Mechanical Loading is the Key to Hypertrophy, not just Motor Unit Recruitment

 In Coaching, Olympic Weightlifting, Powerlifting, Programming, Strength Training

Researchers continue to find that mechanical loading is the primary mechanism for hypertrophy. However, I come across people who believe that reaching full motor unit recruitment is the best way to achieve muscle growth.

First, we must address the obvious question.

What is the different between “mechanical loading” and “motor unit recruitment?”

These two mechanisms are different in two very important aspects. These two aspects will also show that mechanical loading is the key for hypertrophy, and motor unit recruitment.

First, 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. Hurray!! Science!!!!

Second, the amount of mechanical loading that is put on a muscle fiber in an active muscle contraction is different 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. These are called Low-Threshold motor units. Others control large, strong groups of muscle fibers that produce a large amount of force when activated. These motor units are called High-Threshold motor units.

What is important to remember is that according to the size principle, low-threshold motor units are ALWAYS recruited before high-threshold motor units.

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. So, the high-threshold motor units are only recruited when the CNS identifies that they are needed.

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 determines when to use high-threshold. The CNS detects that force production provided by the recruitment of the low-threshold motor units is not sufficient, and utilizes high-threshold motor units.

First, 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 the motor units in a prime mover muscle. This seems to be the obvious one after the discussion we just had on the definition of motor recruitment.

Second, 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 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.

Third, when we lift a light weight with a faster bar speed, the CNS recruits most of our motor units. Performing movements like throwing a medicine ball, doing jump squats, or plyometric pushups all very likely achieve (nearly) maximal motor unit recruitment.

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

So, when lifting a heavy load, it is the maximal force of all muscle fibers to work together. When lifting to failure, it is the maximal force of the muscle fibers controlled by high-threshold motor units take over from other, fatigued muscle fibers that are no longer producing enough force. And when lifting a light load with a fast bar speed, it is the fact that each individual muscle fiber produces port of its maximal force when moving quickly, compared to moving slowly.

Mechanical Loading for Hypertrophy

By now you know that muscle growth occurs when we lift heavy weights or go to failure. We also know that we do not see it in plyometric of ballistic training.  So, we see that motor recruitment is not the end all be all of hypertrophy. But, why does it not achieve hypertrophy? The answer is the amount of mechanical loading on each individual muscle fiber.

This force is the mechanical loading experienced by the fiber.

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. Therefore, the mechanical loading experienced by each individual muscle fiber is high, and a large enough exposure to this leads to muscular hypertrophy.


Muscle growth is not determined by 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. This combination leads to enough simultaneous cross-bridges forming in the muscle fibers controlled by high-threshold motor units.

This can be achieved by either lifting heavy weights or lifting light weights to muscular failure.

What can we do with this information in programming for hypertrophy.

  • Eccentrics with a slow tempo can allow for mechanical loading and enough motor recruitment
  • Max Rep sets with a High RPE will also lead to high mechanical loading with motor recruitment. This method can lead to a dramatic increase in muscle growth
  • Joker sets. Using a joker set at the conclusion of an exercise achieves the mechanical loading by going until failure


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