Motor systems I: Lower systems
The effect of stimulation rate on muscle tension. (A) At low frequencies of stimulation, each action potential in the motor neuron results in a single twitch of the related muscle fibers. (B) At higher frequencies, the twitches sum to produce a force greater than that produced by single twitches. (C) At a still higher frequency of stimulation, the force produced is greater, but individual twitches are still apparent (unfused tetanus). (D) At the highest rates of motor neuron activation, individual twitches are no longer apparent (fused tetanus).