The recruitment order of motor units during increasing muscle tension is described by which principle?

The recruitment order of motor units during increasing muscle tension is accurately described by the size principle. This principle posits that smaller motor units, which innervate slow-twitch muscle fibers, are activated first because they are more easily depolarized. As the demand for force increases, larger motor units, which control fast-twitch muscle fibers, are recruited to generate greater tension.

This approach ensures an efficient response to varying levels of activity. Initially, with low-level muscle contractions, the smaller, less powerful units are sufficient. As more force is needed, larger units are systematically enlisted to meet the demand, thereby optimizing muscle performance and preventing fatigue of the slower fibers.

The other principles mentioned do involve important aspects of muscle function but do not specifically relate to the recruitment order of motor units. The force-velocity relationship focuses on the interaction between the speed of contraction and the force produced, while the tension-time relationship pertains to the time course of tension development during muscle contractions. The sliding filament theory explains the molecular mechanism of muscle contraction at a cellular level but does not address motor unit recruitment directly.