This is misuse at its worst and is almost always preventable. Each area involves different muscles and the motions of bending and extending the wrist and extending the elbow. Concentric contraction occurs when the muscle is being shortened, and eccentric contraction occurs when the muscle is being lengthened. The injury to the elbow in tennis elbow is usually due to eccentric contraction, which causes tearing of the tendon and muscle. An example of this is when the forearm is being straightened to hit the ball, and the same forearm muscles while being stretched, are contracted when the ball is hit.
Injury occurs as the muscle is being stretched while the muscle fibers are being contracted.
Another way that hitting the tennis ball can injure the elbow is when the fibers of muscle inserting into the bone are pulled away from the bone by too much force from hitting the ball. Injury can also be chronic , with injury time after time to the muscles, causing shortening of the muscles due to scarring from repeated injury. The reason for the flares is that the elbow never gets completely well and is subject to reinjury. If your pain is where the muscles insert into the bone of the elbow either on the sides or the back, you probably have pulled fibers of tendon from the bone. Injection in the elbow with steroids is at times helpful but not as successful as injections in the shoulder.