How Motor Units Work With Your Muscles

Young woman lifting weights
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As you dive into weight lifting, cardio, or any other exercise program, you may hear the term "motor unit." What does that mean exactly? Basically, a motor unit controls the skeletal muscles and is the driving force behind every movement you make. This includes voluntary movements like walking or running as well as involuntary ones like breathing.

What Is a 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.

A motor unit is made up of a single nerve cell or neuron that innervates a group of skeletal muscles. The neurons receive signals from the brain and stimulate all the muscle fibers in that particular motor unit.

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 They Work

As soon as a motor unit gets a signal from the brain, all of the muscle fibers in that unit contract at the same time with full force. You can't go halfway with motor units—it's all or nothing. 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 say you're picking up a bowling ball. You're using the same motor units but this time 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 faster and more efficient.

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.

Use It or Lose It

The key to teaching your body to recruit more motor units, generate more force, and maintain muscle mass is to lift weights on a regular basis.

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