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.
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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