supply meets demand, your aerobic system can sustainably power the effort at a constant rate (up to around 70% of your maximum heart rate).

“The speed at which your heart beats is no different between elites and recreational riders,” says La Gerche.

“No matter how hard you train, you can’t change that. But you can change its size. In general terms, the fitter you get, the bigger your heart is. A normal heart is around 300g – the size of a fist – while a Tour de France rider’s is up to a kilogram.”

Consistent training thickens the heart walls and increases chamber size. This is important during your sportive adventure due to stroke volume, which is the volume of blood pumped from the heart with each beat.

“During exercise your heart has about a 70% efficiency rate at pumping out blood. A 400g heart can accommodate around 200ml of blood, which means you’re pumping out around 140ml of blood with every beat,” says La Gerche.

“Professional cyclists’ chambers fill with around 400ml of blood. Apply that 70% figure and that’s 280ml of blood with every beat.”

Its importance becomes clear when you apply it to cardiac output, which is the amount of blood pumped out each minute.

If both our recreational and elite examples are riding at 135bpm, the former’s sending out 27l of blood each minute, while the latter’s pumping out 54l.

And as blood carries the oxygen and nutrients to serve your working muscles, you can see how a bigger, stronger heart can boost cycling performance.

This benefits higher-intensity efforts, too, because that greater blood flow also means swifter recovery between efforts, such as when the ascent undulates.

Which begs the question: during a tough ride, does your heart fatigue? “After five or six hours of intense exercise, we can put an ultrasound on the heart and see that the heart is fatigued,” says La Gerche.

“It’s easy to squeeze your leg muscles and see that they’re sore. But the effect on your heart is underestimated because it’s a silent partner. Just respect the challenge.”

As the day rolls on, your muscular system also suffers, especially if you push it well beyond what you manage in training.

This is down partly to increasing the production of reactive oxygen species, or ROS, that results in oxidative stress in the blood and skeletal muscle itself.

Essentially, it’s down to the huge number of muscular contractions required to send you sky-rocketing over the next mountain and has been associated with accelerating muscle fatigue.

It’s also mooted to be one of the reasons behind delayed onset muscle soreness (aka DOMS). If you haven’t trained as much as you’d hoped, a well-balanced post-event diet, including antioxidant-rich fruit and veg, will help.

Power carbs

Why did our early ancestors eat each other? Because they saw each other as a nutritious snack, according to a 2017 study by James Cole, an archaeologist at Brighton University.

According to Cole’s calculations, the average human body contained 125,822cals. Much of that figure is down to fat’s high calorie content, with 1g furnishing 9cals. Even a WorldTour rider, who weighs only 60kg and has 7% body fat, has more than 37,000 stored fat calories.

So why do we bonk in a sportive? Well, that’s down to exercise intensity and the fact that you’re burning predominantly carbohydrates to avoid being swept up by the broom wagon.

“Your most readily accessed carbohydrate store is muscle glycogen, which is mooted at around 500g or 2,000cals,” says Tim Podlogar, nutritionist at Tudor Pro Cycling. “Riding at a high intensity, you can burn through this in two-and-a-half hours.”

This is why Podlogar recommends filling up your glycogen stores the day before. “That means multiple meals that are high in carbohydrate and low in fibre,” he says, “easy-to-digest foods such as white rice, white pasta and even Haribos.”

Also, avoid a gentle ride late on the eve of your big day because that’ll eat into your glycogen levels.

When it comes to race day, a high-carb breakfast further boosts glycogen stocks. Porridge, a buttered bagel or white toast and jam are popular.

And then it’s riding time. Many of you will already consume gels, bars and energy drinks in search of glucose replenishment, following a traditional 60-to-90g-an-hour strategy. You might be capable of more, however, especially if you’re one of the fitter cyclists out there.

“Our research shows that many amateur riders can handle 120g carbohydrate an hour,” he says, which goes some way to compensating for the 6,000cals-plus burned during a long day in the saddle. “At an elite level, we had many riders at last year’s Giro d’Italia who were consuming way more than that.”

Feeding should start from your first pedal stroke and, according to Podlogar, comprise sugar from solids only, keeping fluids to water because separating makes it easier to monitor how much sugar and fluid you’re consuming. You can go for an electrolyte drink, albeit Podlogar says you can tick this off from most gels.

But when should you feed to optimise the sugar hit? “I recommend to my riders that they fuel well downhill.

“You’re not pedalling so it’s easier to digest; the bloodflow to the intestines increases and so absorption improves.”

Three energy hits per hour – one bar and two gels, for instance – will boost your muscle glycogen levels. Not only should this prevent a sugar low, it could prevent muscle degradation, too.

“We know that your muscle is damaged if there’s insufficient glycogen around it; it’ll break down protein to a higher degree from the muscle itself in search of energy.”

• Read more: Cycling nutrition – everything you need to know, according to the latest science

Core values

Should your event be one of the big Alpine challenges, you could well be enduring temperatures over 30°C.

That means you’ll heat up like a nuclear reactor, which isn’t great because a swathe of biochemical reactions and nerve conductivity are optimised at around the 37°C mark.

But you can’t argue with science or, more precisely, the body’s inefficiency. “It’s around 20% efficient at generating energy,” says Chris Jones of body-temperature sensor outfit Core.

“That means for every 5cals of energy you burn, only 1cal is used for mechanical work. The remaining 4cals are released as heat energy.”

This means you’re almost combustible on steep ascents. Take WorldTour rider Simon Clarke, who Core monitored on stage 14 of the 2021 Tour de France, specifically a 100km stretch that included the ascent of the Col de Montségur: 4.2km at an average gradient of 8.7%.

“This section and the following ascent up the Col de la Croix de Morts saw the fastest increase in Clarke’s core body temperature, where it hit 39.2°C,” says Jones.

“The steeper the ascent, the faster the increase in core body temperature. This is because of the increased physical intensity and because there is less cooling from the wind at slower speeds.”

On the flipside, the rapid descent of Col de Montségur saw Clarke’s core drop to 38.1°C, which was below what it was when he started the climb.

This highlights that pacing is even more integral to reaching your goals when it’s hot, although your body will try its best to lose heat via the same way it gains it, namely radiation, convection and conduction.

Sweating is key to maintaining balance in the body. Around 580cals per litre of heat energy is needed to turn water from liquid to gas so it can evaporate from your skin (or your breath).

So, whenever water leaves your body via evaporation, a corresponding amount of heat also leaves your body into the surrounding air, helping you cool down. So, it’s great for cooling but clearly not wonderful for your water stores.

You can sweat over 1.5l an hour on a sweltering Alpine sportive. Covering those losses is difficult on two fronts: the practical side of drinking from a water bottle and the physical side of absorbing so much fluid.

It might be manageable for an hour, but the deeper a rider digs into a ride, the more uncomfortable the gut becomes. If you can manage 750ml an hour, that’s a good start.

Again, training will help cool your engine because low aerobic fitness makes you more susceptible to heat illness.

There’s a strong correlation between an individual’s aerobic capacity and their ability to offload heat. Studies show that if someone’s working at half their aerobic capacity, their core temperature reaches around 38°C.

If you and I do the same amount of work and you’re twice as fit as me, your core temperature will be lower and you’ll suffer less.

Beyond training, acclimatise from home by having a 15-minute hot bath after a ride. Doing this five or six times in the final two or three weeks before your event will really help.

“Dressing lightly will also help,” says Jones. “And keep it old-school by pouring water over your body and head. That can change your perception of the pain.”

Mental maths

It’s not just your body that suffers during a sportive – your mind does, too, and you can be on the back foot from the start, according to a 2017 study, The Effects of Mental Fatigue on Physical Performance: A Systematic Review.

The authors showed that seven of eight key studies “reported that endurance performance was negatively affected by mental fatigue, which was evidenced by a decrease in time to exhaustion”. In short, don’t cause yourself unnecessary pre-race stress or you’ll be drained before the off.

Of course, even the most diligent planner can see motivation drop when they’re halfway up a mountain, miles from the finish line.

This, according to round-the-world record-holder Mark Beaumont, is to be expected and he calls it the ‘psychological arc’.

“You start fresh, dig deep at the end and finish strongly but there’s that bit in the middle where you’re far from starting and, in your head, far from finishing,” he told us. This is true whether you’re riding 50 miles or 500 miles.

To ride the arc rather than sink in, try some psychological games. Chunking is one of the most common and involves breaking the event down into smaller, more mentally manageable chunks.

If your next ascent features 10 hairpins, for example, tick off each hairpin. Many events supply a route sticker to smooth onto your top tube that can show natural chunks.

Ultimately, the biggest driver of your mental state is motivation. Take a 2008 study by Professor Samuele Marcora that involved 10 rugby players cycling at a fixed power output based on 90% of their VO2 max.

When they could no longer maintain that wattage, he had them ride as hard as they could for five extra seconds.

From an average power output of 242 watts in the test, despite apparent fatigue, they generated 731 watts.

The results convinced Marcora that fatigue is no physical phenomenon – if it was, how could the cyclists generate such power when they’d slowed down?

Instead, Marcora says your decision to slow or quit stems from two key areas – motivation and perception – and lays the foundations for his psychobiological model of fatigue.

When effort is perceived as maximal or when effort required eclipses the amount of effort you’re willing to exert, then you stop. In essence, fatigue is an increase in time of perception of effort.

Whether you agree with Marcora or not, one thing’s clear: despite your body’s suffering, it’s the mind that’ll often decide whether you reach the finish line or not.