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.|
When a knee cannot be fully extended due to lengthening or weakening of the quadriceps muscle at the front of the thigh, it is said to have an 'extension lag'. An extension lag of three degrees means that the person needs to gain three degrees of extension for their knee to reach their full range of extension.