A movement diagram shows bones, a joint and muscles at one moment, with an arrow for the movement. Your job is to say which muscle shortens, which one relaxes, and where the force acts.
This lesson sits in support and movement. If the muscle names are not yet secure, revisit relating bones, joints and muscles first.
What is the routine for reading a movement diagram?
Ask four questions in order. Each one gives you a sentence you can write.
- Which joint is the bone turning around?
- Which way does the arrow show the bone moving?
- Which muscle, pulling on that bone, would cause this direction?
- What is the partner muscle doing at the same time?
The routine works because it turns a picture into a chain of cause and effect. A chain of cause and effect scores better than labels alone.
Worked example: a diagram described in words
Imagine a side view of an arm with the upper arm bone fixed at the top. Muscle P runs along the front and joins the radius below the elbow. Muscle Q runs along the back and joins the ulna behind the elbow.
An arrow shows the forearm moving up towards the shoulder.
Question: name P and Q, and explain the movement.
- The joint is the elbow, a hinge joint.
- The forearm moves up, so the angle at the elbow gets smaller.
- The muscle at the front that can pull the forearm up is P, the biceps. It contracts and shortens.
- Q is the triceps. It relaxes and lengthens, so it does not resist the movement.
A full answer reads: “P is the biceps and Q is the triceps. The biceps contracts and pulls the radius upwards, while the triceps relaxes. The elbow joint acts as the pivot, so the forearm bends.”
What does a weak answer look like?
A weak answer labels the diagram and stops. A strong answer explains what each labelled part does.
| Part of the answer | Weak | Strong |
|---|---|---|
| Naming | “P is biceps, Q is triceps” | Same, plus which contracts |
| Direction | Missing | “Forearm bends upwards” |
| Partner muscle | Missing | “Triceps relaxes” |
| Joint | Missing | “Elbow acts as the pivot” |
Most of the marks go to the explanation, not the names. Leaving out the partner muscle is an easy mark to lose.
Reversing the movement
Now the arrow points down and the forearm straightens. The same four questions give a different answer. The triceps contracts and pulls the ulna, and the biceps relaxes.
A quick check: the muscle that contracts is always the one whose pull matches the arrow. If your answer has both muscles contracting, stop and reread the arrow.
Check yourself
A diagram shows a lower leg at the knee. Muscle X at the back of the thigh joins the bone below the knee. The arrow shows the lower leg swinging backwards so the heel moves towards the thigh. Name the movement at the knee and say what X and the muscle at the front of the thigh are doing.
Answer
The knee is a hinge joint, and the leg is bending. Muscle X (a hamstring) contracts and pulls the lower leg backwards. The muscle at the front of the thigh relaxes, so it lets the leg bend.
If you wrote that the front muscle also contracts, reread the arrow: two muscles pulling in opposite directions would stop the movement.
What to study next
Go deeper on the pair action in explaining antagonistic muscle action, then test yourself with the support and movement practice set. The biological data interpretation trainer gives extra reading practice.
For a teacher to check your diagram explanations as you write them, see online one-to-one Biology tuition.