Sports Nutrition Basics – Fatigue

It doesn’t seem so obvious, but one of the most basic aspects of sports nutrition is to understand how fatigue limits performance during sport. Think about it – nutrition is often promoted as helping to delay or prevent fatigue, or improve athletic performance. So if this is correct, then there must be an interplay between nutrition and how the body develops or experiences fatigue.

Fatigue – just a theory

There are several proposed theories to explain how fatigue occurs in the body during exercise. I must emphasise that they are only theories, since you cannot definitively prove that these things exist. You can however show that something doesn’t exist by disproving it. Many of the explanations you’ll find in textbooks or on the internet have been around since the 1930s, and regarded as “proven” because they have not been shown otherwise. However in the past decade or so a series of research has been published that now challenges some of the fundamental principles of these traditional explanations.

The theory of fatigue I will explain here is the most recent based on available research, and best explains what happens in the body during exercise both in the lab and from observation of competition. The theory is known by a few names, but the most commonly used name currently is the “Anticipatory regulation” theory. It was developed largely by sports scientists in South Africa, two of which did their PhD’s on the topic. These two guys have given a very detailed description on their blog – my aim here is not to rewrite their story, but to explain the theory briefly before looking at the interactions with nutrition. If you’re interested in the details and science behind it I’d strongly encourage you to read their blog by clicking here. Align Center

Anticipatory Regulation of Fatigue

The “anticipatory regulation” theory of fatigue was proposed from some simple observations that could not be properly explained by previous explanations of fatigue:

  • Why is it that athletes can be “fatigued” at the end of a race such as a marathon, but actually speed up in the last few hundred metres?
  • When people exercise in suboptimal conditions that would be expected to cause fatigue more quickly (eg. in the heat, at altitude or if the athlete has a maximised storage of carbohydrate in the body), why is it that they start slowing down early on during exercise, without even realising it, and before there is any sign of fatigue or discomfort?
The anticipatory theory proposes that the body “anticipates” fatigue based on a variety of signals from the body and the environment. These include information about:

  • Core body temperature, the speed with which core temperature is rising and how quickly it is likely to rise given the environmental conditions.
  • The body’s blood glucose level, the availability of carbohydrate based on how much is stored in the body (as glycogen), the rate at which it’s being used, how much is coming in from food, and how much will be needed to complete an exercise session whilst maintaining a safe blood glucose level.
  • The pH (acidity) of the body’s cells and in the blood, how much the acidity is likely to increase (pH reduces as acidity increases) and whether this is safe for the body.
  • The concentration (or dilution) of sugar and electrolytes in the blood, how this is changing during exercise through sweating and drinking fluid and whether severe dehydration is likely to result during the exercise session.

Pacing to prevent failure – how the body avoids catastrophe

Failure of one or more body systems occurs when the maximum tolerated level of exercise is exceeded, and thus the athlete is physically unable to continue. These failures include:

  • Core body temperature is too high causing collapse due to heat exhaustion. Of course you’ll start feeling the effects of increased core temperature before you get to this point.
  • Hypoglycaemia (low blood sugar) causing collapse or unconsciousness. This is preceded by “hitting the wall”, “bonking” or “hunger flatting” which causes a rapid drop in performance. This occurs when an athlete paces so that they use their carbohydrate stores up before the end of an event (without adequately replacing it).
  • Failure caused by severe dehydration - blood becomes too concentrated in sugar and electrolytes because of the absence of water. Preceded by thirst and feeling of dehydration.
  • Metabolic acidosis, causing collapse and vomiting – the body’s cells and blood have become too acidic from high intensity exercise, which causes an accumulation of lactate and acid (NB. it’s the acid NOT the lactate that causes fatigue – a topic for another post). This is what occurred to Australian sprinter Sally Pearson as she crossed the finish line in the 4 X 400m relay at the 2010 Commonwealth Games.
  • Injury caused by overuse of muscles and/or joints. Preceded by physical pain in the relevant muscles or joints.
It is interesting to note that in sport these extremes of fatigue are fairly rare. Our brains are simply too smart to let us hurt ourselves – we experience such a strong unpleasant sensation or feeling that we are forced to stop exercise BEFORE we do damage. In exceptional circumstances exceptional athletes can push deeper into this feeling of unpleasantness, such as in Olympic competition. This either results in spectacular performances or spectacular failures.

The anticipatory theory proposes that the body “anticipates” fatigue by comparing its current situation against what it expects to be coming. In other words the current pace, the required distance to go and how much “reserve” is left to get them through to the end. This provides the athlete with the “feeling” of how hard they are working, and they then adjust their pace to prevent their “reserve” being used up before the end of the exercise session.

Perceived exertion – how athletes “feel” fatigue

At the core of the anticipatory theory is that athletes in the real world unconsciously pace themselves based on how they “feel”, and this feeling is the body’s response to all of the signals mentioned above (and probably more). When studies are done of athletes exercising in race-like conditions (eg. told to run 15km in the fastest time they can), this “feel”, which is known as Rating of Perceived Exertion (RPE), virtually always starts at the lowest score and finishes at the highest score. Exceeding the maximum RPE means that one or more of the failures mentioned above occur, resulting in the inability to continue exercising. If an athlete doesn’t reach maximum RPE when the exercise finishes then they still have a “reserve” that hasn’t been completely utilised. In other words the athlete hasn’t maximised their potential.

Because athletes alter their pace to prevent exceeding maximum RPE before the exercise finishes, a marathon run in the heat (or at altitude) will be slower than in cooler conditions. But regardless of the conditions, the pace changes made unconsciously by the athlete will ensure that the rise in RPE remains the same. In other words, if you’ve paced yourself accordingly you won’t “feel” any different; you’ll just be faster or slower. This is an interesting point to consider when looking at factors that improve performance such as nutrition. When you optimise nutrition you won’t “feel” any different, but you will set a better pace and improve your performance.

What's the relevance to nutrition?

The main reason for introducing the concept of fatigue is because nutrition strategies can play a role in most of them. Hypoglycaemia can be prevented or delayed in endurance exercise with adequate carbohydrate, both before and during exercise. Nutritional supplements have the ability to buffer against the lowered pH in both the blood and cells, allowing athletes to perform better in high intensity exercise. Novel nutrition strategies to slow the rise in core body temperature were developed by sports dietitians in the lead up the the Beijing Olympics, and the importance of hydration has been understood by athletes for years.


So to wrap this up, we know that the body can theoretically fail in several different ways from overexercising – these are the ultimate end points of fatigue. But we also know that in the real world of competition (and studies where athletes are free to set their own pace) athletes rarely experience this type of fatigue, because they unconsciously alter their pace to suit the demands of the exercise session.

Nutrition has been shown to have a significant impact on many of these physiological limits. In the coming posts I’ll start to describe how we can use nutrition strategies to maximise performance by providing a greater “reserve”, allowing us to set a faster pace. You won’t “feel” it working, because the increase in RPE will remain the same, but you will be producing a higher speed or power output for the same level of RPE. And the ultimate proof will come as you cross the finish line, or finish the match.


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