HOW OUR BODIES RECRUIT MUSCLE FIBERS

Even the smaller muscle groups have over 100,000 muscle fibers. A motor neuron is what stimulates our muscles to contract. It carries impulses (messages) from our brain and spinal cord to our muscles. One motor neuron controls anywhere from 2-2,000 muscle fibers. A single motor neuron and the fibers it stimulates are called motor units. Each motor unit fires with a frequency that is conducive to the fibers it stimulates. Simply put, a slow twitch motor neuron will cause the muscles in it to contract SLOWLY while a fast twitch unit will fire QUICKLY.

The quicker the motor neuron fires, the more power it produces! If the activity is light, it will stimulate mostly type I muscle fibers. When it becomes too intense it will call upon type IIa muscle fibers. And finally, for the highest intensity movements, our muscles will recruit the type IIb fibers. This is why type I fibers are called low threshold, and fast type IIb fibers are called high threshold. Low threshold means they are the first muscle fibers to be recruited and high threshold because they are only recruited under the most intense circumstances. Our bodies always activate muscle fibers in this fashion.