Cambridge Lower Secondary · Science 0893 · Stage 8 · Biology

Joints and Antagonistic Muscles

Ball-and-socket and hinge joints, and how antagonistic muscles such as the biceps and triceps move the bones at a hinge joint.

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In the curriculum

Identify ball-and-socket joints and hinge joints in the human skeleton (8Bs.01)
Explain how antagonistic muscles move the bones at a hinge joint, such as the biceps and triceps at the elbow (8Bs.01)
Skills used on this page: analogies and models, means, planning and evaluating (8TWSm.01, 8TWSm.02, 8TWSa.04)

Your skeleton has more than 200 bones. It supports your body, protects organs (the skull protects the brain, the ribs protect the heart and lungs), makes blood cells inside some bones, and — with muscles — lets you move. Bones are rigid, so the body can only bend where two bones meet. These places are joints.

Word / phrase	Meaning / use
skeleton	الهيكل العظمي — all the bones of the body
joint	مفصل — where two bones meet
ball-and-socket joint	مفصل كروي (كرة وتجويف) — moves in all directions, e.g. shoulder, hip
hinge joint	مفصل رزّي (زي مفصّلة الباب) — moves in one direction, e.g. elbow, knee
fixed joint	مفصل ثابت — no movement, e.g. skull
cartilage	غضروف — smooth layer on bone ends; stops rubbing
ligament	رباط — joins bone to bone
tendon	وتر — joins muscle to bone
contract	ينقبض — the muscle gets shorter and pulls
relax	ينبسط / يرتخي — the muscle stops pulling and can be stretched
antagonistic pair	زوج متضاد — two muscles that move a bone in opposite directions
biceps / triceps	العضلة ذات الرأسين / ذات الثلاث رؤوس — front / back of the upper arm

Types of joint

At Stage 8 you must identify two types of moving joint: ball-and-socket and hinge. You should also know that some joints, such as those between the bones of the skull, are fixed: they do not move at all.

Simplified diagrams of the two moving joints. The blue arrows show the movement each allows.

	Ball-and-socket joint	Hinge joint
Shape	A rounded end (ball) of one bone fits into a cup-shaped hollow (socket) of another.	A rounded end of one bone fits against a curved groove in the other, like a door hinge.
Movement	In all directions (forwards, backwards, sideways) and it can rotate.	In one direction only — bending and straightening (one plane).
Examples	shoulder, hip	elbow, knee (also the joints in the fingers and toes)
Analogy	a joystick or a ball in a cup	a door on its hinges

X-ray of a human shoulder joint — At the shoulder, the rounded top of the upper arm bone (the ball) sits in a shallow cup of the shoulder blade (the socket), so the arm can move in all directions.Photo: Mikael Häggström / Wikimedia Commons, CC0

Where the main joints are in the human skeleton (simplified).

X-ray of a bent elbow seen from the side — At the elbow, the end of the upper arm bone fits into a curved notch in a forearm bone like a door hinge, so the arm can only bend and straighten.Photo: Mikael Häggström / Wikimedia Commons, CC0

Inside a moving joint

Cartilage covers the ends of the bones. It is smooth and slightly soft, so the bones do not rub (less friction) and shocks are absorbed when you run or jump.
Ligaments are strong, slightly stretchy bands that join bone to bone and hold the joint together.
A slippery liquid inside the joint (synovial fluid) lubricates it, like oil in a door hinge.
Tendons are strong cords that join muscle to bone. They do not stretch, so when a muscle shortens the pull is passed to the bone.

X-ray of a knee seen from the side — The knee is a hinge joint. The gap between the thigh bone and the shin bone looks empty because cartilage does not show up on an X-ray.Photo: Ptrump16 / Wikimedia Commons, CC BY-SA 4.0

Memory trick: Ligament Links bone to bone. Tendon Ties muscle to bone.

Muscles can only pull

A muscle moves a bone by contracting: it gets shorter and fatter and pulls on the bone through its tendon. When it relaxes, it stops pulling and can be stretched back to its long, thin shape. A muscle can never push. So one muscle can move a bone in one direction only. To move the bone back again, you need a second muscle on the other side of the joint.

Two muscles that move the same bone in opposite directions are called an antagonistic pair. When one muscle contracts, the other relaxes.

Biceps and triceps at the elbow (a hinge joint)

The biceps and triceps are an antagonistic pair. Each is joined by tendons to the shoulder and to a forearm bone.

Movement	Biceps (front of upper arm)	Triceps (back of upper arm)	What happens to the forearm
Bending the arm (raising the forearm)	contracts — shorter and fatter	relaxes — stretched	The biceps pulls the forearm up; the elbow (hinge) bends.
Straightening the arm (lowering the forearm)	relaxes — stretched	contracts — shorter and fatter	The triceps pulls the forearm down; the arm straightens.

The brain sends a signal to the biceps.
The biceps contracts (gets shorter).
Its tendon is attached to the forearm just below the elbow, so it pulls the forearm up.
The elbow joint bends — like a door swinging on its hinge.
The triceps relaxes and is stretched as the arm bends.
To straighten the arm, the triceps contracts and pulls the forearm down, and the biceps relaxes.

The knee works the same way. The hamstrings at the back of the thigh contract to bend the knee; the quadriceps at the front of the thigh contract to straighten it. Hamstrings and quadriceps are another antagonistic pair.

العضلة بتشدّ بس

العضلة عمرها ما بتزق العظمة — هي بتنقبض (contract) فتقصر وتشدّ. عشان كده العضلات بتشتغل أزواج متضادة (antagonistic pair): في الكوع، الـ biceps قدّام بتنقبض فتثني الدراع، والـ triceps ورا بتنقبض فتفرد الدراع. لما واحدة تنقبض، التانية ترتخي (relaxes).

Model (analogy): you can model the arm with two pieces of card joined by a split pin (the elbow) and two elastic bands or strings for the muscles. Pulling the front string raises the "forearm"; pulling the back string lowers it. The model shows why two muscles are needed. Its limitations: strings do not contract by themselves, the card joint has no cartilage or ligaments, and the real biceps is attached at the shoulder by two tendons.

Worked examples

Worked example 1 — identifying joints Name the type of joint at (a) the hip, (b) the elbow, (c) the knee. [3]

(a) ball-and-socket
(b) hinge
(c) hinge

Common mistake: calling the knee a ball-and-socket joint because it is big. Ask: "Can it move in all directions?" The knee only bends and straightens.

Worked example 2 — explaining a movement Explain how the muscles in the upper arm bend the arm at the elbow. [3]

The biceps contracts (shortens) [1].
It pulls on the forearm (through a tendon), so the forearm moves up / the arm bends [1].
At the same time the triceps relaxes [1].

Common mistake: "The triceps pushes the arm up." Muscles never push.

Worked example 3 — why pairs? Muscles usually work in pairs. Explain why. [2]

Muscles can only pull (contract); they cannot push [1].
So a second muscle is needed to pull the bone back the other way / one muscle bends the joint and the other straightens it [1].

Worked example 4 — the knee A student kicks a ball. Just before the kick her knee is bent; then the leg straightens. (a) Which muscle contracts to straighten the leg? [1] (b) What happens to the hamstrings? [1] (c) What type of joint is the knee? [1]

(a) the quadriceps (front of the thigh)
(b) they relax (and are stretched)
(c) a hinge joint

Worked example 5 — a model and its limitation Omar builds a model arm with card, a split pin and two strings, A on the front and B on the back. (a) Which string models the triceps? [1] (b) What does the split pin represent? [1] (c) Give one way the model is different from a real arm. [1]

(a) B — the triceps is at the back of the upper arm.
(b) the elbow (hinge) joint.
(c) Any one: strings do not contract on their own; muscles get fatter when they contract but strings do not; there is no cartilage/ligaments in the model; the model only moves on a flat card.

Worked example 6 — comparing joints The shoulder can move the arm in a full circle, but the elbow cannot. Explain this using the types of joint. [2]

The shoulder is a ball-and-socket joint, which allows movement in all directions / rotation [1].
The elbow is a hinge joint, which only moves in one direction (bending and straightening) [1].

Common mistakes

Saying a muscle pushes. Muscles only pull (contract).
Saying a relaxed muscle "pushes back" or "gets longer and pulls". Relaxing means it stops pulling and is stretched by the other muscle.
Mixing up biceps (front, bends the arm) and triceps (back, straightens the arm).
Mixing up tendons (muscle to bone) and ligaments (bone to bone).
Calling the knee or elbow a ball-and-socket joint.
Writing "the muscle contracts and relaxes" without saying WHICH muscle does which.

How Checkpoint asks this

A diagram of an arm with muscles labelled with letters: "Which muscle contracts to raise the forearm?" — find which side of the joint pulls the forearm up.
"Name the type of joint at the shoulder / knee." One word or phrase — spell ball-and-socket in full.
"Explain how antagonistic muscles…" — 2 or 3 marks: name the muscle that contracts, say it pulls the bone, say the other relaxes.
Draw lines to match joints to examples, or tick the hinge joints in a list.
Model questions: what each part of a model represents, and one limitation.

Practice

Remember

Ball-and-socket joints (shoulder, hip) move in all directions and rotate; hinge joints (elbow, knee) move in one direction only.
Muscles can only pull (contract); they cannot push.
Antagonistic pair: when one muscle contracts, the other relaxes.
Elbow: biceps contracts → arm bends; triceps contracts → arm straightens.
Tendons join muscle to bone; ligaments join bone to bone; cartilage stops bones rubbing.

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