The mainstream fitness industry is *plagued* by myths and superstitions about how muscle growth happens, and about how we should structure workouts to maximize gains.
In contrast, the serious bodybuilding and professional strength and conditioning communities are far better educated, and very well grounded in the science of hypertrophy.
Even so, there is still one misconception that presents a barrier to the understanding of muscle growth of even the most well-read strength coaches and personal trainers, which is how the degree of motor unit recruitment affects subsequent hypertrophy.
Fortunately, we can fix this problem by comparing the effects of high-velocity strength training programs, such as those that involve jumping or plyometrics, and conventional bodybuilding programs.
Before we get to that, let’s go over the basics of motor unit recruitment.
What is motor unit recruitment?
Motor units are defined as motor neurons and the muscle fibers that they innervate. There are typically hundreds of motor units in any given muscle, but the exact number can vary quite widely between muscles.
When the central nervous system causes an action potential (an electrical signal) to travel along a motor neuron, this “recruits” the motor unit, and causes all of the muscle fibers governed by that motor unit to be activated.
Once muscle fibers are activated, they immediately produce force, and try to shorten as quickly as they can.
How are motor units recruited?
Motor units are recruited in order of motor neuron (not muscle fiber) size, which can be identified by the amplitude of the action potential, because larger motor neurons display large action potentials.
This is called “Henneman’s size principle.”
We can measure the level of force at which individual motor units are recruited during muscular contractions, and this provides a measurement of the recruitment threshold. This threshold is simply the force (in Newtons) or torque (in Newton meters) at which the motor unit is first switched on by the central nervous system.
Motor units that are recruited earlier in sequence, often at relatively lower levels of force, are called “low-threshold motor units” and they govern a small number of muscle fibers. Motor units that are recruited later in sequence, at higher levels of force, are called “high-threshold motor units” and they govern a large number of muscle fibers.
How does motor unit recruitment relate to static force production?
When non-fatigued muscles are prevented from shortening, such as when we perform a static (isometric) contraction, the amount of force they can produce is determined by (1) the number of motor units that are recruited, and (2) the frequency of the action potential discharges.
It was also once believed that the degree of motor unit synchronization could be altered to affect the level of muscle force, but we now think that this is fixed, and does not change.
Muscle force increases dramatically with increasing motor unit recruitment, with the recruitment of some high-threshold motor units involving 100 times more force than the recruitment of the early low-threshold motor units. This increase in force happens mainly because *more* muscle fibers are activated with increasing recruitment levels, and this happens in two ways.
Firstly, as more motor units are recruited, all of the previous motor units remain active. Therefore, there is an incremental increase in the number of active muscle fibers as the number of recruited motor units increases.
Secondly, the number of muscle fibers controlled by a motor unit increases exponentially with recruitment order. While there are hundreds of thousands of fibers inside a muscle, the number of fibers controlled by each motor unit varies widely, from a handful up to a couple of thousand. Therefore, the amount of force that a low-threshold motor unit can produce is far smaller than the amount of force exerted by a high-threshold motor unit.
In addition to the number of active muscle fibers, force production is also affected by the size and type of the fibers themselves. Larger diameter, more fatiguable fibers can produce slightly more force than smaller diameter, less fatiguable fibers. While it is true that low-threshold motor units tend to be associated with smaller diameter, less fatiguable (slow twitch) fibers, there is no exact association between fiber type and motor unit threshold.
How does motor unit recruitment relate to dynamic force production?
When non-fatigued muscles are able to shorten, as in normal strength training and in most other kinds of movement, the amount of force they can produce is determined by the force-velocity relationship, as well as by the same factors that influence static (isometric) force.
Importantly, we know from studies performed using single muscle fibers that the force-velocity relationship is determined by the fiber itself. When a single fiber shortens slowly, it is capable of producing a high level of force. When a single fiber shortens quickly, it can only exert a low level of force.
The force-velocity relationship inside a muscle fiber is determined by the number of actin-myosin crossbridges that are attached at any one time, because the actin-myosin crossbridge is the engine that produces force. When muscle fibers shorten slowly, they can form many simultaneous crossbridges, but when they shorten quickly they can only form a fraction of this number of crossbridges at the same time. This is because the detachment rate of myosin motors from actin filaments is lower at slower velocities.
This means that the faster we try and move, the less force each individual muscle fiber can produce, to contribute to total muscle force.
To compensate for this, the central nervous system accelerates the rate at which motor units are recruited, as movement speed increases, which increases the number of activated muscle fibers. This means that the recruitment threshold (force level) at which any given motor unit is switched on is *lower* in a fast contraction than in a slow contraction.
In fact, the recruitment threshold of a motor unit in fast movements can be just 10–30% of the force level required to recruit the same motor unit in a static (isometric) contraction. In practice, extremely high levels of motor unit recruitment can be reached with light loads and fast bar speeds, which is why plyometrics increase voluntary activation levels after training.
How does motor unit recruitment change with fatigue?
When muscles experience fatigue at the same time as they are producing force, the amount of force they can produce is determined by the level of fatigue, as well as by the same factors that influence dynamic force.
Although the ways in which fatigue leads to a reduction in force are highly complex, the underlying mechanism by which fatigue affects force production is a reduction in the ability of the single muscle fibers to produce force. So in this way, fatigue affects muscle force in a similar way to the force-velocity relationship.
To compensate for the reduced amount of force produced by each (fatigued) muscle fiber governed by low-threshold motor units, the central nervous system recruits high-threshold motor units.
This means that the recruitment threshold (force level) at which any given motor unit is switched on is *lower* when fatigue is present than when fatigue is not present. In fact, computer models of the effects of fatigue on motor unit recruitment suggest that very high levels of recruitment can be achieved even with quite low forces, just like they can in high-velocity movements.
How does motor unit recruitment during strength training stimulate muscle growth?
Until recently, it was believed that we needed to lift heavy weights to achieve muscle growth.
Over the last decade, it has become increasingly clear that this is not the case. In fact, similar muscle growth is achievable with light and heavy loads, so long as the sets with light loads are performed to muscular failure, which involves a high level of fatigue.
In studies comparing the effects of heavy and light loads, you will often read researchers suggesting that the mechanism by which muscle growth occurs is a high level of motor unit recruitment. After all, when lifting heavy weights, the level of motor unit recruitment is high because there is a need to produce a high level of force, while when lifting light weights to failure, there is a need to recruit high-threshold motor units to compensate for the reduced force produced by each muscle fiber governed by the low-threshold motor units.
Unfortunately, we can see by looking at high-velocity movements like jumping that this explanation cannot possibly be correct. Although fast movements involve extremely high levels of motor unit recruitment, long-term research into the adaptations produced by jumping reveal that it causes little or no muscle growth.
All three training methods (heavy loads, light loads at fast speeds, and light loads under fatiguing conditions) involve very high levels of motor unit recruitment, and yet only two of these methods lead to meaningful muscle growth (heavy loads and light loads under fatiguing conditions).
Clearly, reaching a high degree of motor unit recruitment is not *sufficient* for producing the necessary stimulus that leads to hypertrophy. As we can see by this comparison, a slow muscle shortening velocity is *also* required.
N.B. Time under tension
Some people have argued that the factor that prevents high-velocity strength training from stimulating muscle growth is a short length of time under tension (and not a fast muscle shortening velocity), because such movements are completed very quickly. However, if time under tension were the key factor, instead of muscle shortening velocity, then we would be able to achieve meaningful hypertrophy by doing a large number of jumps with long rest periods between them, over the course of a whole day. If you really do think that this training approach would accomplish large muscle gains, feel free to ignore everything that follows).
How does muscle shortening velocity during strength training stimulate muscle growth?
As I explained earlier, similar muscle growth is achievable after strength training with light or heavy loads, so long as the sets with light loads are performed to muscular failure, which involves a high level of fatigue. In contrast, muscle growth is very limited after high-velocity strength training with light loads.
This tells us that even when muscle fibers are activated, they still need to shorten at a slow speed in order for them to be stimulated to grow.
Clearly, heavy loads cause muscle fibers to shorten at a slow speed because of the force-velocity relationship. In order to produce the required amount of force to lift a heavy load, the fibers cannot do anything other than contract slowly. The slow contraction velocity allows a greater number of actin-myosin crossbridges to be attached at any one time, and the actin-myosin crossbridge is the engine that produces force.
This higher level of muscle fiber force that is permitted by a slow contraction velocity is detected as mechanical tension by mechanoreceptors on the muscle cell membrane. This triggers the molecular signaling cascades that lead to elevated muscle protein synthesis, and causes an increase in the protein content of the muscle fiber, which we record as hypertrophy.
Similarly, strength training with light loads to failure causes muscle fibers to shorten at a slower speed because of accumulated metabolic stress. This is reflected in a fairly similar pattern in the reduction in bar speed, which reaches the same velocity regardless of the relative load used.
As the set progresses, and new, higher threshold motor units are recruited, their muscle fibers contract slowly, and the slow contraction velocity allows a large number of simultaneous actin-myosin crossbridges to be attached at any one time, which produces a high level of muscle fiber force. This force is detected as mechanical tension by mechanoreceptors on the muscle cell membrane, triggering molecular signaling cascades, increasing muscle protein synthesis rates, and producing increases in muscle fiber size.
N.B. Slow tempos
Some people have suggested that slowing down the tempo during strength training with light loads should increase muscle growth, because it increases the mechanical tension on the working muscle fibers. While this is true, the slow bar speed also dramatically increases the motor unit recruitment threshold, meaning that the high-threshold motor units that are the ones that increase most in size after training are not recruited, which is probably why most long-term studies report that tempo has little effect on hypertrophy. It seems likely that light load strength training does not stimulate muscle growth until fatigue begins to cause increased motor unit recruitment, at which point it reduces muscle shortening velocity as well.
What are the practical implications?
Importantly, the size of the weight used does not affect the bar speed of the final rep when training to failure. Regardless of what weight we use on an exercise, we end up moving at the same speed by the end of the set.
Given that this is the same speed as we move in a 1RM effort, it probably happens because it is the speed that allows the maximum force-producing capacity of the recruited motor units.
However, since muscle growth can be achieved without training to failure, there must be a (slightly faster) threshold bar speed at which a set begins to trigger muscle growth, because this corresponds to a threshold level of tension experienced by each muscle fiber.
Before this point, we are either training with a submaximal bar speed, and therefore using mainly low-threshold motor units that will not grow substantially after training, or we are training with maximal bar speed but moving too quickly for the working muscle fibers to achieve the necessary levels of mechanical tension that stimulate muscle growth.
This threshold “hypertrophy velocity” will probably correspond to the speed we can move when lifting slightly heavier weights than have traditionally been used for bodybuilding, because such weights are lifted under fatiguing conditions. Since full motor unit recruitment is typically reached at 85–90% of 1RM, we might speculate that the speed we can move without fatigue with this weight in a given exercise is the threshold speed we need to reach in order to trigger muscle growth, although whether motor unit recruitment increases and bar speed decreases in exactly the same way is unclear.
Since training to failure leads to more muscle damage than avoiding failure, monitoring bar speed during a set could be a valuable way of stopping a set after the hypertrophy stimulus has been triggered but before too much muscle damage accumulates, thereby allowing faster recovery post-workout, and a higher training frequency. Naturally, this would only work if all reps were performed with maximal effort.
Once we identify it, this “hypertrophy velocity” will likely correspond to a given number of reps in reserve, given the close relationship between bar speed and reps in reserve that has been observed, and this would be the easiest way of implementing this finding in practice.
What is the takeaway?
Muscle fibers increase in size when they are activated and shorten at a slow contraction velocity. Only this state allows enough actin-myosin crossbridges to form and produce a high enough level of mechanical tension to stimulate the mechanoreceptors on the muscle cell membrane, which then trigger the molecular signaling cascades that lead to elevated muscle protein synthesis, and therefore an increase in the protein content of the muscle fibers of high-threshold motor units.
This state can be reached by strength training with either heavy loads or light loads under fatiguing conditions, but not by high-velocity strength training or plyometrics, which involve high levels of motor unit recruitment but fast muscle contraction velocities.
Since we can achieve muscle growth without training to failure, and since the bar speed of the final rep in a set to failure is the same regardless of the relative load we use, there must be a threshold bar speed below which hypertrophy is stimulated (so long as maximal effort is used on all reps). Since training to failure and stopping short of failure produce similar muscle growth, and since training to failure takes longer to recover from than avoiding failure because it causes more muscle damage, this could be a valuable way to increase training frequency.