Activity 3 of the Skeletal Muscle Physiology virtual lab. Stimulate again before relaxation ends and watch the twitches sum. Twitches after full relaxation ar…
Background
If a second stimulus arrives after the muscle has fully relaxed, it produces an identical twitch. If it arrives before relaxation is complete, the second contraction builds on the first and reaches a higher peak: wave summation. Ca²⁺ is still elevated and the elastic components are already stretched, so more force reaches the transducer.
The shorter the interval, the greater the summation. Once all motor units are recruited, frequency, not voltage, is how force is increased further.
Steps
With the voltage at 8.5 V, click Single Stimulus. When the twitch has returned to baseline, click Single Stimulus again. Record Data.
Click Clear Tracings. Give a single stimulus, then another while the trace is still falling (before it returns to baseline). Record Data.
Clear Tracings, then click Single Stimulus several times in quick succession. Record Data.
Clear Tracings, raise the voltage to 10 V and give a single stimulus. Compare its peak with the 8.5 V twitch.
What you should find
Twitches after full relaxation are the same height. A second stimulus during relaxation gives a higher second peak (wave summation), and rapid stimuli give even more. 10 V gives the same single twitch as 8.5 V. (The simulation does not reproduce treppe.)
Check your understanding
A second stimulus delivered before the muscle has relaxed produces a higher peak. This is:
Why does the second contraction produce more force?
All motor units are already recruited at 8.5 V. How can force be increased further?