The arm as a lever: the biceps beats the load

What the experiment shows

The forearm is a lever with its fulcrum at the elbow; the muscle pulls close to the fulcrum and the load is far away. So the muscle must pull far harder than the weight of the load.

How to run it

1. Press “Run”: both arms hold the load and the muscle spring stretches.
2. Compare the muscle stretch with 1 kg and 3 kg.
3. Move the muscle attachment farther from the elbow and see how the stretch changes.

Questions to discuss

• How many times larger is the muscle force than the weight of the load?
• Which is better: attaching the muscle nearer to or farther from the elbow?

The science

Balance of torques about the elbow: F_muscle · d = m·g·L. With the attachment 40 cm from the elbow and the hand 190 cm away, the muscle pulls about 4–5 times the weight. In return, a small contraction gives a large hand movement.

Try it at home

Bend your arm and feel your biceps tendon with the other hand near the elbow — it attaches very close to the joint.

What to change in the editor

• Make the muscle stiffer (k = 400).
• Put 5 kg in the hand.