The Knee Learning Resource
Extension means 'straightening'. When the knee is straight it is said to be extended. Extension is when the angle between the thigh bone and the shin bone is 0 degrees. 'Hyperextension' is when the knee continues to bend a few more degrees on from this straight position.
A normal range of motion at the knee includes flexion, extension and a small amount of hyperextension.
This knee is fully extended for this person. It is at about minus one degree of flexion. A straight knee is at 0 degrees.
This knee is fully extended for this person, too - it is at about minus ten degrees, ie quite markedly hyperextended.
Some people normally have a few degrees of hyperextension. People with hypermobility syndromes (eg Ehlers Danlos) may have marked hyperextension without having had a specific injury. The knee can be injured when it is forced into too much hyperextension.
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