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How Motor Unit Summation Develops Muscle Tension

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  • 0:05 Whole Muscle Contraction
  • 1:11 Motor Unit
  • 2:47 Motor Unit Summation
  • 5:00 Motor Unit Physiology
  • 6:08 Strength Conditioning
  • 6:43 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Did you know that skeletal muscles are composed of motor units? This lesson describes the nature of a motor unit and how motor unit recruitment regulates whole muscle contraction. Additionally, this lesson describes the effect of strength conditioning on muscle.

Whole Muscle Contraction

We all know that muscles contract. Furthermore, muscles contract harder if we try to move a heavier load. For example, the biceps brachii contracts more to lift 50 pounds compared with only 20 pounds. How do our muscles contract more to lift heavier loads?

In short, we use only as much of a muscle as needed in order to move a load or get a job done. In this sense, a muscle works like a group of people attempting to push a car. If one person can't do it alone, maybe he'll call for another person to help. If the two of them together can't do it, then they'll call for yet another, and so on until enough persons are recruited to move the car. Once they're recruited, nobody else is needed. This lesson describes the nature of a motor unit and how motor units are recruited in order to regulate whole muscle contraction and, therefore, to move objects.

Motor Unit

The motor units of skeletal muscles.
Muscle Motor Units

A skeletal muscle is an organ composed of multiple muscle cells or fibers, just like any organ is made up of a whole bunch of cells. These fibers are arranged in motor units, each of which is composed of a single motor neuron and all the muscle fibers that that motor neuron innervates. Each motor unit contracts in an all-or-none fashion. In other words, if the motor neuron is excited, it will stimulate all of the muscle fibers to contract - that is, all of the muscle fibers within that particular motor unit.

It's important to know that not all motor units are the same size. Different muscles contain different sizes of motor units - that is, more or less muscle fibers innervated by the same neuron. Larger motor units generate more tension as they're composed of more muscle fibers. Generally speaking, larger muscles responsible for larger movements contain larger motor units.

For example, our biceps brachii is a relatively large muscle, and it's responsible for relatively large movements. Its motor units are quite large with a single motor neuron innervating thousands of muscle fibers. Extraocular muscles, on the other hand, generate small contractions and, thus, small movements of the eye. These muscles contain small motor units each of which contains maybe 10 muscle fibers. That's not very many compared to a thousand.

Motor Unit Summation

Now that we understand how motor units work, we can discuss how these motor units regulate muscle contraction at the organ level, and we call that whole muscle contraction. Motor unit summation, sometimes referred to as spatial summation, is the recruitment of additional motor units to generate more contractions. Let's do an experiment. This will demonstrate how motor unit summation works.

First of all, remove the gastrocnemius muscle from a frog - that's the frog's calf muscle. After you remove the muscle, hook it up to a force transducer. Force transducers are devices that record the force or tension development when the muscle contracts. Now, let's get an adjustable power source to stimulate the muscle with different levels of intensity.

Now, we're ready to begin. If we administer a small stimulus, we observe a small contraction in response - I think that makes sense. What happens if we administer a stronger stimulus? As you can see below, greater stimulation results in a larger contraction. How do we account for this observation? Simply put, additional motor units are activated with the increased stimulus, thus generating more contraction.

More contraction is generated when the stimulus increases, up to a point.
meter as second stimulus is administered

If we increase the stimulus strength yet again, we observe an even greater response. Let's increase stimulation yet one more time and see what happens. Even though a greater stimulation was just administered, if you notice below, the same amount of contraction occurred in response.

Muscles can be stimulated until they reach their maximum contraction.
Muscle Stimulation Maximum

How do we account for this observation? Our muscles contain a finite number of motor units. Once all the motor units are activated within a particular muscle, a maximum contraction is produced. Increasing the stimulus strength has no further effect as there are no more motor units to recruit.

Motor Unit Physiology

What do the results of our little experiment here have to do with real life? Our muscles generate tension as needed to move loads; for example, to lift heavy weights. They generate more tension by recruiting motor units until enough force is developed to move the opposing load. Once sufficient force is generated within the muscle, motor unit recruitment stops.

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