What effect does increasing the voltage of a stimulus have on muscle contractions?

Increasing the voltage of a stimulus leads to stronger muscle contractions due to the principle of motor unit recruitment. When a muscle is stimulated, the strength of the muscle contraction depends not only on the frequency of the stimuli but also on the number of motor units activated. As the voltage increases, more motor units are recruited to respond to the stimulus. Each motor unit consists of a motor neuron and the muscle fibers it innervates. With greater voltage, additional muscle fibers are activated, resulting in a more forceful contraction.

This principle is also described by the size principle, which states that smaller motor units (which activate weaker muscle fibers) are recruited first, followed by larger motor units (which activate stronger muscle fibers) as the stimulus intensity increases. This process allows the muscle to generate a graded response, matching the force of contraction to the demands placed on it, which is why increasing voltage results in stronger contractions.