ECU stands for the extensor carpi ulnaris tendon, one of the major wrist tendons, located on the ulnar side of the wrist, which is the same side as the small finger. ECU tendonitis can cause wrist pain and swelling, and may not have a clear cause. ECU snapping or subluxation can cause the tendon to move out of its groove and cause snapping sounds.
Source: Verywell Health
