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When the knee is straight it is said to be extended. A normal range of motion at the knee includes flexion, extension and a small amount of hyperextension.
The kneecap or patella is part of a chain of structures that help to straighten the knee.
Range-of-motion is a key concept in knee rehabilitation. This paper discusses how it is measured.
Full arc extensions are early exercises to strengthen the quadriceps muscle - this article offers a video and photos to explain the exercise.
Exercise to increase range of movement.
An explanation of how to perform the exercise of short arc extensions, with video.