Your arm is made up of two segments: the upper arm and the lower arm (forearm). These two segments meet and join at your elbow. The bone in your upper arm is called the humerus while your two bones in the forearm are called the radius and the ulna. There are bony bumps at the bottom of your humerus, and these are called the epicondyles. The epicondyle situated on the outside (lateral side) of your elbow is called the lateral epicondyle. In lateral epicondylitis, primarily the muscles and tendons of your forearm are involved. Your forearm muscles extend your wrist and fingers, while your forearm tendons attach your muscles to your bone, specifically on the lateral epicondyle. The tendon which is usually involved in tennis elbow is called the Extensor Carpi Radialis Brevis (ECRB).