IN WHAT ORDER DO WE RECRUIT OUR MUSCLE FIBERS? (Henneman's size principle)
Anyway, the central nervous system (CNS) is responsible for the orderly recruitment of motor neurons, beginning with the smallest motor units. Henneman's size principle indicates that motor units are recruited from smallest to largest based on the size of the load. For smaller loads requiring less force, slow twitch, low-force, fatigue-resistant muscle fibers 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.
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 muscular failure, things are a little different:
Quite in fact, to compensate for the reduced amount of force produced by each (fatigued) muscle fiber governed by low-threshold motor units, the central nervous system will recruit high-threshold motor units.