The Skeletal Muscle Length–Tension Relationship (Virtual Lab)

Activity 6 of the Skeletal Muscle Physiology virtual lab. Change the resting length and record active, passive and total force. Active force is highest at 75…

Background

Here the muscle contracts isometrically at a fixed length, and the variable is its length before stimulation. Active force comes from crossbridge cycling and depends on how much the thick and thin filaments overlap: it is highest at the optimal length and falls when the muscle is shorter or longer.

Passive force comes from stretching the resting muscle: the elastic recoil of the tissue, mainly the protein titin. It is zero at and below the optimal length and rises steeply with stretch. Total force = active + passive.

Steps

1. Set the voltage to 8.5 V and the muscle length to 75 mm. Click Stimulate, then Record Data.
2. Shorten the muscle in 5 mm steps down to 50 mm, stimulating and recording at each length.
3. Click Clear Tracings. Lengthen the muscle in 5 mm steps from 75 mm up to 100 mm, stimulating and recording each time.
4. Click Plot Data to see active, passive and total force against length.

What you should find

Active force is highest at 75 mm and falls at shorter and longer lengths. Passive force is zero up to 75 mm and climbs steeply beyond it. Total force dips just past the optimum, then rises again as passive force takes over.

Check your understanding

1. Active force is greatest at 75 mm because:
2. Passive force in a stretched resting muscle comes mainly from:
3. At 100 mm active force is low, yet total force is high. Why?