A wearable joint-action sensor detects actions of a joint that links a first body segment to a second body segment by using a proximity sensor worn on the first body segment to detect a separation between the proximity sensor and the first and/or second body segment. Actions of a hip joint can be detected by wearing the joint-action sensor on the waist, and actions of a finger joint can be detected by wearing the joint-action sensor on a finger. A waist-worn joint-action sensor incorporating a proximity sensor for detecting hip-joint actions and a triaxial accelerometer for detecting body actions can detect common physical activities, such as sitting, standing, dancing, push-ups, and sit-ups, that cannot be detected by a prior-art waist-worn or wrist-worn pedometer. A detected joint action can also be used for controlling a computing device, or for reminding a user of poor posture or of sitting for too long.
