Which factor is a key determinant of the strength of a muscle contraction?

The strength of a muscle contraction is significantly influenced by the number of motor units recruited. Motor units consist of a motor neuron and the muscle fibers it innervates. When more motor units are recruited during a contraction, a greater number of muscle fibers are activated. This results in a stronger overall force being generated by the muscle.

Recruitment of motor units follows the size principle, where smaller motor units are activated first and larger motor units are recruited as greater force is needed. Thus, if a heavy load is encountered, the nervous system responds by activating additional motor units to produce the necessary strength for the contraction.

The other factors relating to the type of muscle fibers, inhibiting muscle action, or inhibiting motor units do not directly correlate with the overall strength of contraction in the same manner. While the type of muscle fibers can indeed affect the characteristics of a contraction (such as endurance versus power), the immediate determinant of contraction strength remains the number of motor units engaged.