Epicondylitis (commonly referred to as Tennis elbow) is a medical condition where the outer part of the elbow becomes painful and tender.
In 2371, Lieutenant Walter Baxter was in sickbay on board the USS Voyager being treated for localized tenderness to his ulnar bone by The Doctor after working out in the ship's gymnasium. He asked Kes, who was in training to become a nurse, for a possible diagnosis. Kes suggested epicondylitis. (VOY: "Eye of the Needle")
Guinan claimed to seek treatment from Dr. Beverly Crusher for the condition in 2369 as a ruse to counsel Dr. Crusher, who was relieved of duty while investigating the suspicious death of Ferengi scientist Dr. Reyga. This was later admitted to be a ruse, however, to encourage Beverly to discuss the circumstances leading to her relief. (TNG: "Suspicions")