The graph below was generated by students in a physiology lab. The top trace (myogram) shows contraction force, the bottom (EMG) shows the electrical stimulus. On the graph, label a muscle twitch. What property of muscle is being tested here? What caused the increase in force (and subsequent decrease)?
The clearest single twitches are the first few and last few peaks on the myogram. The muscle is being stimulated at an increasing rate (frequency), as evidenced by the decreasing time between subsequent stimulus pulses. This caused fusion of the twitches (summation), 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.
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