What goes first – the legs or the head?

When fatigue strikes and you can pedal no more, it is purely a physical phenomenon or is it your brain forcing you to slow down?

Call it what you want – the bonk, meeting the man with the hammer – we’ve all experienced that moment on a tough ride when you just can’t turn the pedals any more. But what part of you has reached its limit? Is it that your legs have genuinely reached a point of fatigue that renders them incapable of any more work, or is it your brain calling a halt? Could you eke out more revolutions if only you could muster the will?

Certainly both physical and mental strength have their parts to play in conquering a savage ride, but it’s not always obvious what is going on beneath the surface that makes your body grind to a stop, so let’s examine the two systems separately, starting with the physical.

Running on empty

‘You’re in a situation where you’ve been working at an intensity and duration that has seen you run out of glycogen. As a result you can’t contract the muscle enough to sustain the effort,’ says exercise physiologist Garry Palmer of Sportstest. To keep moving, you need fuel, and that comes in the form of glycogen.

When you take on carbohydrates (sugars), your body stores it as glycogen in the muscles and the liver, and then converts it to glucose during exercise, especially at high intensity. Your body can store up to 500g of glycogen, so for a 70kg athlete riding steady-state over an 80km time-trial and burning 5g of glycogen per minute, they’ll be empty after about 100 minutes. As an event typically takes upwards of two hours to finish, there’s a clear shortfall. That’s where taking on food during the ride comes into play but, even then, around 90g of carbs per hour has been proven to be the maximum the body can absorb without the probability of stomach cramps or diarrhoea. If the energy requirement is higher than this, the rider could well then become glycogen depleted and potentially come to a standstill.

Over-heating and dehydration have also been identified as a cause of fatigue. ‘70% of the energy produced in the muscle is dissipated as heat,’ says Palmer. ‘That means you must cool more as exercise intensity increases.

This leads to increased sweat rate and, if you’re not replenishing quickly enough, you will be dehydrated, which can lead to fatigue. Things are worse when it’s hot or there’s little windchill, like when riding uphill.’

Theoretically, you can require 2-2.4 litres of fluid per hour during hot, intense rides, which isn’t really feasible to either consume or carry. ‘The situation’s worse for heavier riders because they generate more heat, so
one way to lessen the potential problem of fatigue is to shed a few kilos,’ Palmer adds.

Feel the burn

There is another common way your body can physically stop working, even when there is plenty of fuel left in the tank.

‘That’s down to acidosis,’ says David James, professor of exercise physiology at University of Gloucestershire. ‘When you’re cycling at high intensity, you need to produce energy rapidly by anaerobic means, ie without oxygen. Ultimately, this creates higher acidic levels in the muscle.’

We’re all aware of lactic acid. It’s a by-product of anaerobic metabolism and is cited as a reason for ‘the burn’. Yet it’s actually the hydrogen ions that come with the increasing levels of lactate that create the problems. When exercise intensity is high, lactate can’t be broken down quickly enough and builds up along with the hydrogen ions. The point at which acid tips over into the bloodstream is your anaerobic threshold, with blood lactate levels of 4mmol/l regarded as a rough guide.


‘Regular training helps with clearance,’ says James, ‘and that’s down to a mix of longer, lower-intensity rides and shorter, high-intensity efforts. Bicarbonate loading has also been mooted as one way of lowering the acidic level in the blood.’

Ultimately, whether it’s lack of fuel, poor hydration or lactic overload, it’s plain that the human body has a physical point beyond which it can’t function properly. But do we ever actually reach that point, or does the brain step in to bring you to a halt before the body arrives at its physical limit?

For years, it was assumed that these physical limiters were the reason for ceasing a session early, until exercise physiologist Tim Noakes proposed his central governor model of fatigue (CGM) in the late 1990s. Noakes believes the point in the training session when you feel you’ve given everything isn’t a physical phenomenon but actually a signal from the brain to slow down to preserve your body from overheating. Noakes would argue each of us has a cerebral black box that judges the danger of physical exertion. By training right, you can demonstrate to the brain that the body will survive ever longer and more intense sessions and so break down these mental constraints.

It’s a theory that has divided opinion among exercise physiologists, including Samuele Marcora, professor of exercise physiology at Kent University. ‘It’s bollocks,’ the genial Italian says bluntly. ‘It doesn’t add up because you can’t test it. And if you can’t test it, in my opinion it’s not science.’

Proof and perception

Marcora’s background is clinical psychology. In 2008 he proposed his alternative model, entitled ‘the psychobiological model of fatigue’. Marcora’s theory suggests that it’s not the physical limits or a subconscious message that stops us but our perception of fatigue.

Marcora had 10 male athletes perform a simple exercise protocol on stationary bikes. They rode as hard as they could for five seconds. The subjects then rode at a fixed power output until they could no longer sustain the wattage – for around 12 minutes on average – before immediately repeating the five-second max test. Marcora found that power output from the second max test was roughly 30% lower than the first, but was still three times greater than the power generated when riding to exhaustion. So how is it that the subjects couldn’t sustain 250 watts at a steady pace, for example, but then managed to knock out five seconds at an average of 750 watts?

‘It’s down to motivation, which impacts perception of effort,’ says Marcora. ‘When the effort is perceived as maximal or when the effort required eclipses the amount of effort you’re willing to exert, you stop.’

Unlike Noakes’ subconscious model, the psychobiological model is a conscious awareness of the brain and neural system, which sends signals to the active muscles and ties in with the physical models.

‘If you sense your muscles have weakened, you have to increase the activity of central motor command to compensate. This is perceived as an increase in effort and will stop you. Signs of “weaker muscles” are physical aspects such as lower glycogen levels or acidosis. However, they have an indirect impact rather than directly stopping you. It’s why caffeine is used. It’s been shown to lower the perception of effort.’

As well as encouraging exercising, Marcora has devised cerebral activities designed to reduce your perception of effort. The first focuses on subliminal messages. These affect your subconscious brain and are based on cognitive science. In one study, Marcora showed that positive words like ‘go’, ‘energy’ and ‘lively’ motivated the group more than negative words like ‘stop’, ‘toil’ or ‘sleep’, extending their workout time by 17%.

Then there’s brain endurance training, a project Marcora’s received a grant from the MOD to develop. Key is ‘response inhibition’, which works in an area of the brain called the anterior cingulate cortex (ACC) and is linked to motivation and effort, as shown by studies into mice that had this part of their brain removed. It turned them ‘lazy’ and they grew fat. Marcora showed that by stimulating the ACC three times a week via ‘focusing’ exercises, athletes perceive less fatigue. Marcora and his team are currently working on apps that will exploit this.

If he’s right, it seems the brain is the first to push the stop button when the going gets tough, but that it can be tricked into letting you tap more energy reserves. And, if all else fails and you’re struggling at the Fred Whitton, simply take a lead from American researchers who showed that swearing increases pain tolerance. Whether the marshalls will perceive that as an ergogenic tool remains to be seen.