Lateral epicondylitis or "tennis elbow" as it is more commonly known is caused by a tear or inflammation in the tendons on the outside of the elbow. This common injury often happens when you repeatedly overuse your arm leading to inflammation and tiny tears in the tendon.
Patients often experience pain on gripping or twisting things such as turning a door handle or shaking hands. Playing tennis three times a week when you normally play once a week will often result in this type of injury. However it is also very common in people who don't play tennis and other activities may be the aggravating factor such as using a screwdriver, using vibratory work equipment or even a keyboard.