Assisted passive extensions are knee rehabilitation exercises.

The assistant places a rolled-up towel under the knee and then helps to straighten the leg to increase extension.
The importance of regaining extension
The last few degrees of extension are important in allowing the knee to lock back when standing still in a relaxed position.
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CLINICAL PUBLICATIONS
Osteoarthritis After Anterior Cruciate Ligament Reconstruction: The Importance of Regaining and Maintaining Full Range of Motion. Shelbourne KD, Freeman H and Gray T. ELS Sports Health. 2012 Jan; 4(1): 79–85.
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