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• The skeleton is made up of more than 200 bones.
• Biomechanics is the study of how the skeleton moves at its joints when muscles exert force on bones.

The most common type of joint in your body are synovial joints. These have the widest range of movement. Examples include hinge joints, pivot joints and ball and socket joints.

Bones are very strong but will eventually wear away if they touch. To stop this happening there is cartilage between them. This is a tough but smooth tissue found at the ends of bones which reduces friction when they rub together. To help reduce this further a liquid called synovial fluid surrounds the ends of the bones.

Ligaments are strong cord-like tissues which attach to the ends of bones either side of the joint. They hold the bones in the correct place. Tendons are also strong cord-like tissues but these attach the bones in the joint to muscles which contract and relax to move them.

Athletes occasionally damage their tendons or ligaments when they twist joints beyond their normal range. Repeated stress on joints can eventually wear away the cartilage causing bones to rub together. This often happens in older people and causes pain.

The elbows are hinge joints. These act like the hinges on a door and only allow movement in two directions. Elbows allow the movement of the humerus in the upper arm and the radius and ulna in the lower arm to pivot.

The knee is a hinge joint which allows the femur bone in the thigh and the fibula and tibia in the lower leg to move. Unlike the elbow, knees also allow some rotation. Fingers also have hinge joints.

The radius and ulna in the lower arm allow the elbow to hinge. However, these bones are also joined together in a pivot joint. This means they can rotate around each other. To see this stretch your arm and hand out then turn your hand palm face up then face down. There is another pivot joint in the neck.

The hips are ball and socket joints. These allow the greatest amount of movement. At the top end of the humerus bone in the upper arm is a ball shape. This sits in a depression in the shoulder blade (scapula bone) which allows it to rotate. The same is true at the top end of the femur bone which sits within a depression of the pelvic bone in the hip.

Muscles can only pull and cannot push. This would be a problem if the joints were controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. This problem is solved by having muscles in pairs, called antagonistic muscles.

For example, the elbow joint has two muscles that move the forearm up or down. These are the biceps on the front of the upper arm and the triceps on the back of the upper arm:

• to raise the forearm, the biceps contracts and the triceps relaxes
• to lower the forearm, the triceps contracts and the biceps relaxes

Find out more about antagonistic muscles and how they move the skeleton.

Many of the muscles in the body are different shapes and sizes and cause movement of bones in different directions. It is therefore difficult to compare which muscle applies the biggest force for its size.

The jaw muscle possibly applies the greatest force. Without this joint food couldn't be eaten!

The turning effect of a force is called a moment. Moments are calculated by multiplying the force applied by the distance from the pivot.

Find out more about moments.