Final answer:
The rule that determines the order in which motor units are recruited is the size principle. It dictates that smaller motor units with lower activation thresholds are recruited first, followed by larger units as intensity increases. Other options like Hooke's law pertain to mechanics and materials, not muscle recruitment.
Step-by-step explanation:
The rule that determines the order in which motor units are recruited is known as the size principle. This principle states that motor units are recruited from smallest to largest based on their threshold for activation. Smaller motor units are recruited for low-intensity contractions, while larger motor units are activated as the intensity of the muscle contraction increases. This allows for a gradation of muscle force production and helps to prevent fatigue.
Hooke's law, Boyle's law, and Archimedes' principle are unrelated to the recruitment of motor units. They are linked with physics, describing elastic properties of materials, the behavior of gases under varying pressure and volume, and buoyancy in fluids, respectively.