Motor Unit Recruitment in Training

Motor units act as connections for our muscles and the nervous system that operates them. Motor unit recruitment relies on a process of orderly recruitment according to the size principle. This has important implications for strength training.


  • Motor units act according to the all-or-none law.

For a motor unit to get used, it must meet a threshold according to the demands of an activity. If it meets this demand, all muscle fibers in the motor unit contract as hard as possible. Fatigue will reduce its force though.

The all-or-none response applies only to each motor unit. The fibers within a unit cannot function on their own. Each unit consists of a single fiber type that focuses on strength, endurance, or a balance of the two.

The muscle fibers of each unit also spread out in bundles. This explains why you feel a contraction throughout the muscle and not just in a small region. Inactive units still move but do not contract when a range of motion takes place.

Many experts state that we must use a very heavy resistance to recruit all the fibers and move that weight as fast as possible too. They apply the all-or-none law on an incorrect basis. They apply a law specific to each motor unit for the whole system.

This will make more sense with the next statement.

  • Units are activated in a ramp-like pattern according to the intensity needed.

As the intensity exerted by a muscle increases or continues, more motor units become activated and remain active until the demands decrease. Demands that have a low force rely on slow-twitch muscle fibers that have endurance and attach to the smallest motor units.

When they no longer meet the demands, fibers with a balance of endurance and strength join the fray.

Finally, if the efforts of all these fibers fail, the big fast-twitch muscle fibers come into play. The growth of these fibers has the greatest effect on strength and size. They require the highest intensities to become involved.

This indicates that for every action we must involve the slow-twitch muscle fibers. This happens before recruiting the larger order motor units that control the fast-twitch muscle fibers. The least active muscle fiber types in our everyday lives therefore have the most strength potential.

Some speculation exists that fast-twitch fibers may activate earlier with explosive contractions but this does not have enough proof. Very fast contractions may bypass the smaller motor units to activate the larger ones as seen in animal motions such as the flick of a cat’s paw. This remains to be seen in humans though.

Here is the important lesson. This intensity can come from a combination of more force, speed, and duration. Heavy weights are not the only way. A lighter weight with more reps would eventually recruit the big fibers. Using a lighter weight carries safety advantages as well.

Ensuring that these fibers activate therefore requires a high effort, not a heavy weight. This is mostly true but with higher durations of exercise, other factors come into play that set an upper limit. We also need slower contractions.

  • Fast-twitch muscle fibers drop out first.

After you strain with the utmost effort for long enough, these motor units drop out from fatigue first. This explains why advanced techniques are unneeded and possibly harmful. They develop fatigue that does not affect the fast-twitch muscles fibers most responsible for growth.

Strength training in general affects the endurance of the slow-twitch fibers less since the duration remains very low. These fibers have a limited capacity for strength and size though. Intervals develop them better.

This shows that proper strength training remains specific and will not develop the endurance needed for longer tasks.

Motor Unit Recruitment Lessons

Screw it, let’s do it!

– Richard Branson

You must work hard sooner or later. Whether you work hard with 3 reps or 12 reps makes little difference in motor unit recruitment. The big fibers demand an effort before they help out.

Using the heaviest weights to develop strength and size is therefore unnecessary. How heavy of a weight you choose and the number of reps appears much broader than some experts may have you believe to gain strength. Nonetheless, there is an upper limit due to energy systems. You still have a liberal range but likely should avoid much beyond 60 seconds for how long a set should last.

Work hard with a fairly heavy weight and it matters little how heavy it is.

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