1Types of joint
A joint is where two bones meet. A hinge joint bends one way, like a door. A ball-and-socket joint turns most ways. Fixed joints, in the skull, don't move.
jointhinge jointball-and-socket jointfixed joint
Lesson 13.2 notes
Joints and muscles
Hinge and ball-and-socket joints, what is inside a joint, and how muscles work in antagonistic pairs, like the biceps and triceps.
2Joints in your body
Your shoulders and hips are ball-and-socket joints, so your arms and legs can swing round. Your elbows and knees are hinge joints. The bones of your skull meet at fixed joints.
hinge jointball-and-socket joint
3Inside a joint
Smooth cartilage covers the bone ends. A membrane makes synovial fluid, which lubricates the joint. Ligaments join bone to bone; tendons join muscle to bone.
cartilagesynovial fluidligamenttendon
4Smooth joints
Cartilage is smooth and slippery, and cushions the bones. Synovial fluid lubricates the joint, so the bones glide. If the cartilage wears away, bone rubs on bone, which hurts.
cartilagesynovial fluid
5Muscles can only pull
A muscle contracts: it gets shorter and pulls on a bone. When it relaxes, it can't push the bone back. A second muscle pulls the other way: an antagonistic pair.
contractrelaxantagonistic pair
6Biceps and triceps
To bend the arm, the biceps contracts and the triceps relaxes. To straighten it, the triceps contracts and the biceps relaxes. They are an antagonistic pair.
bicepstricepsantagonistic pair
7More pairs
At the knee, the hamstrings bend the leg and the quadriceps straighten it. At the ankle, the calf muscle points the toes and the shin muscle lifts them.
hamstringsquadriceps
Explore: Muscle pair lab
Bend and straighten the elbow, the knee or the ankle, and see which muscle of the pair contracts.
Common mistake: Muscles can only pull
It is easy to think the triceps pushes the arm straight. In fact a muscle can only pull. The triceps straightens the arm by pulling on the forearm behind the elbow. That is why muscles work in pairs.
Remember
- A joint is where two bones meet. Hinge joints (elbow, knee) bend one way; ball-and-socket joints (shoulder, hip) turn most ways.
- Cartilage and synovial fluid let joints move smoothly. Ligaments join bone to bone; tendons join muscle to bone.
- Muscles contract to pull. In an antagonistic pair, one contracts while the other relaxes, like the biceps and triceps.
Going further at GCSE