Mechanical loading and *not* motor unit recruitment is the key to muscle growth

You have probably come across people who argue that reaching full “motor unit recruitment” is the key to muscle growth.

Yet, researchers have repeatedly observed that *mechanical loading* is the primary mechanism by which hypertrophy occurs, with metabolic stress likely also playing a role in some way.

So how is “mechanical loading” different from “motor unit recruitment?”

Well, it differs in two quite important ways, both of which demonstrate that mechanical loading is the key to muscle growth, and not the degree of motor unit recruitment.

Firstly, mechanical loading can occur whether the muscle is active (with motor units recruited) or not (without motor units being recruited). And studies have shown for many years that both *active* loading (by muscular contractions) and *passive* loading (by static stretching of a relaxed muscle) can increase muscle size, in both animals and humans.

Secondly, the amount of mechanical loading that is imposed on a muscle fiber in an *active* muscle contraction differs according to the contraction velocity. This difference in mechanical loading explains why we can achieve full or nearly full motor unit recruitment during a very fast contraction, and yet achieve little or no muscle growth after a strength training program made up of high-velocity exercises.

Let me explain.


What is motor unit recruitment?

Motor unit recruitment is (a big part of) the way in which the central nervous system (CNS) controls force production in muscles. There are many motor units for each muscle, and each motor unit acts as the control system for a group of muscle fibers.

Importantly, motor units differ in size. Some motor units control small, weak groups of muscle fibers that only produce a small amount of force when activated. Others control large, strong groups of muscle fibers that produce a large amount of force when activated.

Motor units controlling small, weak groups of muscle fibers are called “low-threshold” motor units. Those that control large, strong groups of muscle fibers are called “high-threshold” motor units.

And according to the size principle, low-threshold motor units are *always* recruited before high-threshold motor units. If you have been in the sports science field for more than a few years, you may recall that at one point, researchers thought that the size principle might not apply under certain, specific circumstances, but now we know better.

This “recruitment” of motor units occurs in response to a need determined by the CNS. If the CNS detects that more force is needed, more motor units are recruited.

And this means that the high-threshold motor units (and their associated muscle fibers) are only recruited when the CNS identifies that they are needed.

So when are high-threshold motor units recruited?


When does the CNS identify that high-threshold motor units need to be recruited?

There are three circumstances when the CNS detects that force production provided by the recruitment of the low-threshold motor units is not sufficient to meet the demands, and therefore it recruits some high-threshold motor units. These are:

  • Firstly, when we lift a heavy weight (anything between 80–90% of maximum force), the CNS usually recruits all available motor units. So every rep you perform with a heavy weight will typically recruit all of the motor units in a prime mover muscle.
  • Secondly, when we lift a light weight to muscular failure, the CNS usually recruits all available motor units towards the very end of the set. So most of the reps you perform with a light weight will typically recruit all of the low-threshold motor units in a prime mover muscle, while the final few reps will recruit all of the high-threshold motor units as well.
  • Thirdly, when we lift a light weight (between 30–40% of maximum force) with a fast bar speed, the CNS recruits most of our motor units. Throwing a medicine ball, doing jump squats, or plyometric push ups all very likely achieve (nearly) maximal motor unit recruitment.

In each case, the CNS detects that more motor units must be recruited in order to meet the needs of the muscle action.

Obviously, when lifting a heavy load, it is the maximal force of all muscle fibers *working together* that is required. When lifting light loads to failure, it is the maximal force of the muscle fibers controlled by high-threshold motor units that are needed to *take over* from other, fatigued muscle fibers that have ceased producing sufficient force. And when lifting a light load with a fast bar speed, it is the fact that each individual muscle fiber produces a *small fraction* of its maximal force when moving quickly, compared to moving slowly.

We know that a similar amount of muscle growth can be achieved by (1) lifting heavy weights, and (2) lifting light weights to muscular failure. Yet, lifting light weights quickly, such as by using plyometrics or ballistic strength training (which both achieve recruitment of many high-threshold motor units) does not cause very much muscle growth.

So how is lifting light weights quickly different from the other two methods? Well, as I noted above, the answer turns out to be the amount of mechanical loading on each individual muscle fiber.


Why is high-velocity strength training different?

When lifting light weights quickly, there is far less mechanical loading on each individual muscle fiber despite the recruitment of all motor units, because of the force-velocity relationship.

The force-velocity relationship states that as force increases, velocity must decrease. Similarly, it states that as velocity increases, force must decrease.

We currently think that force decreases during high velocity contractions because there is less overlap between the strands (myofilaments) inside the muscle fibers, which are the structures that move against one another to produce force by the creation of crossbridges.

As the muscle fibers contract more quickly (as they must do in high-velocity contractions), crossbridges between myofilaments have to detach more quickly, and this reduces the amount of overlap between the myofilaments that exists at any one point in time, and this in turn reduces the force produced by each individual muscle fiber.

This force is the *mechanical loading* experienced by the fiber.

When (1) lifting heavy weights, and (2) lifting light weights to muscular failure, this decrease in force produced by each individual fiber does not happen. In the case of lifting heavy weights, all muscle fibers produce a high level of force concurrently. In the case of lifting light weights to muscular failure, each individual fiber gets a turn at producing a high level of force, when compensating for other, fatigued fibers.

But in both cases, the contraction velocity is low, so the force exerted by each individual fiber can be high. Thus, the *mechanical loading* experienced by each individual muscle fiber is high, and a large enough exposure to this stimulus is what triggers the signaling cascades that lead to muscular hypertrophy.


What is the takeaway?

Muscle growth is not determined by the degree of motor unit recruitment, but by the mechanical loading experienced by each muscle fiber.

To achieve the necessary level of mechanical loading, contraction velocity must be both maximal and slow, because only this combination leads to enough simultaneous crossbridges forming in the muscle fibers controlled by high-threshold motor units.

This state can be achieved by either (1) lifting heavy weights, or (2) lifting light weights to muscular failure.

Recruiting high-threshold motor units does not work if the velocity is not slow (like when lifting light weights quickly, and not to failure), as the mechanical loading on each individual fiber is insufficient, because the crossbridges detach too quickly after forming.