What Is a Motor Unit?

Young woman lifting weights
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A motor unit controls the skeletal muscles and is the driving force behind every movement you make. This includes voluntary movements like walking or lifting weights, as well as involuntary ones like breathing. As you lift weights, your body adapts to motor unit needs. This means you'll have to increase the challenge and be consistent to keep seeing progress.

What Is a Motor Unit?

A motor unit comprises a single nerve cell or neuron that supplies nerves to (innervates) a group of skeletal muscles. The neurons receive signals from the brain and stimulate all the muscle fibers in that particular motor unit.

The muscles of the human body are extremely complex and responsible for every movement we make. Exercise can make them stronger, and inactivity will weaken them.

Muscles are made up of different types of fibers. They are attached to the bones with connective tissue, and this has to be even stronger than the muscle itself.  Each muscle is made up of many fibers and multiple motor units that are dispersed throughout the muscle. Motor units help make sure muscle contraction force is evenly spread through the muscle.

Motor units are different depending on where they are and what they do. They also come in different sizes. There are small motor units that may only stimulate five or ten fibers to do things like blinking or sniffing. You also have motor units that include hundreds of muscle fibers. These are responsible for big movements like kicking or jumping.

How Motor Units Work

Motor units measure the number of motor neurons that are activated in a muscle for a task. A higher amount of motor neurons are needed for stronger muscle contractions, while fewer are required for less effort.

As soon as a motor unit gets a signal from the brain, all of the muscle fibers in that unit contract simultaneously with full force. The amount of force you generate at any given time depends on how many motor units your body is calling for.

For example, if you're picking up a pencil, your motor units will generate only as much force as you need to pick up that pencil. But if you're picking up a heavy barbell, you're using the same motor units, but you need much more force to pick up the heavier weight.

You can generate more force when you have bigger, stronger muscles. This can happen if you lift weights on a regular basis and focus on overloading your muscles with more weight than they can handle.

Motor Units and Adaptation

The purpose of lifting weights is to challenge your muscles. By doing so, they adapt to the new challenge and grow stronger. Motor units are a big part of that adaptation.

When you first start strength training, your brain responds by recruiting more and more motor units every time you contract a muscle. As you continue working out, you're able to generate more force, and your motor units fire at a more rapid pace. This makes your movements more efficient. You can increase motor unit recruitment by consistently increasing the weight, challenging your muscles further.

Once you generate this relationship between your brain, muscles, and motor units, that relationship remains, even if you stop working out. The pathway will always be there when you come back to training.

No matter how long of a break you take, your body will always remember how to do, say, a bicep curl or a squat. Of course, that doesn't mean your muscles will have the same strength. You still need to build back any strength or endurance you may have lost. It is the memory of that movement that remains.

How to Build and Maintain Muscle

The key to teaching your body to recruit more motor units, generate more force, and maintain muscle mass is to lift weights regularly. The general guidelines suggest lifting weights for all muscle groups two to three nonconsecutive days a week. Consistency is the real key to maintaining muscle mass, and regular progression guarantees you won't hit a frustrating plateau.

If you're just getting started, give a beginner total body workout a try. You'll feel the difference in about a week and will have a good start on training those motor units.

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Article Sources
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  1. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. The Motor Unit. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

Additional Reading
  • American Council on Exercise. ACE Personal Trainer Manual, 4th ed. San Diego, A: American Council on Exercise, 2010.