Sprain

A sprain is an injury to a joint caused by its being overstretched beyond its limit capacity. It most often happens to the ankle, knee, fingers, wrist, and toes. Although they can be caused by a sudden trauma, they are often caused by fatigue of the supporting muscles or by sudden physical exertion after inactivity.
Sprains are easy to diagnose, although a physician may recommend an X-ray to insure there are no fractures.
Sprains will resolve themselves unless a ligament becomes detached. Rest, icing, compression, partial immobilization, and elevation will all speed healing. Analgesics are recommended for any pain. Full immobilization (such as a cast) isn't recommended as this will usually slow healing. Mild exercise is recommended once pain and swelling decrease (after 1–3 days in most cases).
To avoid sprains, a period of warming up is recommended before intense exercise such as running, and exercise should cease if fatigue sets in.
Sprain at Wikipedia
Sprain