A client may report difficulty bending the knee fully, reduced comfort when kneeling, stiffness during squatting, or a side-to-side difference after a period of reduced loading, symptoms or training interruption.
The Supine Heel to Butt Test gives a simple way to assess knee flexion using a practical position. It can be recorded as a distance measure, an angle measure, or a qualitative comparison.
The result does not explain the cause of reduced knee flexion on its own, but it provides useful baseline information when interpreted alongside pain, swelling, symptoms, strength, quadriceps capacity, hip range of motion and functional tests.
Test name: Supine Heel to Butt Test
Purpose: Assess knee flexion range, heel-to-butt distance and side-to-side comparison
Movement: Bending the knee so the heel moves toward the buttock
Joint/body region: Knee, anterior thigh and lower limb
Plane: Sagittal plane
ROM type: Active ROM, passive ROM or assisted ROM
Score: Heel-to-butt distance, knee flexion degrees, side-to-side difference or qualitative result
Equipment: Tape measure, goniometer, inclinometer or Measurz ROM recording workflow
Best used with: Knee flexion ROM, knee extension ROM, squat, sit-to-stand, lunge, calf raise, gait and lower-limb strength tests
Key limitation: Distance and ROM values vary by body size, thigh/calf contact, hip position, symptoms, swelling, device and measurement method
The Supine Heel to Butt Test assesses how far the knee can bend while the client is lying face-up.
The heel is moved toward the buttock either by the client actively bending the knee or by the professional assisting the movement. The result may be recorded as:
heel-to-butt distance
knee flexion angle
whether the heel can touch the buttock
side-to-side comparison
pain or symptom response
movement quality
This test is useful when a practical field comparison is needed, especially when tracking change over time.
The test is used to establish a baseline, compare sides and monitor change in knee flexion over time.
It may help inform:
knee mobility monitoring
anterior thigh and quadriceps mobility context
squat and kneeling assessment
lower-limb progress tracking
side-to-side comparison
symptom response during knee flexion
return-to-training decisions
exercise selection for knee and lower-limb programmes
The test measures how much knee flexion is available in a supine heel-to-butt position.
It may be influenced by:
knee joint range of motion
anterior thigh and quadriceps tissue tolerance
calf and hamstring bulk
swelling or effusion
pain or symptoms
hip position
strength and motor control
warm-up
previous activity or loading history
measurement device
professional technique
client effort or guarding
A reduced heel-to-butt result provides movement information, but it does not explain the cause on its own.
Active supine heel to butt measures how far the client can bend the knee using their own muscle control.
Passive supine heel to butt measures how far the knee can move when guided by the professional while the client relaxes.
Comparing active and passive results can help separate available movement from control, strength, pain inhibition, confidence or guarding.
Passive ROM should be applied gently and should not force symptoms.
This test may be useful for:
runners
field sport athletes
gym clients
older adults
clients monitoring knee movement
clients returning to squatting, kneeling or lower-limb training
clients with a side-to-side knee flexion difference
people tracking lower-limb mobility over time
It is also useful when a simple retest method is needed for home, gym, field or app-based assessment.
Firm surface or treatment table
Tape measure if recording heel-to-butt distance
Goniometer or inclinometer if recording knee flexion angle
Pain scale
Measurz for recording ROM, side, distance, pain and progress
Optional towel roll
Optional comparison side notes
Optional Measurz AR measurement for distance or setup consistency
Optional Measurz inclinometer if recording angle
Position the client lying on their back with both legs relaxed.
Use the same surface and position for each retest.
The test leg starts extended or in a comfortable starting position. The opposite leg may remain straight or bent, but this should be recorded and kept consistent.
Stand or sit beside the test leg so the knee, hip and foot position can be observed.
Keep the thigh reasonably stable and avoid allowing excessive hip rotation or pelvic movement.
Stabilise the thigh if needed and avoid allowing the whole leg to rotate outward or inward during the movement.
For active ROM, ask the client to slide or bring the heel toward the buttock as far as comfortably possible.
For passive ROM, guide the heel toward the buttock gently until the first firm endpoint, symptom limit or agreed end range.
Choose one consistent method:
measure heel-to-butt distance in centimetres
measure knee flexion angle in degrees
record whether heel-to-butt contact is achieved
record side-to-side difference
record symptom response
For distance, measure from the heel or posterior calcaneus to the buttock using the same landmarks each time.
For goniometry, commonly align the axis near the lateral femoral epicondyle, the stationary arm toward the greater trochanter and the moving arm toward the lateral malleolus.
Ask about pain, stretch, stiffness, pressure, pinching, symptom location and whether the movement feels familiar.
Stop if pain increases sharply, symptoms spread, the client guards strongly, swelling or pressure limits movement, or the test is not tolerated.
Record side, active or passive method, distance or angle, pain score, symptom location, hip position, opposite-leg position, device used and compensation.
One to three trials may be used. Record the best, average or selected trial consistently.
Use the same position, device, landmarks, warm-up, endpoint and scoring method each time.
The result may be recorded as:
heel-to-butt distance in centimetres
knee flexion angle in degrees
side-to-side difference
heel-to-butt contact achieved or not achieved
pain score
symptom location
movement quality
A smaller heel-to-butt distance generally indicates more available knee flexion in this position. A larger distance indicates less knee flexion or reduced tolerance under the tested setup.
If measured in degrees, a higher knee flexion value generally indicates more knee flexion.
Interpretation is stronger when combined with:
pain score
symptom location
active versus passive comparison
left versus right comparison
knee extension ROM
swelling or effusion
quadriceps strength
hamstring strength
squat, step, lunge or kneeling findings
gait or running findings
The result does not explain the cause of reduced movement by itself. It helps guide exercise selection, monitoring and further assessment decisions.
Evidence level: Level 3 — limited exact norms for this specific supine heel-to-butt protocol; use practical comparison guidance.
Common knee flexion teaching references often describe knee flexion values around 135 degrees, but values vary by protocol, position, device, age, body structure, symptoms and measurement method.
For a heel-to-butt distance test, universal norms are limited because results are influenced by thigh size, calf size, heel shape and testing position.
Practical benchmarks:
compare left and right sides
compare active and passive results
compare baseline to retest
track pain at end range
track symptom location
track distance and/or angle consistently
use related squat, kneeling, step and gait findings
If heel-to-butt contact is achieved comfortably and symmetrically, this may indicate good practical knee flexion for many general tasks. However, sport, occupational and individual requirements vary.
ROM reliability improves when the same measurement position, landmarks, device and endpoint are used.
Research on knee ROM measurement shows that accuracy and minimum detectable differences vary by device. Digital inclinometers and long-arm goniometers may provide more accurate repeated measurements than visual estimation or short-arm goniometers.
For heel-to-butt distance testing, reliability is likely to improve when:
the same surface is used
the same measurement landmarks are used
the same active or passive method is used
hip and pelvic position are controlled
the same endpoint is used
symptoms are recorded
the same assessor or method is used where possible
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 active to passive testing without recording it
changing the hip position
allowing excessive hip rotation
forcing end range
measuring from inconsistent landmarks
comparing distance and angle results directly
not recording pain or symptoms
not recording which side was tested
comparing results from different protocols
using the test as a diagnosis
Limitations include:
body size affects heel-to-butt distance
calf and thigh contact may limit distance
pain and swelling may limit movement
active control may differ from passive capacity
distance measurement is not the same as joint angle
protocol differences affect comparison
the test does not identify tissue source
the test does not determine readiness for sport or work on its own
Use the Supine Heel to Butt Test to:
establish baseline knee flexion
compare sides
monitor symptom response
track mobility progress
guide lower-limb exercise selection
support squat, kneeling and lunge progressions
decide whether related tests would add context
compare ROM with function and strength findings
It is most useful with:
knee flexion ROM
knee extension ROM
squat assessment
sit-to-stand testing
lunge testing
step-down testing
calf raise testing
gait or running assessment
lower-limb strength testing
In Measurz, record the baseline result using the chosen method.
Record:
active or passive test
side tested
heel-to-butt distance or knee flexion angle
pain score
symptom location
testing position
hip position
opposite-leg position
device used
endpoint definition
compensation notes
retest date
Use the Measurz inclinometer if recording knee flexion angle. Use Measurz AR measurement if recording heel-to-butt distance or setup consistency.
Track progress across sessions and compare both sides. Add related lower-limb strength, calf endurance, squat, step, gait or running findings when relevant.
Knee Flexion ROM
Knee Extension ROM
Prone Heel to Butt Test
Ely Test
Squat Assessment
Sit To Stand - 30 secs
Step Down Test
Single-Leg Calf Raise Test
Hamstring Raise
Knee Pain Outcome Measures
It measures how close the heel can move toward the buttock in a face-up position, giving information about knee flexion range and symptom response.
It is closely related. It can be recorded as knee flexion angle or as heel-to-butt distance.
Both can be useful. Active testing shows what the client can control. Passive testing shows available movement when guided.
Many teaching references describe knee flexion around 135 degrees. For heel-to-butt distance, side-to-side and baseline comparison are usually more useful than a universal value.
It means the heel remains farther from the buttock under the tested setup. It does not explain the cause by itself.
No. It provides movement information but does not diagnose the cause of knee pain.
Record side, active/passive method, distance or degrees, pain, symptoms, test position, device and compensations.
Use the same position, landmarks, device, endpoint and scoring method each time.
Supine Heel to Butt assesses practical knee flexion in a face-up position.
It can be recorded as distance, angle or qualitative side-to-side comparison.
Active and passive results should be labelled separately.
Broad knee flexion references are useful, but side-to-side and baseline comparison are often more practical.
Body size, pain, swelling, hip position and protocol influence results.
Measurz should capture distance or degrees, side, pain, symptoms, device, position and progress.
Clarkson, H. M. (2020). Musculoskeletal assessment: Joint range of motion, muscle testing, and function (4th ed.). Wolters Kluwer.
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
Norkin, C. C., & White, D. J. (2016). Measurement of joint motion: A guide to goniometry (5th ed.). F. A. Davis.
Sancheti, K. H., Sancheti, P. K., Shyam, A. K., Joshi, R., Patil, K., & Jain, A. (2013). Factors affecting range of motion in total knee arthroplasty using high flexion prosthesis: A prospective study. Indian Journal of Orthopaedics, 47(1), 50–56.
