The notion of fatigue

For athletes, winning involves the notion of pushing past their limits. Their physical resources are limited, however, and pushing beyond these boundaries sometimes requires a significant investment in terms of training.

In too many cases, the additional exercise volume only brings minimal results.

Only some people realize that fitness is one of many factors behind achieving athletic excellence. Muscle fatigue is the other dimension of athletic success.

Obviously, fatigue is affected by the level of physical fitness, but several other fairly complex mechanisms are also incorporated into the equation. For an athlete to reach new levels of performance, it becomes necessary to understand and effectively manage muscle fatigue mechanisms.

All athletes inevitably reach their maximum capacity. At that point, they can no longer maintain the same level of effort, which defines muscle fatigue.

From this angle, we perceive fatigue as an absolute and unavoidable reality. However, it is essential to realize that fatigue always appears due to a specific task or performance.

Types of muscle fatigue

Thus, there are several types of muscle fatigue, each presenting its own set of particular characteristics. In short, with an understanding of the specific context in which athletes deploy physical effort, the determinant factors of fatigue can be more accurately identified so they can be "manipulated."

First, let's look at the definition of muscle fatigue in physiological terms. You will see that it is more easily explained than you might think. Obviously, how fatigue is expressed depends on the activity. Running a marathon, playing hockey, or playing football involve completely different biomechanical, physiological, and muscular mechanisms, even though these sports rely on the same fundamental components, i.e., muscles and their ability to produce energy.

When two people practice the same sport with the same intensity of effort, we usually predict that the most physically fit athlete will tire less quickly.

VO2max

But is this always the case? Consider, for example, endurance sports such as running, cycling, biathlon, triathlon, cross-country skiing, etc. With these sports, the level of physical fitness is determined by the maximum oxygen consumption capacity, referred to as the VO2 max. If we compare two cyclists, the first with a VO2max of 75 and the second with a VO2max of 72, we could assume that the fittest cyclist, with a VO2max of 75, would tire less quickly than the one with a VO2max of 72 for the same intensity of effort.

This conclusion, however, does not consider other factors that determine physical fitness: the gas exchange threshold (GET - beginning of the intense effort zone where you burn carbs) and the maximal lactate steady state (MLSS - beginning of the severe effort zone, where you burn carbs and accumulate blood lactate).

For example, our athlete with a VO2max 72 may reach his GET and MLSS later than the other cyclist, producing a higher power with less fatigue. Thus, for the same intensity of effort, the athlete with a VO2max of 75 will have reached his severe effort zone, which inevitably leads to exhaustion, while the other is still in a zone he can sustain more easily. The level of physical fitness (VO2max) alone cannot explain muscle fatigue.

Let's think now of a marathon. Because of fatigue, a runner's speed drops gradually and constantly throughout the race. So why can marathon runners increase their speed over the last few kilometers of a race? Usually, muscle fatigue would make such a phenomenon technically impossible. However, this renewed energy is commonly seen in all sports and disciplines.

Let's stay with endurance sports for a moment. How long can you maintain your maximum effort while running or cycling? Most people say 2, 3, or 5 minutes at the most. But imagine if I set you up on a treadmill and asked you to run at a speed that would enable you to reach your VO2max. Once you have reached your maximum level, I progressively reduce the treadmill speed. You can then keep running at VO2 max for another 30 minutes. Here, the notion of fatigue at high intensity takes another hit.

The origins of fatigue

The fact remains that every person eventually reaches their limit. Fatigue takes over the body, and no human can sustain a given effort indefinitely. To decipher the fatigue process, science has explored various avenues to understand its origins.

The systems under study include the central nervous system, reduction in glycogen reserves, muscle energy reserves, and reduction in ATP (adenosine-triphosphate) reserves.

At this point, energy substrates (carbohydrates, lipids, and proteins) are used in ATP synthesis. ATP provides the chemical energy required for muscle contraction. Although energy substrate levels diminish under effort, ATP synthesis remains constant. Energy production is therefore not diminished. Once again, we need a more complete explanation than the reduction in energy reserves.

And what about blood lactate? Long considered as metabolic waste, it is actually a source of energy because it is used by muscles and other organs such as the heart to produce energy. Beyond the MLSS, we understand that, in the end, the maximum oxygen consumption capacity will inevitably be reached, quickly followed by a reduced capacity to sustain the effort. Blood lactate appears to reflect effort intensity and the body's limits in using this source of energy rather than the reason for muscle fatigue.

Muscle cells tire under effort. What is often overlooked, however, is that, at the same time, other cells are recovering. Again, we are struck by an oddity of nature – muscle fatigue and recovery co-occur under effort.

The central nervous system's ability to send an optimum signal to the muscles is also reduced, but the signal is never completely eliminated. Furthermore, a less-than-optimum signal does not necessarily lead to diminished performance.

Thus, you might expect a work team that is physically and mentally tired after working long hours to deliver a poorer performance than the relief team that comes in fully refreshed. The apparently tired team is often more efficient than the relief team since they can maintain better-coordinated actions. Muscle fatigue does not always lead to diminished capacities; it can help achieve the ideal performance rhythm.  

In short, the physiological dimension only sheds partial light on the nature of muscle fatigue. What about the psychological aspect? Could the way we think explain the end-of-race renewal of energy mentioned above?

The psychological side of fatigue

Our laboratory looked at several perceptual, sensory, and cognitive factors related to athletic performance. We thus manipulated several variables that affect human and motor behavior: fear, stress, anticipation, expectations, sensory perceptions, perception of time, and perceived pain.

In terms of endurance, our observations indicate that in doing so, we do not directly improve an athlete's physical fitness (VO2max). However, we manage to delay the onset of fatigue or alter its significance for the athletes, considerably increasing their performance.

Fear, worry, stress, anticipation, negative emotions, and interpretation of sensory information significantly impact athletes' physiological and biomechanical reactions. Thus, as athletes' brains are "hijacked" by information they perceive as threatening, they will experience fatigue prematurely.

Although this may seem strange to you, I believe there is a vast difference between a 1000-meter runner who fears the pain he will suffer after 600 meters and a 1000-meter runner who is asked to be sure to run at a level of intensity so that the onset of fatigue occurs at the 800th meter. In the first case, the fear of pain is constantly on the athlete's mind, hindering his performance. Fear is not a factor in the second case since the problem has been addressed and incorporated into the race plan. In addition, the "perceived" time of onset of fatigue has been "delayed."" Fatigue is, therefore, experienced very differently by these two athletes.

Let's play some more with your perceptions... I take you to a lab, place weights into a crate, and ask you to lift it. We repeat this exercise until you have reached your maximum lifting capacity. You can lift at most 120 lbs. One of my assistants a little further away places a 120-weight in another crate of the same size and color. I ask you to give me a hand to move this crate that contains the same load you just lifted because my assistant is not strong enough to lift it by himself. You go over to the crate, pick it up, and place it where my assistant asks you to. But before completing this additional task, I didn't tell you that the crate weighs 15 lbs. Therefore, you lifted 135 lbs rather than the 120 lbs you had just identified as your maximum physical lifting capacity... Indeed, what your mind thinks or what it doesn't know affects your muscle capacity.

Most athletes spend enormous amounts of time planning their training, which is essential. Rarely, however, do we meet athletes who "plan their thoughts and physiological reactions" to enable their bodies to react with maximum efficiency.

Considering muscle fatigue as a resource

I consider fatigue as a relative rather than an absolute concept. We thus define fatigue based on the task to be performed. As a result, the type of fatigue felt, its onset, duration, and intensity become variables that can be manipulated to enable athletes to control them completely. From this perspective, muscle fatigue is based on tangible notions that are quantifiable and measurable for the athlete and can be incorporated into the athlete's performance plan. Muscle fatigue is no longer abstract, and no mental or physical battles are required. This vision transforms physical, physiological, and psychological reactions related to fatigue.

When considering muscle fatigue as a resource that can be managed and manipulated rather than a detrimental consequence, surprising possibilities can suddenly become within our reach.

Yes! Human beings are fascinating!