Tennis elbow is typically caused by the motion of a tennis downstroke. The muscle of the forearm becomes weak from overuse and small tears form in tendon fibres that attach the muscle to the elbow, causing pain and swelling. As the elbow bends and straightens, the muscle rubs against bony bumps in the forearm, which can cause wear and tear of the muscle over time.
Tennis elbow can happen to anyone who uses their forearm muscle regularly. It is common in painters, plumbers, carpenters, cooks and butchers.