This graph was generated by students in a physiology lab. The top trace shows contraction force, the bottom shows the electrical stimulus. On the graph, label a muscle twitch. What is happening to the muscle and why?
The clearest single twitch is the first few and last few peaks on the myogram. The muscle is being stimulated at an increasing rate, as evidenced by the decreasing time between subsequent stimulus pulses. This caused fusion of the twitches, producing partial then complete tetanus, as well as an increase in peak force produced. Though the stimulus is maintained, the peak force starts to fall as the muscle fatigues. The students then decreased the stimulus frequency, allowing recovery from fatigue, and the muscle was able to contract again.