The tendon is the part of a muscle that attaches to the bone. Forearm tendons attach the forearm muscles to the outer bone of the elbow. Tennis elbow often occurs when a specific muscle in the forearm the extensor carpi radialis brevis (ECRB) muscle is damaged. The ECRB helps raise (extend) the wrist.

Repetitive stress weakens the ECRB muscle, causing extremely tiny tears in the muscle's tendon at the point where it attaches to the outside of the elbow. These tears lead to inflammation and pain.