Introduction: Patients with osteoarthritis (OA) of the knee have quadriceps weakness and arthrogenous muscle inhibition (AMI). While total knee arthroplasty (TKA) reliably reduces pain and improves function in patients with knee OA, quadriceps weakness persists after surgery. The purpose of this investigation was to assess contributions of AMI to quadriceps weakness before and after TKA and to assess the effect of pain on AMI.

Methods: Twenty‐eight patients with unilateral, end‐stage, primary knee OA were tested an average of 10 days before and 26 days after TKA. The mean age at time of operation was 63 years (range 49–82 years). Measurements on the involved and uninvolved knees were performed using the burst‐superimposition technique, where supramaximal electrical stimulation is superimposed on a voluntary contraction. Knee pain during contraction was measured using a numeric rating scale.

Results: The involved quadriceps were significantly weaker than the uninvolved prior to TKA (p < 0.05). Quadriceps strength decreased by 60% (p < 0.001) and activation decreased 17% (p < 0.001) after TKA. Changes in muscle activation accounted for 65% of the variability in the change in quadriceps strength (r² = 0.65) (p < 0.001). Knee pain during muscle contraction accounted for a small, but significant portion of the change in voluntary activation (r² = 0.22) (p = 0.006).

Discussion: Exercise regimens that emphasize strong muscle contraction and clinical tools that facilitate muscle activation like biofeedback and neuromuscular electrical stimulation may be necessary to reverse the quadriceps activation failure and weakness in the patients with knee OA that worsens after TKA. The failure of current rehabilitation regimens to directly address activation deficits within the first months after surgery may explain the persistent quadriceps weakness in patients after TKA. © 2003 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.