In a portable exercise tracking system, a method and apparatus for measuring power output of exercise motion by a user. The system includes a wireless motion sensor and a wireless central mass unit. The wireless motion sensor detects, measures and transmits motion data associated with a first motion by the user. The central mass unit detects and measures motion data associated with a second motion by the user. The central mass unit also computes the power output of the first motion and the second motion as a function of the first motion data and the second motion data. The disclosed method and apparatus provides a more effective means of computing total power output of a user performing free body exercises.