Mobility

Muscles
Muscles pull on your bones to move your skeleton. Different tissues join muscles to bones, and strap bones together. Each of these tissues is designed for the particular job it has to do.

Joints
Sports can be tough on your joints. The ends of the bones are under great force when you run or land from a jump. The tissues that hold bones together can be twisted by a sudden change of direction, or loss of balance. A really bad twist can even pull a muscle away from its bone. Physiotherapists can speed recovery from injury but it is better to be aware of the risks and take steps to avoid them.

Joints and movement
Two or more bones meet at a joint. Different types of joint allow different sorts of movement. Ball-and-socket joints, at your hip and shoulder, are the most versatile. These joints move in every direction, like a computer joystick. Hinge joints, such as the knee and elbow, move in just two directions – back and forwards.
Tough, fibrous bands called ligaments hold the bones in place, and limit how far the bones can move. Cartilage stops bones from knocking against each other as they move. It forms a rubbery shock-absorbing coat over the end of each bone. This stops the bones from damaging each other. Cartilage is smooth, but friction could still wear it down. To reduce this, as far as possible, the joint is lubricated with oily synovial fluid.
Tendon: a tough band of inelastic tissue attaching muscle to bone.
Ligaments: bands of tough elastic tissue holding bones to each other.
Cartilage: a smooth protective surface covers the bone ends, providing easy movement.
Synovial fluid: this lubricates and nourishes the tissues in the joint capsule.
Synovial membrane: this tissue lines the joint capsule and secretes synovial fluid.

The knee joint. Like most joints in the body, this is a synovial joint.

How muscles move bones
Muscles can only pull a bone for movement. They cannot push it. A muscle contracts to pull on a bone and move it at a joint. After contracting the muscle is only stretched again when the bone is pulled back by another muscle. So at least two muscles must act at every joint:
• One contracts to bend the joint.
• The other contracts to straighten it.

Muscles which work opposite each other are called an antagonistic pair.
There are over 600 muscles attached to the human skeleton. They make up almost half the total body weight.

The skeleton has three main functions:
Support - the skeleton provides the framework which supports the body and maintains its shape.
Movement - powered by skeletal muscles which are attached to the skeleton at various sites on bones.
Protection - the skeleton protects many vital organs.