MOTOR UNIT RECRUITMENT
Muscles are innervated by named nerves, which are in turn collections of alpha motor neurons
When the nervous system activates pools of alpha motor neurons in the spinal cord's ventral horn, motor neurons with the smallest cell bodies depolarize first. Because smaller motor neurons generally innervate motor units with smaller numbers of muscle fibers, this distinctive order of depolarization causes the smallest motor units to be recruited first every time the motor system activates motor neuron pools.
When more force is necessary, the neuromotor system depolarizes more and larger alpha motor neurons, thereby recruiting successively larger motor units. This stereotyped order of motor unit recruitment follows the "size principle."