Final answer:
Henneman's size principle states that motor units are recruited in order of their thresholds, beginning with smaller, more excitable units for light contractions, and progressing to larger units for greater force.
Step-by-step explanation:
The principle that states motor units are recruited according to their recruitment thresholds and firing rates is a. Henneman's size principle. This principle explains the orderly and gradual activation of motor units, which are comprised of muscle fibers and the motor neurons that innervate them. Initially, smaller motor units with lower-threshold, more excitable motor neurons are activated to produce a gentle muscle contraction. As the need for force increases, larger motor units with higher-threshold motor neurons are recruited, resulting in a stronger muscle contraction. This sequence allows for smooth increases in muscle tension, from lifting a feather to picking up heavier weights with the same muscle, such as the biceps brachii.
To avoid muscle fatigue, motor units are not all active at the same time; they alternate between activity and rest, allowing for prolonged muscle contraction. This is part of the nervous system's efficient regulation of muscle fiber recruitment and muscle contraction strength.