Knee extension is important for standing posture, walking, running, squatting, stepping, landing and efficient lower-limb loading. Even small losses of knee extension can influence gait, quadriceps function, comfort and movement confidence.
The Knee Extension (cm) assessment is useful because distance-based measures can make subtle extension differences easier to track, especially when comparing one side with the other. It should be interpreted with pain, swelling, symptoms, strength, gait and functional tests.
Test name: Knee Extension (cm)
Alternative names: Heel-Height Difference, Knee Extension Deficit Test, Prone Knee Extension Test, Knee Extension Gap
Category: Knee range of motion / mobility assessment
Primary score: Difference or gap in centimetres
Best use: Knee extension baseline, side-to-side comparison and retesting
Key limitation: Results depend on body size, setup, landmark choice, support position, swelling, pain, relaxation and measurement method.
The Knee Extension (cm) assessment measures knee extension using a distance method rather than only an angle.
Common options include:
prone heel-height difference
supine heel-height difference
distance from knee to surface
distance from heel to surface
side-to-side comparison
extension deficit in centimetres
hyperextension difference in centimetres
The method must be recorded clearly because different setups measure different things.
The test may be used to assess:
knee extension range of motion
knee extension deficit
side-to-side difference
hyperextension asymmetry
baseline and retest change
pain or stiffness at end range
swelling or guarding influence
movement restrictions that may affect gait, squatting or lower-limb loading
It is particularly useful when small changes in extension matter and a simple clinical or field measure is needed.
The test measures how close the knee is to full extension, usually using centimetres.
It may reflect:
knee extension ROM
extension deficit
hyperextension difference
hamstring or posterior knee tolerance
swelling or effusion effects
pain or guarding
quadriceps inhibition context
side-to-side mobility difference
end-range knee comfort
It does not diagnose the cause of extension loss or identify the exact structure limiting movement on its own.
The test may be useful for:
athletes
runners
field and court sport athletes
gym clients
older adults
clients with knee mobility goals
clients monitoring knee extension after changes in symptoms or loading
professionals monitoring lower-limb ROM and movement capacity
It may need modification if the client cannot comfortably lie prone or supine, has high pain at end range, cannot relax the limb, or has symptoms requiring further medical review.
Treatment table, plinth or firm surface
Tape measure or ruler
Optional towel roll or bolster
Optional Measurz AR measurement for distance
Optional Measurz inclinometer if also recording angle
Pain scale
Measurz/MAT platform to record side, distance, symptoms and retest comparison
Optional MAT tools such as Anker, Gripper or Muscle Meter for related lower-limb strength testing
Measurz can store this test alongside knee ROM, lower-limb strength, balance, orthopaedic tests, outcome measures and endurance assessments within the broader Measurz/MAT test library.
Choose the measurement method
Decide whether you will measure prone heel-height difference, supine heel-height difference or another consistent knee extension distance measure.
Prepare the surface
Use a firm table, plinth or mat. Record the setup so it can be repeated.
Position the client
For a prone heel-height method, position the client lying face down with the knees and thighs supported consistently and the lower legs positioned so heel height can be compared.
For a supine method, position the client lying face up with the legs relaxed and supported according to the chosen protocol.
Allow the limb to relax
Ask the client to relax the thigh and knee. Guarding can change the result.
Standardise support
Use the same towel roll, bolster, table edge or support position each time.
Measure the distance
Measure the selected distance in centimetres, such as heel-height difference between sides or knee extension gap from a defined landmark to the surface.
Compare sides
Record both sides where possible, especially if one side is used as the reference.
Record symptoms
Ask about pain, stiffness, posterior knee stretch, anterior knee discomfort, swelling sensation or guarding.
Repeat consistently
Repeat one to three times if appropriate and record the selected value consistently.
Common scoring options:
heel-height difference in centimetres
extension gap in centimetres
left-right difference
pain or symptom response
hyperextension difference
compensation notes
angle in degrees if measured alongside distance
A smaller extension deficit generally indicates a knee closer to full extension. A larger gap or heel-height difference may indicate greater extension limitation or asymmetry under the tested setup.
Interpretation is stronger when combined with:
pain score
symptom location
swelling or effusion notes
active versus passive extension
left versus right comparison
quadriceps strength
gait findings
squat or step findings
functional mobility tests
related knee ROM findings
The result does not explain the cause of reduced extension by itself. It helps guide monitoring and further assessment decisions.
Normal knee extension is often described as approximately 0 degrees, with some people having a small amount of hyperextension.
For centimetre-based testing, universal norms are limited because values depend on:
body size
thigh and calf shape
table setup
support position
measurement landmark
prone versus supine position
relaxation
symptoms
whether hyperextension is present
Practical field guidance:
aim for side-to-side symmetry where appropriate
record whether the knee reaches neutral extension
monitor changes from baseline
note any extension deficit in centimetres
record pain or stiffness at end range
use angle measurement if a more direct ROM score is required
interpret with gait, strength and functional tasks
Heel-height difference has been used as a practical way to document subtle extension deficits, especially where small changes are important. Some ACL-related literature has used heel-height difference because it can evaluate small variations in knee extension deficit, with one cited conversion reporting that 1 cm of heel-height difference is approximately equal to 1.2 degrees of knee extension limitation. This should be treated as method-specific guidance, not a universal conversion for every setup.
Distance-based knee extension measures can be useful when standardised.
Research on heel-height difference has shown a high correlation with standard ROM measurement and supports its use as a practical way to document knee flexion contracture or extension deficit. It has also been described as a meaningful and easier method for detecting subtle knee flexion contractures of less than 10 degrees.
Reliability improves when the same:
body position
surface
support position
measurement landmark
side comparison method
relaxation instructions
endpoint
measurement tool
symptom scale
are used each time.
Knee extension measurement studies also highlight that definitions, methods and client presentation can vary. Visual estimation, short-arm goniometers, long-arm goniometers, digital devices and distance-based measures should not be treated as interchangeable.
Small changes should be interpreted cautiously unless they are repeated, exceed likely measurement variation and align with symptoms, function or related testing.
Common errors include:
changing from prone to supine testing
changing the support position
measuring from inconsistent landmarks
not recording the reference side
not allowing the limb to relax
forcing extension
not recording pain or symptoms
ignoring swelling or guarding
comparing centimetres with degrees without context
assuming the result explains the cause of extension loss
Limitations include:
body size affects distance values
prone and supine methods differ
hyperextension can complicate interpretation
pain and guarding may affect results
swelling can limit extension
distance is not the same as joint angle
the test does not identify tissue source
the test does not determine sport or work readiness on its own
The Knee Extension (cm) assessment can help professionals:
monitor knee extension progress
track subtle extension deficits
compare sides
record end-range symptoms
support gait and lower-limb assessment
guide knee mobility programming
decide whether related tests would add context
combine knee ROM findings with strength, balance and functional performance
It is most useful when paired with:
Knee Flexion ROM
Supine Heel to Butt Test
Sit To Stand - 30 secs
Gait Speed
Step Down Test
Squat Assessment
Knee Extension Strength
Lower-limb strength testing
Outcome measures
Record:
test name: Knee Extension (cm)
method used
side tested
distance in centimetres
reference side
prone or supine position
support used
pain or symptoms
swelling or stiffness notes
compensation notes
retest date
Use Measurz AR measurement for distance, the Measurz inclinometer if also recording knee extension angle, and notes for symptoms and compensations.
A useful Measurz note may read:
“Knee Extension (cm), prone heel-height difference. Right heel 3.0 cm higher than left, indicating reduced right extension compared with left under this setup. Mild posterior knee stretch, no pain. Same table and support used for retest.”
It measures knee extension using a distance-based method, such as heel-height difference or knee extension gap.
No. Centimetres and degrees are different measurement methods. They can both be useful, but they should not be compared directly without context.
There is no universal centimetre value because the result depends on body size, position and measurement method. Side-to-side comparison and baseline tracking are usually most useful.
Yes. Side-to-side comparison is very useful, especially when one side is the reference.
No. It measures extension position but does not diagnose the cause of restriction.
Yes. Pain, stiffness, swelling and guarding all change interpretation.
Knee Extension (cm) records knee extension using a distance-based measure.
Heel-height difference can help document subtle extension deficits.
Centimetres and degrees should be recorded as separate measurement types.
Side-to-side and baseline comparison are often more useful than universal norms.
Standardisation is essential.
Measurz can record distance, method, side, symptoms, setup and progress.
Hancock, G. E., Hepworth, T., & Wembridge, K. (2018). Accuracy and reliability of knee goniometry methods. Journal of Experimental Orthopaedics, 5, 46. https://doi.org/10.1186/s40634-018-0161-5
Movement Assessment Technologies. (n.d.). Knee extension test. https://www.matassessment.com/blog/knee-extension-test
Norkin, C. C., & White, D. J. (2016). Measurement of joint motion: A guide to goniometry (5th ed.). F. A. Davis.
Shelbourne, K. D., Gray, T., & Haro, M. (2002). Reliability of heel-height measurement for documenting knee extension deficits. Knee Surgery, Sports Traumatology, Arthroscopy, 10(5), 279–284.
Webster, K. E., & Feller, J. A. (2023). Persistent knee extension deficits are common after anterior cruciate ligament reconstruction: A systematic review and meta-analysis. Knee Surgery, Sports Traumatology, Arthroscopy, 31, 4331–4342. https://doi.org/10.1007/s00167-022-07299-3
