Muscle Stimulation Frequency

Depending on the demand, motor neurons control the strength of a muscle's contraction by altering the frequency of action potentials delivered to the motor units.

When the neuron fires at a low frequency, the muscle fibers can fully relax between each stimulus, causing twitch contractions.

However, motor units in active muscles require a series of action potentials to perform work.

When the neuron fires before the muscle fibers fully relax after the first twitch, the second contraction is added atop the previous one. This phenomenon, termed wave summation, makes the overall contraction stronger.

Suppose the motor neuron increases its signaling frequency and stimulates the muscle fibers 20 to 30 times per second.

In such cases, muscle fibers partially relax between twitches, resulting in a wavering contraction called incomplete tetanus.

Further, if the stimulation frequency increases to 80 to 100 times per second, the muscle fibers get no time to relax.

As a result, the individual twitches are indistinguishable and fuse into a sustained plateau of maximum contraction known as complete tetanus.