Quadriceps Active Test

For Medical Students

What It Tests

The quadriceps active test looks for the tibia reducing forwards when the patient contracts the quadriceps, which is what happens only if the tibia was sitting back to begin with.

Every other posterior cruciate test applies a force from outside. This one asks the patient to supply it, and the finding is the movement that force produces.

The anatomical basis is the line of pull of the patellar tendon. With the knee flexed to about ninety degrees that tendon pulls the tibia forwards, so an intact quadriceps reduces a posteriorly subluxed tibia. 3.8.2 covers posterior stability.

How It Is Performed

Position the patient supine with the knee flexed to about ninety degrees and the foot flat on the couch.

Stabilise the foot lightly, enough that it does not slide and not so much that the patient cannot move.

Look along the profile of the knee from the side and note where the tibia is starting, which will usually be a visible sag. 4.11.1 covers the sag.

Ask the patient to slide the foot gently down the couch without lifting it, or to tense the thigh as if straightening the leg against your hand.

Watch the front of the tibia as the muscle contracts, and keep watching the profile rather than the foot.

What a Positive Means

A visible forward shift of the tibia as the quadriceps contracts is a positive test and indicates a deficient posterior cruciate.

The movement is the finding. A knee with an intact ligament has nothing to reduce, so the tibia does not move.

The test says the tibia was subluxed, which is the same information the sag sign gives, reached by a different route and without the examiner applying any force. 4.11.1 covers it.

A negative test in a patient who cannot or will not contract the quadriceps is uninformative rather than reassuring. The test depends on the patient and it fails silently.

MRCS Level

How It Is Performed

In a viva this is the test to name when asked what you would do in an acute knee that cannot tolerate a posterior drawer, and that is usually why it is being asked about.

The expected points are the flexion angle, the light stabilisation, the instruction given to the patient, and watching the tibial profile rather than the leg.

Say that the test needs the patient relaxed before the contraction and cooperative during it, because both halves of the comparison are needed.

Say that you would use it alongside the sag sign rather than instead of it, since one reads the resting position and the other reads the correction. 4.11.1 covers the sag.

Say that a knee held in extension gives nothing, because the patellar tendon is then pulling along a different line.

What a Positive Means

The reasoning expected is that the angle of the patellar tendon to the tibial plateau changes through the flexion range, and that at around ninety degrees its pull is anterior.

The same geometry explains why a contracted quadriceps masks a sag in a knee examined soon after injury, and why asking a guarding patient to relax is part of the examination rather than a courtesy. 4.11.1 covers that trap.

Near extension the tendon pull is close to parallel with the plateau and the test loses its basis, which is the anatomical reason for the flexed position rather than a convention.

The test distinguishes a posteriorly subluxed tibia from a normally positioned one. It does not distinguish an isolated cruciate injury from a combined one, and nothing about the movement tells you which. 4.11.7 covers the combined knee.

How Good It Is

The quadriceps active test is the most useful posterior cruciate test in the acutely injured knee, because guarding does not defeat it. Guarding is the mechanism it uses.

It is less useful in the chronic knee, where the sag is obvious and the posterior drawer can be performed properly, so it adds little to either. 4.11.6 covers the chronic knee.

It is more specific than sensitive in most teaching, because a knee with nothing to reduce produces no movement, and a positive result is therefore hard to produce by accident.

Its weakness is dependence on the patient rather than on the examiner, which is the opposite of the posterior drawer's weakness and the reason the two sit well together. 4.11.2 covers the drawer.

No figures appear for any of these comparisons, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level this test contributes to the diagnosis in a narrow window and to the indication almost not at all.

The window is the acute knee in the first weeks, where the question is whether the posterior cruciate is torn and the drawer cannot answer it. That question matters because early bracing in extension is the treatment most likely to be offered and most likely to be missed.

It also has a place in the patient who cannot be examined conventionally for reasons of size or pain, where the examiner's force is unreliable and the patient's own is not. 4.23.4 covers those patients.

Beyond those cases it is a confirmation rather than a decision, and a confirmation of something the inspection has usually already shown.

Where It Misleads

The commonest error is calling a test positive on the movement of the foot or the whole leg rather than on the tibia moving relative to the femur.

The second is accepting a negative in a patient who did not contract, which is an absent test rather than a normal one.

The third is performing it at the wrong angle, where the result is negative for a reason that has nothing to do with the ligament.

The fourth is expecting it to grade the injury. It is a yes or no observation and treating it as a grade imports a precision it does not have.

Fellowship Level

Where It Misleads

In referred practice the test appears most often as a line in somebody else's notes, and the question is what it was worth at the time.

A positive test recorded acutely is good evidence that the tibia was subluxed then, which matters when a patient presents years later with a chronic deficiency and a disputed history. 4.11.6 covers the presentation.

A negative test recorded acutely is weak evidence of anything, because the commonest reason for it is a patient who could not cooperate, and that reason is rarely written down.

After reconstruction the test loses its meaning rather than changing it. A graft that holds the tibia reduced leaves nothing to reduce, so the test is negative whether the graft is excellent or merely adequate, and the drawer and the step off carry the assessment instead. 4.11.2 covers it.

In the multiligament knee the test can be positive from a combined injury in which the corner contributes most of the subluxation, so a positive result here never licenses the word isolated. 4.11.7 covers the sequence.

In medicolegal work the distinction between an absent test and a negative one is the distinction that matters, and it is usually unrecoverable from the record. 4.23.7 covers that setting.