Motor Unit - What is a Motor Unit?

Illustration of muscle cells
Illustration of muscle cells. Callista Images/Getty Images

You already know that muscles are responsible for all of the movements the human body can make and that we can make then stronger by doing things like lifting weights and doing cardio

But muscles are complex, made up of different types of fibers and attaching to bones with connective tissue that has to be even stronger than the muscle itself.

Muscles also include something called motor units. Thes units control the skeletal muscles, making them the driving force behind every movement you make, both voluntarily, like walking or running, and involuntarily, like breathing or digesting a meal.

What is a Motor Unit?

A motor unit is essentially a single motor nerve or neuron. Each motor neuron can control muscle cells that are collectively called a motor unit.

These motor neurons receive signals from the brain and all the muscle fibers in that particular motor unit are stimulated at the same time.

Motor units are different depending on where they are and what they do. They even come in different sizes. There are small motor units that may only stimulate 5 or 10 fibers, which would be things like blinking, sniffing, or moving your eye.

You also have motor units that are made up of up to 1,000 muscle fibers, which are responsible for big movements like kicking or jumping.

So, each time you do a squat jump, for example, you've got motor units firing all of those muscle fibers all at once, allowing you to move fast and furious.

How Do Motor Units 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 when it comes to your muscle fibers contracting. That means 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 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 that heavy weight.

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

Motor Units and Adaptation

You probably know that the purpose of lifting weights is to challenge your muscles so that they grow stronger, adapting to that new challenge. 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 keep working out, you're able to generate more force and your motor units fire more quickly, making your movements faster and more efficient.

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

So, no matter how long of a break you take, your body will always remember how to do, say, a biceps curl or a squat. That doesn't mean the muscles stay at the same strength, of course. You still need to build back any strength or endurance you may have lost, but the memory of that movement 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, of course, to lift weights on a regular basis. 

The general guidelines suggest lifting weights for all muscle groups 2-3 nonconsecutive days a week. Consistency is the real key to maintaining muscle mass and progressing on a regular basis guarantees that you won't hit a frustrating plateau.

If you're a beginner, start with this Beginner Total Body Workout and you'll feel the difference in just a week.

Source: American Council on Exercise. ACE Personal Trainer Manual, 3rd Edition. San Diego: American Council on Exercise, 2003.

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