How to Measure Knee Flexion At Home: Knee Range of Motion

Bending your foot at the knee seems obvious.  But to what degree can you bend it? Again, can you straighten your leg without any difficulties? You can get answers to such questions if you how to measure knee flexion at home.

Knee flexion is typically the degree to which you can bend your knee. If you the limit at which your knee can flex, then you can assess the range of motion. This is helpful, especially if you are recovering from surgery or an injury.

A full range of motion at the knee is significant, as it makes movement easier. Bending and straightening at the knee makes it possible to run, wall, support weight of the body, to control body balance as well as to absorb shock.

Sometimes it is quite challenging to extend the knee fully. Such as state is called a flexion deformity of the knee.  Simply put, it is the inability to straighten the knee fully and is also commonly called as flexion contractual.

The standard motion range (AROM) of a knee is 0° extension and 140° flexion. In the case of deformities, then the normal active range of motion is reduced. And, this occurs when knee flexors fail to lengthen as you extend your leg. Such is common in people who have insufficient physical therapy. A deformity can be temporary or permanent. Nonetheless, with adequate rehabilitation, i.e., getting active and passive mobilization at the affected joint can return the full motion range.

Measuring the knee flexion at home is easy. You can use a goniometer or a phone app that simulates a goniometer. But also you can use your fingers and a measuring tape, to estimate your knee flexion.

Understanding knee motion

Three motion ranges exist:

  • Active Knee ROM: how much the knee muscles can bend without help, i.e., the field in which the knee can flex and extend on its own
  • Passive Knee ROM: the extent to which a knee can bend assisted by external forces.
  • Active Assisted ROM: extend to which a knee can bend when you are in pain, or it’s at its weakest point. This is usually with the help of someone, i.e., a therapist.

What is the Normal Knee Range of Motion?

The knee usually bends and straightens. But it also rotates to some extent. Doctors use three range motions:

  • Knee Flexion: 135-degree i.e., fully bent
  • Knee Extension: 0-degree i.e., entirely straight
  • Internal Knee Rotation: 10-degree

How to Measure Knee Flexion At Home

The right tool to use is the goniometer; it is typically a special type of protractor that can accurately measure angles at the joints. However, you can get your knee flexion without a goniometer.

Using a goniometer

The goniometer has a circle representing 360 degrees and two extending arms. One arm is movable while the other is stationary. To get an accurate range of motion, identify the following 3 points:

  • The midpoint of the outer knee, i.e., Lateral Epicondyle
  • The bony lump on your ankle (outer side), i.e., Lateral Malleolus
  • A bony lump below the hip joint on the outer side of your thigh, i.e., Greater Trochanter –


  • Find a firm surface, e.g., floor. I suggest you use a mat and then lie on your back. A firm surface gives your body a perfect alignment.
  • Align the goniometer against your leg, ensuring that the center of its disc is against the midpoint of your knee. Position the stationary arm on your outer side of your thigh, ensuring that it aligns with the greater trochanter. Then position the other arm with the bony lamp on the outer side of your ankle – lateral malleolus.

This process is quite challenging, and you may need additional hands to help you.

  • Straighten your leg – the back of your knee should touch the floor. Straighten slowly, especially if you have limited motion. Sometimes your leg might not touch the floor; straighten it a far as you can. Check the reading and record. After the full extension, it should read 0-degrees. However, some people have hyperextended legs, which might read up to -5 degrees or lower while others have an under-extended, which can be 5 degrees or more.
  • Bend your knee and push your feet towards your buttocks. Push as far as you can go, i.e., the maximum point of knee flexion. Check the readings and records. Repeat the measurement on the other knee. I suggest you repeat the procedure at least twice.

There is a range of values that are termed normal. For instance, the American Medical Association defines normal knee flexion range to stand at 150 degrees. The American Academy of Orthopedic Surgeons, on the other hand, states it as 141 degrees ±5.3 degrees.

Measure a Goniometer

  • Lie on your back
  • Straighten your leg to touch the floor
  • Try to slide your fingers, palm, or hand underneath your knee
  • Difficulty to slide fingers – 00 extension
  • All fingers sliding underneath – +50 e. lacking 50 extension
  • Whole hand slides – +100e. lacking 100 extension


Understanding movements at a joint can help your deal with injuries as well as the health of your joint. It is easy to learn. The above tips are straightforward. Additionally, you can try using the app, which is free and easy to use too. participates in the Amazon Associates Associates Program, an affiliate advertising program designed to provide a means for sites to earn commissions by linking to Amazon. This means that whenever you buy a product on Amazon from a link on here, we get a small percentage of its price.