Final answer:
The Size Principle is the mechanism by which motor units are recruited in a sequential order based on their thresholds, allowing muscle contractions to range from very gentle to extremely forceful.
Step-by-step explanation:
The principle that states that motor units are recruited in order according to their recruitment thresholds and firing rates is known as the Size Principle. The Size Principle explains how smaller, lower-threshold motor neurons with more excitable fibers are activated first during muscle contraction, followed by larger, higher-threshold motor neurons as more strength is required. This progressive increase in muscle contraction, or recruitment, ensures a wide range of control over muscle action, allowing for precise movements like picking up a feather and more forceful actions such as lifting a heavy weight.As the recruitment process goes on, the force produced by the muscle increases, due to the larger motor units capable of generating significantly more contractile force. Therefore, a diverse range of motor unit sizes within a muscle allows the nervous system to efficiently utilize skeletal muscle without causing complete muscle fatigue. This is achieved by alternating which motor units are active, thus enabling sustained muscle contractions over time.