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BACKGROUND: Evidence exists that conservative rehabilitation protocols fail to achieve full recovery of muscle strength and function after joint injuries. The lack of success has been attributed to the high amount of muscle inhibition found in patients with pathologic conditions of the knee joint. Clinical evaluation shows that anterior knee pain is typically associated with sacroiliac joint dysfunction, which may contribute to the muscle inhibition observed in this patient group. OBJECTIVE: To assess whether sacroiliac joint manipulation alters muscle inhibition and strength of the knee extensor muscles in patients with anterior knee pain. DESIGN AND SETTING: The effects of sacroiliac joint manipulation were evaluated in patients with anterior knee pain. The manipulation consisted of a high-velocity low-amplitude thrust in the side-lying position aimed at correcting sacroiliac joint dysfunction. Before and after the manipulation, torque, muscle inhibition, and muscle activation for the knee extensor muscles were measured during isometric contractions using a Cybex dynamometer, muscle stimulation, and electromyography, respectively. PARTICIPANTS: Eighteen patients (mean age, 30.5 +/- 13.0 years) with either unilateral (n = 14) or bilateral (n = 4) anterior knee pain. RESULTS: Patients showed substantial muscle inhibition in the involved and the contralateral legs as estimated by the interpolated twitch technique. After the manipulation, a decrease in muscle inhibition and increases in knee extensor torques and muscle activation were observed, particularly in the involved leg. In patients with bilateral anterior knee pain, muscle inhibition was decreased in both legs after sacroiliac joint adjustment. CONCLUSIONS: Spinal manipulation might offer an interesting alternative treatment for patients with anterior knee pain and muscle inhibition. Because this clinical outcome study was of descriptive nature rather than a controlled design, biases might have occurred. Thus the results have to be verified in a randomized, controlled, double-blinded trial before firm conclusions can be drawn or recommendations can be made.