Denis Boucher’s Blog: The surprising factors of muscle 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 major investment in terms of training. In too many cases, however, the additional volume of exercise only brings about minimal results that are relatively unstable over time. What few people realize is that fitness is not the only factor behind the achievement of 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 also factor into the equation. 

In order for an athlete to reach new levels of performance, it’s necessary to understand and effectively manage muscle fatigue mechanisms; all athletes inevitably reach their maximum capacity. At that point, they can no longer continue to maintain the same level of effort and this is what defines muscle fatigue. From this angle, we perceive fatigue as an absolute and unavoidable reality. It is important to realize, however, that fatigue always appears as a result of a specific task or performance. Thus, there are several types of muscle fatigue, each of which can present its own set of specific 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 in turn be “manipulated.”

First, let’s look at the definition of muscle fatigue in physiological terms. You will see it is not as easily explained as you might think. Obviously, the manner in which 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.

In the case where two people practice the same sport with the same intensity of effort, we could logically predict the athlete who is the most physically fit will tire less quickly. But is this always the case? Consider, for example, endurance sports such as running, cycling, 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 VO2 max of 75 (Rider 1) and the second with a VO2max of 72 (Rider 2), we can assume the fittest cyclist, with a VO2 max of 75, would tire less quickly for the same intensity of effort.

This conclusion, however, does not take into account other factors that determine physical fitness: the anaerobic threshold (beginning of the intense effort zone) and the critical power zone (beginning of the severe effort zone). It is actually possible that Rider 2 will reach his anaerobic threshold and his critical power threshold later than the other cyclist. Thus, for the same intensity of effort, Rider 1 will have reached his severe effort zone, which inevitably leads to exhaustion. The other cyclist, meanwhile, has reached an intense effort zone and can go on riding for a long time. Therefore, the level of physical fitness (VO2 max) alone cannot explain muscle fatigue.

In addition, Rider 1’s speed drops gradually and constantly throughout the race. So why then do we see marathon runners increase their speed over the last few kilometers of a race? Normally, muscle fatigue would make such a phenomenon technically impossible. This type of renewed energy, however, is commonly seen in all sports and disciplines. Let’s stay with endurance sports for a moment. How long do you think you can maintain your maximum effort while running or cycling? Most people would say two to five 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 maximum oxygen consumption level. Once you have reached your maximum level, I progressively reduce the treadmill speed. You will then be able to keep running at maximum capacity for another 30 minutes. Here, the notion of fatigue at high intensity takes another hit.

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?

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In our laboratory, we look at several perceptual, sensory and cognitive factors related to athletic performance. We manipulate 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 (VO2 max).

We do, however, manage to delay the onset of fatigue or alter its significance for the athletes, which considerably increases their performance. Fear, worry, stress, anticipation, negative emotions and interpretation of sensory information have a significant impact on the physiological and biomechanical reactions of athletes. Thus, as the brains of athletes 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 huge 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, which will hinder his performance. In the second case, fear is not a factor, since the problem has been addressed and incorporated into the race plan. In addition, the “perceived” time of onset of fatigue has been “delayed.” Therefore, fatigue is experienced differently by these two athletes.

Let’s play some more with your perceptions. I take you into my laboratory and place weights into a crate and ask you to lift it. We repeat this exercise until you have reached your maximum lifting capacity – let’s say 120 pounds. One of my assistants a little further away places a 120-pound 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 I omitted to tell you the crate itself weighs 15 pounds. You therefore lifted 135 pounds rather than the 120 pounds 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, nutrition and recovery period, which is obviously essential. Rarely, however, do we meet athletes who “plan their thoughts and their physiological reactions” to enable their bodies to react with maximum efficiency.

In our laboratory, we consider fatigue as a relative rather than an absolute concept. We thus seek to define fatigue on the basis of 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 take full control of them. From this perspective, muscle fatigue is based on tangible notions that are quantifiable and measurable for the athlete and which can be incorporated into the athlete’s performance plan. Muscle fatigue is no longer an abstract idea; this vision transforms physical, physiological and psychological reactions related to fatigue.

By looking at muscle fatigue as a resource that can be managed and manipulated, rather than a detrimental consequence, surprising possibilities can suddenly come within our reach.

Yes. Human beings are fascinating.

Dr. Denis Boucher holds a Ph.D. degree in experimental medicine. He manages an exercise physiology laboratory in Quebec and a human performance consulting company in the United States. He has conducted the pre-season on-ice fitness evaluation program for the Philadelphia Flyers. His clinical expertise is in the fields of exercise physiology, nutrition and sport performance. He currently hosts and produces a weekly radio show on XM172 entitled ‘The Little Scientific World of Doc Boucher’ (in French). He will blog for throughout the season.