How does recruitment change over a set?
Please subscribe to my newsletter to read the future articles in this series!
If you have read anything about the science of hypertrophy in recent years, you will have come across the idea that motor unit recruitment levels increase in tandem with increasing fatigue, during strength training sets that involve a self-selected tempo or a slow, fixed tempo (as is very common in bodybuilding). The levels of motor unit recruitment that are experienced during a rep are important, because it is the level of motor unit recruitment that determines how many muscle fibers are activated, and therefore trained. Thus, the higher the level of motor unit recruitment during a rep, the greater the number of muscle fibers that are trained, and the more hypertrophy the rep is able to stimulate (although the exact magnitude of the stimulus depends on the level of mechanical tension, which varies independently of whether a muscle fiber is activated or not). Therefore, in this article, I plan to explain exactly how fatigue interacts with levels of motor unit recruitment in each rep over the course of a strength training set.
What is motor unit recruitment?
When we want to perform a muscular contraction in order to create a movement, we first generate a signal inside the motor cortex of the brain, which is called the central motor command. The size of this central motor command is set according to how fast we want the weight to move. Larger central motor commands are created when we want to move a weight as fast as we possibly can, while smaller central motor commands are created when we want to move a weight comparatively more slowly.
The central motor command is then sent to two places. Firstly, it is sent to another part of the brain, where it registers as a perception of effort. This means that we have immediate feedback of our intention to produce a large effort, in the form of a perception. Secondly, it is sent to the intended muscle, where it recruits motor units. Each motor unit is a group of muscle fibers (although these muscle fibers are not grouped closely together, but are in fact spread out throughout a specific region of the muscle). When a single motor unit is recruited, all of its muscle fibers are immediately activated. The size of the central motor command determines how many motor units are recruited. Larger central motor commands recruit larger numbers of motor units, and smaller central motor commands recruit smaller numbers of motor units.
Importantly, while activating more muscle fibers allows more muscle fibers to contribute to whole muscle force, simply activating a single muscle fiber does not guarantee that it also produces a high force and therefore experiences the high level of mechanical tension that is necessary to stimulate hypertrophy. The force produced (and experienced) by each individual muscle fiber is determined mainly by the force-velocity relationship, the length-tension relationship, and the extent of peripheral fatigue. We can consider muscle fiber activation as essentially “switching a muscle fiber on” while these other factors determine the mechanical tension that the activated muscle fiber then experiences. Indeed, this is why very fast movements (like jumping and throwing) do not stimulate hypertrophy despite being performed with very high levels of muscle activation.
What determines motor unit recruitment levels?
Although it is common to see strength training commentators refer to motor unit recruitment levels increasing as a result of fatigue, this is not strictly accurate. Central motor commands are generated entirely voluntarily, and therefore they cannot be either created or increased by the presence of fatigue (or by any other factor, for that matter). Additionally (and most importantly), the magnitude of the central motor command depends on how fast we intend to move the weight.
If we are unfatigued and we want to lift a light weight with a very slow tempo, our intention to move in this way will necessarily create a small central motor command (because this small central motor command in turn generates a fairly low level of motor unit recruitment, and therefore only activates a small number of muscle fibers, which is sufficient for the low level of force that is needed to create the small acceleration necessary for a slow bar speed). Conversely, if we are unfatigued and we want to move the same weight as fast as possible, we will necessarily create a large central motor command (because this generates a high level of motor unit recruitment and therefore activates a large number of muscle fibers, which are necessary for the high level of force that is needed to create a large acceleration that is required in order to achieve the very fast movement speed).
What determines effort levels?
Effort during a strength training set is typically measured by the level of perceived effort or the rating of perceived exertion (RPE). Traditionally, the RPE was defined as a self-reported scale for the subjective sensation of work that is naturally associated with any muscular contraction.
As explained above, the central motor command sends a signal to a sensory part of the brain that creates a subjective sensation of work associated with the muscular contraction that is being performed (this signal is called the corollary discharge). In this way, the central motor command directly contributes to the perception of effort. Thus, when central motor command is low, the perception of effort will also often (but not always) be low. In contrast, when central motor command is high, the perception of effort will always be high. The reason that the perception of effort is not always low when the central motor command is low is because other factors can additionally increase the perception of effort as well as the central motor command.
We can now include the perception of effort into the previous example.
If we are unfatigued and we want to lift a light weight with a very slow tempo, our intention to move in this way will necessarily create a small central motor command (because this small central motor command in turn generates a fairly low level of motor unit recruitment, and therefore only activates a small number of muscle fibers, which is sufficient for the low level of force that is needed to create the small acceleration necessary for a slow bar speed). In addition, the small central motor command will also produce a low perceived level of effort or RPE. Conversely, if we are unfatigued and we want to move the same weight as fast as possible, we will necessarily create a large central motor command (because this generates a high level of motor unit recruitment and therefore activates a large number of muscle fibers, which are necessary for the high level of force that is needed to create a large acceleration that is required in order to achieve the very fast movement speed). In addition, the large central motor command will also produce a high perceived level of effort or RPE. This is why we can refer to throwing a ball as fast as possible as being the same thing as throwing a ball with a maximal effort.
Hopefully, by this point it should be clear that fatigue has absolutely nothing to do with either motor unit recruitment levels or effort levels during a muscular contraction. Both are entirely under voluntary control, and are necessarily both under voluntary control, otherwise we would be unable to gauge how much force to produce in order to create appropriate movements.
What is the limit for the maximum level of motor unit recruitment in a muscular contraction?
Typically, it is not possible for us to access all of the motor units in a muscle during a muscular contraction. Moreover, the number of motor units in a muscle that can be accessed varies between individuals (more well-trained lifters are able to access more motor units than less-well trained individuals), between muscles (smaller muscles permit access to access more motor units than larger muscles), and state of fatigue (generally, more motor units can be accessed when fatigue is totally absent compared to when it is present).
Since the number of motor units that are recruited during a muscular contraction is dependent upon the magnitude of the central motor command, if we were able to continue increasing the size of the central motor command without any external limitation, then we would be able to achieve complete motor unit recruitment. As this is not possible, some other factor must be limiting the magnitude of the central motor command that we can create during a muscular contraction.
It seems likely that the limit is the perceived level of effort.
The perception of effort is discomforting. Therefore, we tend to try and avoid experiencing high levels of effort, especially for sustained periods of time. Therefore, we probably have a maximal tolerable level of perceived effort that we are unwilling or unable to exceed (albeit this threshold does probably vary according to our transient motivation levels for the task in hand). The perceived level of effort is increased by the corollary discharge signal created by the central motor command (as well as other factors). Thus, any increase in the central motor command also creates an increase in the level of perceived effort. Since the level of perceived effort has a maximum tolerable level, this also imposes an upper limit on the maximum possible level of central motor command (although again, this depends on the context).
What other factors increase effort levels (and why does this matter)?
While the corollary discharge signal sent by the central motor command to another part of the brain is the main contributing factor for our perceived level of effort, other factors can also contribute to the perception of effort. Such factors increase the perceived level of effort in the same way as the corollary discharge (although without similarly increasing the central motor command). This is important because they cause our perception of effort to reach its maximum tolerable level at a lower level of central motor command. This then causes the maximum possible level of central motor command to be lower, compared to when these other factors are not present.
There are two main factors that increase the perceived level of effort in a strength training set in this way: [1] metabolite accumulation, and [2] the presence of inflammatory mediators. They both seem to work in essentially the same way.
As metabolites accumulate inside a muscle during a strength training set, they stimulate metaboreceptors inside the muscle. These metaboreceptors send afferent feedback to the brain, where they create an increase in the perceived level of effort, alongside burning and fatiguing sensations. While these sensations are probably what contribute to the increased level of perceived effort, they do not also increase the central motor command. Therefore, they cause us to reach our maximum tolerable level of perceived effort at a lower level of central motor command than if they were not present.
Similarly, as inflammatory mediators accumulate inside a muscle during a strength training set, they stimulate other receptors inside the muscle. These receptors send afferent feedback to the brain, where they create an increase in the perceived level of effort, alongside generalized sensations of fatigue. In addition, the inflammatory mediators also enter the bloodstream and can be detected across the blood-brain barrier (from where it also creates generalized sensations of fatigue). In this way, inflammatory mediators can exert effects on perceived effort through two pathways, both of which contribute to an increased level of perceived effort without also increasing the central motor command. Therefore, like accumulating metabolites, they similarly cause us to reach our maximum tolerable level of perceived effort at a lower level of central motor command than if they were not present.
In practice, this means that the presence of metabolites and the presence of inflammatory mediators both reduce the maximum central motor command that can be sent to the muscle. This is essentially supraspinal central nervous system (CNS) fatigue. We are involuntarily prevented from creating the level of central motor command that we would normally be able to attain, and thus we are similarly involuntarily prevented from creating the level of motor unit recruitment that we would normally be able to attain. Clearly, this means that when metabolite accumulation is high (as during light load strength training to failure or when using certain advanced techniques), maximum motor unit recruitment levels will be slightly reduced. Similarly, when inflammatory mediators are present (such as towards the end of a long workout), maximum motor unit recruitment levels will also be reduced, and probably to a greater extent than when metabolites are present.
Importantly, this reduction in the maximum level of motor unit recruitment during exercise then prevents us from achieving gains in the ability to recruit high-threshold motor units in the future (since it is the very high level of central motor command that acts as the stimulus for this adaptation). And it may also slightly reduce the amount of hypertrophy that can be stimulated, since fewer muscle fibers are being trained.
How do recruitment and effort change over the course of a strength training set?
As I explained in my earlier article, peripheral fatigue mechanisms occur over the course of a strength training set. When some of the muscle fibers within a muscle experience peripheral fatigue, they necessarily stop being capable of producing the required force or shortening at the required velocity. For this reason, unless additional muscle fibers are activated to compensate for this reduction in performance (by means of an increase in central motor command leading to an increase in motor unit recruitment level), the bar speed being used during the set will slow down.
Strength training sets can be performed in one of two main ways. Either they can be performed with maximal intended bar speed on every rep (as is the case during velocity-based training) or they can be performed with a self-selected or a slow fixed tempo on every rep.
When a maximal intended bar speed is used on every rep, central motor command (and therefore motor unit recruitment) and the level of perceived effort are both maximal on the first couple of reps of the set. Thereafter, central motor command gradually decreases slightly to its lowest level at the end of the set, whether the set is terminated before reaching muscular failure (as is often the case with velocity-based training) or whether the set is carried out until muscular failure. However, the perception of effort will remain at a maximal level on all reps of the set. The gradual reduction in central motor command occurs mainly due to the accumulation of metabolites, which slowly starts to increase the afferent feedback, leading to an increase in perceived effort due to mechanisms unrelated to the central motor command. For this reason, central motor command must reduce to compensate and maintain maximum tolerable levels of effort at the same level. In addition, the central motor command that is transmitted to the muscle reduces over time, as a result of spinal CNS fatigue (wherein repeated motor neuron firing reduces the extent to which the signal is transmissible down the spinal cord). Thus, contrary to popular belief, the presence of fatigue actually impairs the level of motor unit recruitment during this type of strength training.
When a self-selected tempo or a fixed slow tempo are used on every rep, central motor command (and therefore motor unit recruitment) and the level of perceived effort are both relatively low on the first couple of reps of the set. Thus, not all muscle fibers are activated. Nevertheless, as peripheral fatigue accumulates inside the working muscle fibers, they experience reductions in their ability to support the movement at the intended velocity. For this reason, additional muscle fibers must be activated to compensate, or else tempo will slow down even further. To maintain the intended tempo, the lifter voluntarily chooses to increase central motor command, which in turn increases the number of recruited motor units, which in turn increases the number of activated muscle fibers. This process increases over the course of the set until the maximum central motor command reaches its maximal level towards the end of the set, whether the set is terminated before reaching muscular failure or whether the set is carried out until muscular failure. Nevertheless, given that metabolite accumulation and inflammatory mediators also build up in this set, and given that spinal CNS fatigue also occurs, the maximum level of central motor command (and the maximum level of motor unit recruitment) in the set is not as high as when maximal efforts are used from the first rep of the set. This is why using maximal intended bar speed on every rep of a set is important for maximizing strength gains.
What is the takeaway?
Fatigue has nothing to do with either motor unit recruitment levels or effort levels during a muscular contraction. Both are entirely under voluntary control. Indeed, it is necessary that both are under voluntary control, otherwise we would be unable to gauge how much force to produce in order to create appropriate movements. Nevertheless, the presence of fatigue during a strength training set can influence the level of motor unit recruitment that is achieved. During strength training sets that involve a maximal intended bar speed on every rep, the presence of peripheral fatigue impairs motor unit recruitment levels, by sending afferent feedback to the brain and thereby creating supraspinal CNS fatigue (such that maximal motor unit recruitment is achieved on the first rep and minimal motor unit recruitment is attained on the final rep). In contrast, during strength training sets with a self-selected or slow fixed tempo, the presence of peripheral fatigue prompts the lifter to use a greater effort, which in turn increases the level of motor unit recruitment to its maximal level on the final rep (which is still lower than the levels achieved during strength training with a maximal intended bar speed on every rep).