Skeleton & muscles

Imagine your body had no bones. You would collapse in on yourself like an empty tent without poles. The skeleton gives you support, and the muscles set this frame in motion. The two always work together.

Bones give support and protection

The bones are hard and stable. They carry your body and protect the delicate organs. The skull encloses the brain like a helmet, and the rib cage with its 12 pairs of ribs wraps around the heart and lungs like a cage. Down the middle of the back the spine carries the whole trunk: it is built from about 33 single vertebrae and protects the spinal cord that runs through it. To keep the bones hard your body needs calcium, which is found for example in milk, yogurt, green vegetables and nuts.

Joints and muscles

If the bones were rigidly joined, you could not move. At the joints two bones meet in a way that lets them move against each other, for example in the knee or the elbow. They are moved by the muscles. But a muscle can only do one thing: contract and pull. It cannot push.

The opposing muscle principle

Because a muscle can only pull, every movement needs two muscles that work together. In the upper arm the biceps at the front pulls the forearm up, it is the flexor. The triceps at the back pulls the arm straight again, it is the extensor. The two are opponents: when one pulls, the other relaxes.