The monitoring of range of motion of the knee during rehabilitation is a sensitive tool in recognising the onset of arthrofibrosis.

First published by Dr Noyes in 2008, and reviewed August 2023 by Dr Sheila Strover (Clinical Editor)


The significant feature of arthrofibrosis is that the scarring within the knee joint prevents it from achieving a full range of motion.

This may affect knee flexion and extension, and a patient's ability to perform ordinary daily tasks, such as taking a bath and getting up from a chair. A limp associated with a loss of knee motion can further stress the back and the knee joint, and aggravate an already difficult situation.


What is a normal knee range of motion (ROM)?

The range of motion (ROM) of a knee is that range (in degrees) from full extension to full flexion.

Take a look at the full flexion and extension of these three healthy university students. You can easily see the amount of variation between them. Note that the middle one is a woman, the outer two are men.


knee flexion knee flexion knee flexion
knee extension knee extension knee extension


A straight leg is at 0 degrees. A flexed knee is at about 140 degrees. It is a common misconception that 0-140 degrees is a normal range of motion, but most people have some hyperextension, too, taking their ROM into negative numbers.

  • normal ROM is usually minus 5 to 143 degrees in women
  • normal ROM is usually minus 6 to 140 degrees in men.

Range of motion of the affected knee also needs to be compared to its partner before one can make a judgement on loss of range of motion. Look again at the three NORMAL individuals in the photographs above. There is a wide variabilty between the ROM of the three individuals, but one would find that there would generally be very little difference between the ROM of the two knees of each individual.


ROM required for activities of daily living

You cannot get by with a ten-degree loss of normal knee extension, as this will result in a limp, shortening, and patellofemoral arthrosis.

You use your knee in normal walking from two degrees of flexion to seventy degrees of flexion. That range of motion is what you need to just walk up and down stairs and have a normal gait. If you walk with your knee in ten degrees of flexion (you have lost only eight degrees from the straight position), you will have a limp which can throw off your back because you have a shorter leg. Your limping gait will be very noticeable, not only to yourself, but to other people as well. The most important problem with a loss of flexion and poor gait is that high pressures are placed upon the patellofemoral (kneecap) joint which, over time, will result in damage to the articular cartilage (patellofemoral arthrosis).

Studies of the range of knee motion required for activities of daily living reveal further that -

  • 93 degrees of knee flexion is required for rising from a seated position (ref 1)
  • 106 degrees of knee flexion is required for shoelace tying (ref 1)
  • 135 degrees of flexion is required to properly take a bath (ref 2)

A person can get by with a loss of two to five degrees, where they may not even notice the difference. Obviously, we like zero degrees of difference between knees, or actually less than zero. A normal knee has at least three degrees less than zero (3 degrees of hyperextension), and that is what we really like to see achieved after a knee injury or operation.


Loss of Extension

A difficult situation arises in patients who have a loss of extension of between five and ten degrees. We notice, even at seven to eight degrees, that the patient will say "I've got a limp and I don't like this". So we do everything possible in all of our patients to regain full knee motion. If I have a patient who lacks only five degrees from full extension, I am not happy with them.

Any loss of extension is a problem. In the last few degrees of extension, something called the 'knee-lock' or 'screw-home' mechanism occurs, and the leg is able to support the body weight despite the quads being completely relaxed. This lock-back or screw-home cannot properly occur if there is an extension lag, that is, if the last few degrees of extension are missing. The quads - and the hamstrings, too - are put under enormous strain. The body has to rely on muscle and ligament considerably more for support and stability, resulting in fatigue, biomechanical problems, pain and increased risk of injury or re-injury.

Screw-home Mechanism

popliteus muscle at the back of the knee

The two rounded ends (condyles) of the femur do not have the same radius - the medial one has to turn through a longer distance than the lateral one. After the lateral condyle has completed its excursion, the medial one continues a bit longer - a process known as 'screwing-home'.

In this final phase of extension (as the knee goes into its final few habitual degrees of extension or hyperextension) the anterior cruciate ligaments are taut and so are both collaterals, and the knee is in its maximally stable position, with the leg able to support the body weight despite the quads muscles being completely relaxed.

So the important principle to remember is that this lock-back or screw-home cannot properly occur if the knee cannot fully extend.


1 Laubenthal KN. A quantitative analysis of knee motion during activities of daily living. Phys Ther 52(1):34-43;1972.

2 Rowe PJ et al. Knee joint kinematics in gait and other functional activities measured using flexible electrogoniometry: how much knee motion is sufficient for normal daily life? Gait and Posture 12:143-155;2000.

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