Tennis elbow is caused by either abrupt or subtle injury of the muscle and tendon area around the outside of the elbow. You don’t have to play tennis to get this, but the term came into use because it can be a significant problem for some tennis players. Pain slowly increases around the outside of the elbow and is worse when shaking hands or squeezing objects. (Examples include lifting, using tools, opening jars, or even handling simple utensils such as a toothbrush or knife and fork.)