Muscle recruitment and development is often talked about and largely conjecturized in the world of health and fitness. We hear a lot of recommendations of what we should be doing. “Three sets of 8 reps if you want your muscles to get bigger, but not 7 reps cause then nothing happens”. Obviously these recommendations come from somewhere and are probably based on research and for all intents and purposes are probably effective enough. But what’s the actual process of muscle recruitment and development?
Well, it’s an intricate process involving a lot of different variables. Today I‘ll address 2 aspects of muscle that will play a huge role in how they develop—muscle fiber composition and motor unit activation.
To keep it simple, our muscles are composed of two main types of fibers, type I and type II. Type I are designated as our slow-twitch fibers and type II are fast-twitch. Type I are endurance fibers, not very fatigable, and smaller than type II. Type II are power and strength fibers, very fatigable, and can get much bigger than type I.
Each of our muscles is composed of a different percentage of each of these fibers. As a result, certain muscles will respond more favorably to a certain stimulus. Your muscle fiber composition will largely determine the athlete you are, or the physical activity you’re best at. For example, you can probably guess the fiber type differences between marathon runners and power lifters.
Like I said previously, slow-twitch fibers are smaller than fast-twitch, and this difference in size also provides implications for the size of the motor unit they belong to.
A motor unit is made up of a motor neuron and the muscle fibers that neuron controls. One motor neuron may control hundreds, if not thousands, of different muscle fibers but a muscle fiber will only ever be controlled by one motor neuron. This is extremely important because the size of the motor neuron will basically determine the muscle fiber type it controls.
So, a small motor neuron will control small muscle fibers and a large neuron will control large muscle fibers.
When it comes to performing an activity, which type of motor neuron do you think will be activated first? The small ones—Correct, good job.
That means when we’re performing simple everyday tasks like standing, we’re activating mostly small motor units.
If we’re using mostly small motor units that means we’re using mostly small muscle fibers, and if we’re using mostly small muscle fibers that means we’re using mostly Type I muscles.
Generally, the exact opposite is true for type II. Type II muscles fibers are large and are controlled by large motor neurons. These large motor neurons are not activated as easily as small motor neurons.
They have different thresholds.
A threshold refers to the amount of signaling received from the central nervous system. When we engage in an activity like walking, our central nervous system sends a signal that’s only powerful enough to activate the motor neurons necessary to complete that action.
Consider what this implies for training.
If you don’t put enough demand on your muscle, will you activate all the motor units in that muscle?
And if you don’t activate all the motor units, are certain parts of your muscle being neglected?
How do you think that makes them feel?
It makes them sad.
And then they shrivel up and die.
But it’s something to seriously think about the next time you train.
Let me know what you think.