The skeleton supports and protects the body and produces blood cells. Together with the muscles, it allows the body to move.
The human skeleton has four main jobs:
A joint is where two bones meet. A hinge joint, such as the elbow or knee, allows movement in one plane; a ball-and-socket joint, such as the shoulder or hip, allows movement in many directions.
Ligaments hold bone to bone, tendons attach muscle to bone, and cartilage cushions the ends of bones so they do not grind together.
The force a muscle produces can be measured, and the skeleton acts as a system of levers. Because a muscle usually attaches close to the joint, it must pull with a force larger than the load being lifted.
Muscles pull rather than push, so they work in antagonistic pairs: one contracts to move the bone one way, the other contracts to move it back.
At the elbow, the biceps and triceps are the pair. To bend the arm, the biceps contracts and the triceps relaxes. To straighten it, the triceps contracts and the biceps relaxes. A contracting muscle gets shorter and fatter.
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