When you bend your wrist back or grip with your hand, the wrist extensor muscles contract. The contracting muscles pull on the extensor tendon where it attaches on the outside bump of the elbow, the lateral epicondyle. The forces that pull on these tendons build when you hit a tennis ball in a backhand swing in tennis.
Overuse of the muscles and tendons of the forearm and elbow are the most common reason people develop tennis elbow. Repeating the same action of the backhand swing in tennis over and over again can put too much strain on the elbow tendons.
The repeated actions of the tennis swing may eventually cause the body to undergo an inflammatory response. Special inflammatory cells make their way to the injured tissues to help them heal. Elbow pain and swelling result.