Study Design: Single-group repeated measures design. Objective: To quantify patellofemoral joint reaction forces and stress while squatting with and without an external load. Background: Although squatting exercises in the rehabilitation setting are often executed to a relatively shallow depth in order to avoid the higher joint forces associated with increased knee flexion, objective criteria for ranges of motion have not been established. Methods and Measures:Fifteen healthy adults performed single-repetition squats to 90°of knee flexion without an external load and with an external load (35% of the subject's body weight [BW]). Anthropometric data, three-dimensional kinematics, and ground reaction forces were used to calculate knee extensor moments (inverse dynamics approach), while a biomechanical model of the patellofemoral joint was used to quantify the patellofemoral joint reaction forces and patellofemoral joint stress. Data were analyzed during the eccentric (0-90°) and concentric (90-0°) phases of the squat maneuver. Results: In both conditions, knee extensor moments, patellofemoral joint reaction forces, and patellofemoral joint stress increased significantly with greater knee flexion angles (P Ͻ 0.05). Peak patellofemoral joint force and stress was observed at 90°of knee flexion. Patellofemoral joint stress at 45°, 60°, 75°, and 90°of knee flexion during the eccentric phase, and at 75°and 90°d uring the concentric phase, was significantly greater in the loaded trials versus the unloaded trials. Conclusion:The data indicate that during squatting, patellofemoral joint stress increases as the knee flexion angle increases, and that the addition of external resistance further increases patellofemoral joint stress. These findings suggest that in order to limit patellofemoral joint stress during squatting activities, clinicians should consider limiting terminal joint flexion angles and resistance loads.

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