Isotonic Contractions and the Load–Velocity Relationship (Virtual Lab)

Activity 7 of the Skeletal Muscle Physiology virtual lab. Hang heavier weights on the muscle and measure how fast, and whether, it lifts them. The lightest lo…

Background

In an isotonic contraction the muscle shortens and moves a load. It first contracts isometrically, developing tension until the tension equals the load; only then does it shorten. The time from the stimulus until the load starts to move therefore grows with the load.

The heavier the load, the more slowly the muscle shortens and the less distance it moves. When the load is at least the muscle's maximal force, it cannot be lifted at all and the contraction stays isometric.

Steps

1. Set the voltage to 8.5 V and choose the 0.5 g weight. Click Stimulate, watch the muscle lift the weight, then Record Data.
2. Repeat with the 1.0 g, 1.5 g and 2.0 g weights, stimulating and recording each time.
3. Click Plot Data to see shortening velocity against load.

What you should find

The lightest load is lifted soonest, fastest and furthest. Heavier loads start moving later and more slowly. The 2.0 g weight is more than a single twitch can lift (1.82 g), so the muscle does not shorten: the contraction is isometric.

Check your understanding

1. As the load increases, the shortening velocity:
2. Why does it take longer before a heavy load starts to move?
3. With the 2.0 g weight the muscle does not shorten. The contraction is: