Antagonistic Muscles

A muscle is a bundle of many cells called fibres which contain myofibrils. These are cylinders of muscle protein which make it possible for a muscle cell to contract. Antagonistic means an opposing effect, so antagonistic muscles are muscles which oppose each other. An example of antagonistic muscles is the biceps and triceps.

Muscles only have the ability to contract or relax, they can not stretch or push. This means when a muscle contracts it can not push its self back, instead an opposite muscle contracts pulling the other muscle back to its original position.

2 Examples:

Bicep/Triceps:
The biceps and triceps control the movement of the arm. Individually the muscles are useless as they can’t pull there self back. However working antagonistically with each other they can control the full movement of the arm.

How the arm bends and straightens again:
1.The bicep contacts pulling the triceps upward (triceps relaxes) and the arm bends.
2.The triceps contract pulling the bicep (bicep relaxes) down and straightening the arm.

Iris:
The muscles which control the iris work as antagonistic muscles. Too much light would damage the photosensitive cells in the retina so the amount of light entering the eye is controlled by the iris. The iris has 2 sets of muscles, the circular muscles and radical muscles.

Bright Light: The circular muscle constricts pulling the radical muscles (radical muscles dilate) and slightly closes the iris. This results in a narrower pupil allowing less light into the eye.
Dim Light: The radical muscle constricts pulling the circular muscles (circular muscles dilate) and opens the iris further. This results in a wider pupil which allows more light into the eye.