IN WHAT ORDER DO WE RECRUIT OUR MOTOR UNITS?

IN WHAT ORDER DO WE RECRUIT OUR MOTOR UNITS? ⭕️ ⁣ ⁣⁣⁣⁣⁣
⁣⁣⁣
“The central nervous system is responsible for the orderly recruitment of motor neurons, beginning with the smallest motor units.” [1]⁣⁣
⁣⁣
To make yesterday’s post even cooler, here’s an analogy I personally came up with.. here’s how “pushing a car uphill” can teach how The Size Principle actually works. ⁣
⁣
Henneman’s size principle indicates that motor units are recruited from smallest to largest based on the size of the load but also contraction speed. ⁣⁣
⁣⁣
Slow twitch, low-force, fatigue-resistant muscle fibers (red fibers, so the red guys in the picture) are activated prior to the recruitment of the fast twitch, high-force, less fatigue-resistant muscle fibers. ⁣⁣
⁣⁣
Larger motor units are typically composed of faster muscle fibers that generate higher forces. They’re more white-ish in color because they carry less blood supply through a decreased amount of myoglobin and mitochondria. [2] ⁣⁣⁣⁣⁣
⁣⁣⁣⁣⁣
Secondly, the number of muscle fibers controlled by a motor unit increases exponentially with recruitment order: while there are hundreds of thousands of fibers inside a muscle, the number of fibers controlled by each motor unit varies widely, from a handful (low-threshold ones) up to a couple of thousand (high-threshold ones). ⁣⁣
⁣⁣
Therefore, the amount of force that a low-threshold motor unit can produce is far smaller than the amount of force exerted by a high-threshold motor unit.⁣⁣⁣⁣⁣
⁣⁣⁣⁣⁣
What happens when we move light weights to failure? ⁣⁣⁣⁣⁣
⁣⁣⁣⁣⁣
Just like mentioned above, light loads will recruit low-threshold muscle fiber that exert less force. However, since we’re taking the contraction to task failure, things become a little different:⁣⁣⁣⁣⁣
⁣⁣⁣⁣⁣
Quite in fact, due to an increase in effort perception because of fatigue mechanisms happening within the muscle, the central nervous system will recruit high-threshold motor units.⁣⁣⁣⁣