Joints and Movement:

• Muscles bring about movement at a joint; most movements are produced by the co-ordinated action of several muscles
• Muscles shorten, pulling on the bone and so moving the joint
• Muscles can only pull, they can't push, so at least two muscles are need to move a bone to and fro
• A pair of muscles woking like that are described as antagonistic
• E.g., when you flex your knee by contracting the hamstrings at the back of the thigh, the quads at the front of the thigh relax and so are stretched
• A muscle that contracts to cause extension of a joint is called an extensor
• The corresponding flexor muscle contracts to reverse that movement

Joint Structure:

• The hip, knee and ankle joints are examples of synovial joints; the bones that articulate (move) in the joint are separated by a cavity filled with synovial fluid, which enables them to move freely
• The bones are held in position by ligaments that control and restrict the amount of movement in the joint
• Tendons attach muscles to the bones, enabling the muscles to power joint movement
• Cartilage protects the bones within joints