Total knee arthroplasty (TKA) is performed more than 300,000 times a year in the United States, most often for osteoarthritis (OA). While pain is predictably reduced, function does not typically ever reach that of age-matched, uninjured subjects. Quadriceps weakness has been implicated in the development and progression of knee OA and is a significant problem after TKA. Voluntary exercise has been ineffective at restoring quadriceps strength after TKA. The aims of this study are: 1) to assess the effectiveness of high-level neuromuscular electrical stimulation as an adjunct to ongoing intensive, early rehabilitation in restoring quadriceps strength and improving the functional outcome after primary TKA, and 2) to identify the physiological and morphological bases for improvements in quadriceps strength and functional outcome.