How does load affect how fatigue develops over a set?
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Fatigue is a temporary reduction in exercise performance as a result of previous exercise. We can analyze the development of fatigue over the course of a single bout of exercise. In my previous article, I explained exactly how fatigue develops over the course of a normal strength training set. In this short follow-up article, I clarify how weight on the bar (and therefore the number of repetitions taken to reach muscular failure) affects which types of fatigue are most prevalent.
How does fatigue develop over a set, and why are the individual mechanisms important?
Introduction
Strength training often involves lifting a weight repeatedly until muscular failure. Muscular failure occurs whenever sufficient fatigue is present such that the muscle can no longer produce enough force, even at a very slow velocity, in order to lift the weight. This fatigue can arise due to many different fatigue mechanisms, including mechanisms inside the central nervous system (CNS), which lead to reductions in the level of central motor command that reaches the muscle (thereby reducing motor unit recruitment and motor unit firing rates), and mechanisms inside the muscle itself.
How fatigue develops over a strength training set
During a normal strength training set, various fatigue mechanisms develop over time. Together, these mechanisms reduce the ability of the muscle to produce sufficient force at the required bar speed, and ultimately lead to muscular failure being reached.
The first fatigue mechanism to appear is acidosis, which predominantly reduces muscle fiber shortening speed. This allows single muscle fiber force to remain high despite an increase in effort (and motor unit recruitment) being needed in order to maintain self-selected tempo. Later, calcium ion-related mechanisms also appear that reduce single muscle fiber force, which in turn reduces the magnitude of single muscle fiber mechanical tension. In addition to metabolite-related fatigue and calcium ion-related fatigue, spinal and supraspinal central nervous system (CNS) fatigue increase progressively over the course of a strength training set. These reduce both the level of motor unit recruitment that can be attained (which obviously reduces whole muscle force being produced, essentially by switching off groups of muscle fibers) and the motor unit firing rates that are attained (which reduces single muscle fiber force of the working muscle fibers).
Ultimately, the gradual accumulation of all of these fatigue mechanisms causes muscular failure to be reached. However, what is important for our purposes today is that the relative contributions of each of these types of fatigue differs depending on the weight that is being lifted.
How does load affect how fatigue develops over a set?
Introduction
Although the mainstream fitness industry tends to assume that metabolite-related fatigue is more important during light load strength training to failure and CNS fatigue is more important during heavy load strength training, this is actually the wrong way around. In fact, as we will see, metabolite-related fatigue contributes more to overall fatigue when lifting heavy loads (even though the total amount of metabolites that accumulate is indeed smaller), and CNS fatigue contributes more to overall fatigue when lifting light loads. To appreciate why this might be the case, let’s look more closely at the fatigue mechanisms involved in lifting heavy loads and light loads. Indeed, this analysis is very much worth doing, because it helps us explain why heavy load strength training and light load strength training produce somewhat different adaptations, at least in terms of gains in maximum strength.
Fatigue when lifting heavy loads
When lifting heavy loads (which are greater than or equal to 5RM), muscular failure is reached very quickly. Heavy loads always require maximal efforts and therefore full motor unit recruitment, which means that all of the fast twitch (highly glycolytic) muscle fibers are activated from the first rep of the set. These muscle fibers quickly accumulate metabolites inside them and therefore progress very rapidly through the acidosis and inorganic phosphate phases of metabolite-related fatigue, such that they quickly get to the point where they are unable to contribute to whole muscle force. Since the force produced by these muscle fibers is necessary for the weight to be lifted, muscular failure is essentially reached by the localized fatigue of a small number of muscle fibers (and the heavier the weight, the fewer muscle fibers are in fact fatigued).
In this situation, metabolite-related fatigue is absolutely critical to the attainment of muscular failure when lifting heavy loads, even though the overall muscle does not accumulate a large amount of metabolites. It is only the accumulation of a small amount of metabolites in the fast twitch muscle fibers of the highest high-threshold motor units that causes muscular failure. Indeed, this is one of those situations in which we realize that the terminology “muscular failure” is somewhat misleading because the failure to achieve the task of lifting a heavy load is actually brought about solely by the fatigue of a very small number of muscle fibers.
The lack of a meaningful accumulation of metabolites throughout the whole muscle when lifting heavy loads has an important corollary effect, which is that very little afferent feedback is generated. Thus, contrary to popular belief, the amount of supraspinal CNS fatigue is actually very low during heavy load strength training exercise.
Additionally, it is important to note that the very rapid attainment of muscular failure when lifting heavy loads reduces the contribution of those fatigue mechanisms that depend upon the duration of time for which muscle fibers are activated. Thus, all of the calcium ion-related mechanisms have very little opportunity to contribute, since they require time to accumulate and to stimulate the actions of calpains. Similarly, the contribution of spinal CNS fatigue is much reduced, because it depends upon the repeated firing of the motor neurons in order to take effect.
Overall, we can therefore appreciate that metabolite-related fatigue is the most important type of fatigue during heavy load strength training exercise, even though whole muscle metabolite accumulation is still fairly low. The levels of supraspinal CNS fatigue are low (because whole muscle metabolite accumulation is low). Similarly, levels of spinal CNS fatigue and calcium ion-related fatigue mechanisms are also low, because of the short duration of time for which the working muscles are active. Indeed, this is an important reason why heavy loads are very effective for achieving the largest gains in maximum strength, because the low levels of spinal and supraspinal CNS fatigue permit very high levels of motor unit recruitment to be achieved at muscular failure, which in turn stimulates large gains in the ability to recruit high-threshold motor units in future workouts or strength tests.
Fatigue when lifting light loads
When lifting light loads, muscular failure takes a long time to be achieved. To reach the point at which the whole muscle is unable to exert the force to lift the weight, a large proportion of the fibers inside the muscle must be greatly fatigued. This has two important implications.
Firstly, it means that the total amount of metabolites that accumulate inside the muscle is very substantial (which leads to a large amount of afferent feedback, which we detect as the fatiguing and burning sensations inside the muscle, and which causes a large amount of supraspinal CNS fatigue).
Secondly, it means that the time for which all of the working muscle fibers are active is much longer, which allows sufficient time for calcium ions to accumulate (and therefore to stimulate the actions of calpains and cause calcium ion-related fatigue mechanisms and it also increases the extent of spinal CNS fatigue, which is also time dependent, because it relies upon the repeated firing of the motor neurons in order to take effect.
Thus, in contrast to lifting heavy loads, light load strength training to failure involves much more CNS fatigue (both spinal and supraspinal) and more calcium ion-related fatigue. The high levels of CNS fatigue during sets explain why training with light loads is much less effective for achieving gains in maximum strength, because the high levels of spinal and supraspinal CNS fatigue at muscular failure reduce the maximum achievable levels of motor unit recruitment, which in turn then fails to stimulate gains in the ability to recruit high-threshold motor units.
What is the takeaway?
During a normal strength training set, various fatigue mechanisms develop over time, which together ultimately lead to a reduction in the ability of the lifter to perform the exercise, leading to muscular failure. Importantly, the load used greatly affects the types of fatigue that are experienced. Heavier loads involve a proportionally much greater contribution from metabolites (even though metabolite accumulation is less) and this means that they involve low levels of calcium ion-related fatigue and low levels of CNS fatigue. Since they involve low levels of CNS fatigue, they permit extremely high levels of central motor command at muscular failure, which is how they are able to cause the greatest gains in the ability to recruit high-threshold motor units. Conversely, training with lighter loads involves much more calcium ion-related fatigue and CNS fatigue, which is why they involve smaller gains in the ability to recruit high-threshold motor units.