Repetitive strain comes from repetitive movement, like tendinitis, or like tennis elbow, is pain that keep coming back when certain movements are repeated, usually as part of work or training.
This tendon and where it attaches to the bone become painful and inflamed with this condition.
These are considered overuse injuries which develops over a period of time, but can be the result of a sudden strain as well.
Activities that cause the person to bend the wrist back (like a backhand shot in tennis, or repeated use of screwdriver or wrench) are often the cause, although any repetitive heavy use can cause it. This is in injury that can easily become chronic, lasting months to years if not treated quickly.