KneeSchool/Clinical Examination Academy/Single Leg Squat
Single Leg Squat
Page 4.21.1 · Functional Testing · examination page
The simplest functional test, which exposes control the couch examination cannot see.
The same topic at 4 depths
Each tier deepens the one below it and never repeats it. Choose the level you want.
Single Leg Squat
Choose a depthFor Medical Students
What It Tests
The single leg squat tests the knee under load on one limb, so it measures quadriceps strength, hip control and the knee's tolerance of flexion at the same time. 4.4.10 covers functional movement screening.
The examination is looking at the quality of the movement rather than at whether the patient can do it. 4.3.9 covers muscle wasting.
It matters because a knee that examines normally lying down can lose control as soon as it carries body weight. 4.7.9 covers the functional range.
How It Is Performed
Ask the patient to stand on one leg, with the other foot clear of the ground and the arms free. 4.5.7 covers dynamic alignment.
Ask them to squat as far as they comfortably can and return, watching from the front and then from the side.
Watch the knee in the coronal plane, specifically for it moving inwards relative to the foot. 3.12.7 covers dynamic valgus.
Watch the pelvis, since a pelvis dropping on the unsupported side indicates hip abductor weakness rather than a knee problem. 4.4.5 covers the Trendelenburg gait.
Watch the trunk, because leaning over the stance leg compensates for both.
Note the depth reached and whether pain or apprehension stops it. 4.7.6 covers the painful arc.
Compare both sides, since the test is interpreted by comparison. 4.1.4 covers the structured approach.
What a Positive Means
A controlled squat with the knee tracking over the foot, a level pelvis and an upright trunk, to a similar depth on both sides, is normal.
The knee moving inwards during the descent indicates poor dynamic control, which is a modifiable finding rather than a structural one. 3.12.7 covers dynamic valgus.
A pelvis that drops indicates hip abductor weakness, and the knee is the downstream site rather than the source. 4.4.5 covers the Trendelenburg gait.
A reduced depth with pain at a particular angle localises the symptom to the arc rather than to the squat. 4.7.6 covers the painful arc.
A knee that gives way during the test raises instability rather than weakness. 4.2.6 covers giving way.
Key Learning Points
- The test measures movement quality rather than ability.
- Knee, pelvis and trunk are watched separately.
- Inward knee movement is a modifiable control finding.
- A dropping pelvis makes the hip the source and the knee the site.
- The test is interpreted by comparing the two sides.
MRCS Level
How It Is Performed
In a viva this is the functional assessment candidates skip, and including it distinguishes a complete answer.
The expected points are the setup, the view from the front and the side, the coronal knee movement, the pelvis, the trunk, the depth, the pain and the comparison.
Say that you would watch the pelvis and the trunk as well as the knee, and give the reason. 4.4.5 covers the Trendelenburg gait.
Say that you would record the depth and what limited it. 4.7.9 covers the functional range.
Say that this test informs rehabilitation rather than diagnosis. 4.2.32 covers previous physiotherapy.
What a Positive Means
The reasoning expected is that single limb loading requires the hip, the knee and the trunk to work together, so a failure anywhere shows at the knee. 3.14.7 covers jumping.
Dynamic valgus during loading increases the load on the anterior cruciate and the patellofemoral joint, which links this test to both. 3.7.12 covers loading during landing.
Quadriceps weakness reduces the depth achievable and alters the strategy, which is why the strategy is observed alongside the depth. 4.3.9 covers muscle wasting.
Hip abductor weakness permits the femur to adduct and rotate, which produces the knee's inward movement without any knee pathology. 4.4.5 covers the Trendelenburg gait.
How Good It Is
The test is reproducible for gross abnormalities of control and subjective for subtle ones.
Its best contribution is exposing a control problem that no static test shows, which is the reason it is included. 4.5.7 covers dynamic alignment.
It is not diagnostic for any structure, since the same abnormal pattern follows from several causes.
No imaging corresponds to this test, so it sits alongside the structural assessment rather than competing with it. 4.1.6 covers that relationship.
No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.
Key Learning Points
- Single limb loading needs hip, knee and trunk together.
- Dynamic valgus loads the cruciate and the patellofemoral joint.
- Quadriceps weakness changes the strategy as well as the depth.
- Hip abductor weakness produces inward knee movement with a normal knee.
- No static test shows what this one shows.
FRCS Level
How Good It Is
At decision level the test contributes to rehabilitation planning and to return to sport judgements rather than to diagnosis.
Poor control with a structurally intact knee is a rehabilitation target, and the test identifies it. 4.21.7 covers return to sport testing.
Poor control after a reconstruction indicates the rehabilitation is incomplete whatever the laxity examination shows. 4.20.1 covers the reconstruction assessment.
Dynamic valgus in an uninjured athlete is a risk pattern rather than a diagnosis, and the distinction matters when advising. 3.7.14 covers injury mechanisms.
Pain reproduced at a specific depth localises a patellofemoral or a compartment problem, and the test is then diagnostic by provocation. 4.15.9 covers patellofemoral pain.
Where It Misleads
The commonest error is watching only the knee, so a hip or a trunk cause is attributed to it. 4.4.5 covers the Trendelenburg gait.
A fit patient compensates well enough to look normal, so the test is insensitive in the strong. 4.23.2 covers the elite athlete.
The test is performed once, and fatigue is what exposes the control problem in many patients. 4.21.2 covers the single leg hop.
Pain stops the test and is recorded as poor control.
The comparison side is assumed normal, which it often is not after a bilateral problem. 4.1.4 covers the structured approach.
Key Learning Points
- Watching only the knee attributes hip and trunk causes to it.
- A strong patient compensates and the test is insensitive in them.
- One repetition does not expose a control problem fatigue would reveal.
- Pain stopping the test is recorded as poor control.
- The comparison side is not necessarily normal.
Fellowship Level
Where It Misleads
At fellowship level the single leg squat is a screening observation whose interpretation is less standardised than its popularity suggests.
Scoring is by observation, so the same squat is graded differently by different examiners and across time. 4.5.7 covers dynamic alignment.
The movement pattern is modifiable within a single session by coaching, which means the finding measures current strategy rather than capacity. 4.2.32 covers previous physiotherapy.
Dynamic valgus is associated with injury at a population level and does not predict it in an individual, which is the limit of what the test licenses. 3.7.14 covers injury mechanisms.
Pain and control are confounded, since a painful knee alters its strategy and the altered strategy is then recorded as poor control. 4.7.6 covers the painful arc.
A single plane observation misses the rotational component, which is part of the pattern that matters. 4.19.7 covers torsional assessment.
Key Learning Points
- Observational scoring varies between examiners and over time.
- The pattern is coachable within a session, so it measures strategy not capacity.
- Dynamic valgus predicts at a population level and not in an individual.
- Pain and control are confounded in the recorded result.
- Single plane observation misses the rotational component.
Explore Further
Where this topic came from and where it goes next in the spiral.
- 4.21.2Single Leg Hop
- 4.21.7Return to Sport Testing
- 4.4.10Functional Movement Screening
- 4.5.7Dynamic Alignment
- 4.4.5Trendelenburg Gait
- 4.15.9Patellofemoral Pain Syndrome
References
No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 023 rather than an omission. The comparative standing of these tests is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page, and chapter 3.17 lists the studies the figures would come from. Any measurement here, including a flexion angle, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.
Safety. KneeSchool is educational and does not give individual medical advice. Seek urgent assessment for a knee that cannot bear weight after an injury, a hot swollen knee with a fever, or new numbness, weakness or coldness in the leg or foot.