Final answer:
Electrical stimulation recruits smaller fibers first, controlled by smaller motor units and lower-threshold motor neurons. Larger motor units are recruited for more strength.
Step-by-step explanation:
The electrical stimulation of muscles recruits smaller fibers first. These smaller fibers belong to small motor units in the muscle, which are controlled by smaller, lower-threshold motor neurons. As more strength is required, larger motor units with bigger, higher-threshold motor neurons are recruited, activating larger muscle fibers.