[This blog was written by Matt Jessee. Matt is a former Athletic Development Intern at Athletic Lab and is currently earning his Masters Degree in Exercise Science with an emphasis in Neuromechanics from the University of Mississippi – Ole Miss.]

At Athletic Lab, all athletes are exposed to a variety of intensities. This is by design to get the most out of your training program. Training heavy or explosive movements is not only fun, but necessary according to the size principle. Yes, size really does matter. This concept is based upon intensity levels and motor unit recruitment patterns.

Let’s first take a look at a couple of definitions:

Intensity, in this case, will be defined as a percentage of maximal speed, or force. Therefore to make something more intense would require an increase in speed, force, or both.

A motor unit is a motor neuron and the muscle fibers it innervates. It consists of both a neuron (nerve cell responsible for stimulating a muscle cell) and the muscle cells themselves. Each type of motor unit possesses different characteristics.

Most people have heard the terms fast and slow twitch muscles. This refers to the specific characteristics of the different motor units. Muscles contain some combination of all three fiber types (not just one motor unit is responsible for the whole thing).

Individuals are genetically predisposed to which type of fiber is more dominant throughout their muscles; however, that doesn’t mean you should ignore training across all intensities. This chart will highlight some of the main differences between fast and slow twitch classifications.

size principle chart

The size principle states that motor units will be recruited in order of size from smallest to largest depending upon the intensity. When considering the various properties of the motor units this makes sense. The small units don’t produce much force, they are slow to act, and they are resistant to fatigue. The larger units produce a lot of force, they react quickly, but they are easily fatigued. This allows for smoother, more controlled movements. It also ensures that the body acts as efficiently as possible from an energy standpoint.

To better understand this it helps to think of muscular functions. For example, postural muscles have to stay active all day long. They have a higher concentration of slow motor units, which can stay active for long periods of time, and they won’t tire out easily. They will be able to maintain the small forces necessary to keep you upright, but they aren’t capable of producing the large forces that are required for sport performance, or demanding tasks.

When a lot of force or speed is required, all motor units will be recruited, especially the larger, faster Type IIx. If performance is the ultimate goal, then these motor units should be trained regularly to ensure adaptation. When trained, the neural pathways become more efficient (faster) and are better able to synchronize with other motor units (more force). The muscle fibers themselves will alter gene expression, becoming stronger as a result. Most people never consider these principles, go to the gym day after day, and fail to leave their comfort zone during training. This is not the case at Athletic Lab. Our programming is just another example of science meeting performance development.