Range of Motion: Knee Flexion

The Knee Flexion Test measures how far the knee can bend. It is useful for baseline assessment, side-to-side comparison, progress tracking, return-to-training context and monitoring pain or symptoms during movement.

Introduction

Knee flexion range of motion is one of the most commonly recorded lower-limb mobility measures. It helps professionals understand how comfortably a client can bend the knee during tasks such as squatting, lunging, kneeling, cycling, running preparation and floor-based movement.

A knee flexion result is useful, but it does not explain the cause of a movement limitation on its own. Interpretation is stronger when ROM is compared with the other side, the client’s baseline, pain response, confidence, strength, function and related assessment findings.

Quick Summary

• Measures knee bending range, usually in degrees.
• Can be assessed actively, passively or both.
• Record side, position, pain score, symptoms and compensation.
• Compare to baseline and the opposite side rather than relying on one universal “normal”.
• Small changes may reflect measurement error unless the protocol is consistent.

What Is the Knee Flexion Test?

The Knee Flexion Test measures the available range as the knee bends. It can be performed as:

• Active ROM: the client bends the knee using their own effort.
• Passive ROM: the professional assists the knee into flexion without the client actively producing the movement.

Active ROM gives information about movement control, comfort and available self-generated range. Passive ROM gives additional context about available joint and soft-tissue range when the client is relaxed.

Why It Is Used

The test is used to:

• Establish a knee ROM baseline.
• Compare left and right sides.
• Track progress across sessions.
• Monitor symptoms with movement.
• Guide exercise selection and range exposure.
• Add context for lower-limb strength, function and movement testing.
• Support return-to-training planning, without being used as a stand-alone clearance measure.

What It Measures

The test measures knee flexion in the sagittal plane. The result is usually recorded in degrees using an inclinometer, goniometer or digital ROM tool.

It may be influenced by:

• pain or apprehension
• swelling or stiffness
• quadriceps and anterior thigh mobility
• hamstring or calf bulk
• previous injury or surgery
• warm-up and fatigue
• testing position
• examiner stabilisation
• device placement

Who It Is Useful For

This test is useful for:

• athletes returning to running, jumping, squatting or kneeling
• general fitness clients with lower-limb mobility goals
• older adults where knee bending affects stairs, sitting and transfers
• post-injury or post-surgery monitoring where appropriate
• performance professionals tracking mobility alongside strength and function
• teams or businesses wanting repeatable ROM data

Equipment Required

• Firm plinth, floor mat or testing bench
• Inclinometer, goniometer or digital ROM device
• Measurz recording profile
• Optional towel or bolster
• Pain scale
• Notes field for symptoms and compensations

Step-by-Step Protocol / Practice

1. Choose the test position. Supine is commonly used for assisted knee flexion. Prone may also be used but should be recorded separately because values are not directly interchangeable.
2. Explain the movement. Ask the client to bend the knee as far as comfortable.
3. Set the starting position. The hip and pelvis should stay controlled. Avoid allowing the pelvis to rotate or lift.
4. Active ROM. Ask the client to slide the heel toward the buttock or bend the knee as far as they can without forcing.
5. Passive ROM, if relevant. Gently assist knee flexion until the first firm end point, symptom limit or compensation.
6. Device placement. With a goniometer, align the axis near the lateral femoral epicondyle, the stationary arm with the greater trochanter and the moving arm with the lateral malleolus. With an inclinometer, keep placement consistent and record the method.
7. Ask what the client feels. Record pain score, symptom location, tightness, pressure, apprehension or any unusual response.
8. Record compensations. Watch for hip hiking, pelvic rotation, lumbar extension, foot gripping or the client lifting the hip.
9. Trials. Use one familiarisation trial and one to three recorded trials if precision is important.
10. Retest consistency. Use the same position, warm-up, device, landmarks, side order and instructions next time.

Stop the test if pain increases sharply, symptoms are unusual, the client cannot relax during passive testing, or the movement is not appropriate for their current status.

Scoring and Interpretation

Record knee flexion in degrees. Higher values generally reflect more available knee bending range, while lower values show less available range in that test position.

Interpretation should include:

• active ROM result
• passive ROM result, if tested
• left versus right comparison
• pain score
• symptom location
• end-feel or limiting factor
• compensation pattern
• change from baseline
• related strength and function findings

A lower knee flexion score does not explain the cause by itself. It may reflect pain, guarding, swelling, soft-tissue limitation, joint stiffness, strength control, confidence, body size or testing method.

Normative Data, Benchmarks or Reference Values

Evidence level: Level 2 — closest available reference values.

Common reference values often place knee flexion around 135–150 degrees, depending on source, population and method. The CDC normal joint ROM study provides age- and sex-specific reference values for selected joints and highlights that ROM differs by age and sex rather than being one universal number.  

Use reference values as context, not strict pass/fail criteria. Knee flexion needed for daily function may be less than maximal anatomical range, but sport, kneeling tasks and deep squatting may require greater flexion.

Practical interpretation should prioritise:

• baseline comparison
• side-to-side difference
• whether active and passive ROM differ
• pain and symptom response
• task goals such as squat depth, kneeling or cycling
• consistency across repeat sessions

Reliability and Validity

Lower-limb ROM measurement reliability depends heavily on standardised positioning, landmarks, device placement and examiner consistency. A recent systematic review of lower-limb ROM measurement found that reliability varies across joints, tools and protocols, which supports the need for consistent measurement procedures.  

Clinical goniometric and digital tools can be useful, but measurement error comes from the device, examiner and client. A 2023 study on common goniometric devices emphasised the importance of validity, reliability and acceptable measurement error when ROM data are used for decisions.  

No single MDC or MCID should be applied to all knee flexion testing unless it matches the protocol, population and device. Small changes should be interpreted cautiously unless they are repeated, meaningful to the client and supported by consistent testing.

Sensitivity and Specificity

Sensitivity and specificity are not usually applicable to knee flexion ROM testing because this assessment measures movement range rather than identifying a condition on its own.

Common Errors and Limitations

Common issues include:

• inconsistent starting position
• poor goniometer or inclinometer placement
• measuring hip or pelvic compensation instead of knee flexion
• comparing active ROM with passive ROM as if they are the same
• ignoring pain or symptom response
• using different warm-ups between sessions
• assuming one “normal” value applies to every client
• comparing values from supine and prone testing without noting the difference

Practical Applications

Knee flexion ROM can help with:

• baseline knee mobility testing
• monitoring progress after irritation, injury or surgery when appropriate
• tracking tolerance to squatting, kneeling and cycling
• comparing active and passive mobility
• guiding exercise range selection
• supporting return-to-training decisions alongside strength, balance, confidence and functional testing

How to Record This in Measurz

In Measurz, record:

• test name: Knee Flexion ROM
• side tested
• active or passive ROM
• score in degrees
• device used
• client position
• pain score
• symptom location
• end-feel or limiting factor
• compensations
• comparison side
• baseline score
• best or average trial
• related strength or function findings
• retest date

Example note: “Right knee flexion AROM 128°, supine, inclinometer, pain 2/10 anterior knee, mild hip lift near end range. Left 138°. Retest in two weeks.”

Related Tests / Internal Links

• Knee Extension Test
• Knee Prone Heel-to-Butt Test
• 90/90 Active Knee Extension Test
• Hip Flexion Test
• Squat assessment
• Lower-limb strength testing

FAQs

What is normal knee flexion ROM?
Common reference values are often around 135–150 degrees, but values vary by age, sex, body size, activity history and testing method.

Should knee flexion be measured actively or passively?
Both can be useful. Active ROM shows what the client can produce independently, while passive ROM adds context about available range with assistance.

What does reduced knee flexion mean?
It shows less available knee bending in that test, but it does not explain the cause on its own.

How should knee flexion be tracked over time?
Use the same position, device, landmarks, warm-up and instructions, then compare with baseline, symptoms and function.

Key Takeaways

• Knee flexion ROM is best interpreted with side-to-side and baseline comparison.
• Active and passive ROM should be recorded separately.
• Pain, symptoms and compensations are as important as the degree score.
• Standardised testing improves confidence in progress tracking.

References

Centers for Disease Control and Prevention. (2023). Normal joint range of motion study. https://archive.cdc.gov/www_cdc_gov/ncbddd/jointrom/index.html

Kiatkulanusorn, S., Luangpon, N., Srijunto, W., Watechagit, S., Pitchayadejanant, K., Kuharat, S., Bég, O. A., & Suato, B. P. (2023). Analysis of the concurrent validity and reliability of five common clinical goniometric devices. Scientific Reports, 13, 20725. https://doi.org/10.1038/s41598-023-48344-6

Santos, H. H., et al. (2025). Reliability of range of motion measurements obtained by goniometry, photogrammetry and smartphone applications in the lower limbs: A systematic review. Journal of Bodywork and Movement Therapies.