A muscle can only pull. Movement happens because one muscle contracts and pulls a bone while its partner relaxes, and a joint lets the bone turn.
This lesson is part of support, movement and growth. It builds on comparing support systems in animals.
What does each part do?
Each part of the arm has one job. Keep the jobs apart and explanations become simple.
| Part | Job |
|---|---|
| Muscle | Contracts to pull on a bone |
| Tendon | Joins muscle to bone |
| Ligament | Joins bone to bone at a joint |
| Cartilage | Cushions the ends of the bones |
| Synovial fluid | Lubricates the joint |
Worked example: bending and straightening the arm
The table is original and shows the state of each muscle in two phases.
| Phase | Biceps | Triceps | Forearm |
|---|---|---|---|
| Arm bends | Contracts | Relaxes | Pulled up |
| Arm straightens | Relaxes | Contracts | Pulled down |
Bending. The biceps contracts and pulls the forearm bone upward through its tendon. The triceps relaxes so it does not resist.
Straightening. The triceps contracts and pulls the forearm bone downward. The biceps relaxes.
Joint. The elbow is a hinge joint, so the forearm moves in one plane, like a door on its hinge.
The mistake that costs marks
The common slip is to write that the biceps pushes the arm down when it relaxes, or that the ligament joins the muscle to the bone. Both ideas are wrong: a muscle never pushes, and the tendon is the part that joins muscle to bone.
| Step | Wrong | Right |
|---|---|---|
| Muscle action | Biceps pushes the arm down | Triceps contracts and pulls it down |
| Relaxing muscle | Pushes the arm | Stops pulling, so the partner can act |
| Muscle to bone | Ligament | Tendon |
| Bone to bone | Tendon | Ligament |
The fix is to name the contracting muscle, then the relaxing one, then the bone that moves.
Writing the full explanation
Use four linked parts: the contracting muscle, the relaxing muscle, what is pulled, and the joint type. For the arm bending, this reads: “The biceps contracts and the triceps relaxes. The biceps pulls the forearm upward through the tendon, and the hinge joint at the elbow allows movement in one direction.”
Compare this with the support systems in the previous lesson: an exoskeleton also needs muscles, which attach inside the hard covering.
Check yourself
Explain how the leg straightens at the knee, using the muscle at the front of the thigh (quadriceps) and the muscle at the back (hamstring).
Answer
The quadriceps at the front of the thigh contracts and the hamstring at the back relaxes. The contracting quadriceps pulls the lower leg forward through its tendon, so the leg straightens.
The knee is a hinge joint, so the lower leg moves back and forth in one plane. To bend the knee again, the hamstring contracts and the quadriceps relaxes.
What to study next
Test the whole topic with the support, movement and growth practice set, or return to the hub page for the full route. If the comparison sentences are still weak, go back to comparing support systems.
If you want a teacher to go through movement explanations with you, see online one-to-one Science tuition.