Simply, an Repetitive Strain Injury, or RSI, is an impairment of motion caused by the overuse of a limb in a recurring motion.

Muscle cells in the forearm, for example contract and expand to make movement in the hand, thumb, and fingers possible. 

All muscles have two ends. In the forearm, one end of the muscle attaches near or at the elbow. The other end attaches at the wrist or hand. When muscles contract they pull on their attachments at or around these joints. Movement is the result. 

To work properly, muscles should have oxygen and nutrients readily flowing in and metabolic waste products (acids) flowing out.

Sometimes muscle fibers and cells remain tight, contracted and overly acidic due to overuse, stress hormones, quality of diet, environmental pollutants or other factors.

When muscles get tight, oxygen and nutrients cannot get in and waste acids build up, making muscles tighter, pulling on their attachments at both ends.

Constant pulling creates irritation, causing pain, soreness, loss of strength and reducedrange of motion. This condition is usually referred to as tendonitis.

Over time, overly tight attachments begin to change the working alignment of joints, compressing delicate nerve pathways causing pain, numbness, tingling, a burning sensation or other symptoms. Carpal Tunnel Syndrome and other nerve compression problems can result.
 
Tendonitis, Carpal Tunnel Syndrome and other conditions that are caused by repetitive motions that place stress on joints and muscles are ofte collectively descirbed as Repetitive Strain Injuries.

http://www.repetitive-strain-injury.com/