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