Active Motion

For Medical Students

What It Tests

Active motion is the range the patient achieves using their own muscles, so it tests the joint, the muscles and the patient's willingness together.

It is recorded separately from passive motion because the difference between the two is itself a finding, and the commonest version of that difference is an extension lag. 4.7.8 covers it.

Active motion is also the range that matters functionally, since a patient uses their knee actively rather than having it moved. 4.7.9 covers functional range.

How It Is Performed

Lay the patient supine with both legs straight and nothing under the knees. 4.3.5 covers the supine assessment.

Ask them to straighten the knee as far as they can and measure the angle achieved, stating the method used. 4.1.8 covers the record.

Ask them to bend it as far as they can, sliding the heel towards the buttock, and measure that angle.

Repeat on the other side and record both, since the comparison is part of the measurement. 4.1.2 covers the principle.

Ask them to hold the knee straight against gravity with the leg lifted, which tests the extensor mechanism rather than the range. 4.6.6 covers tendon tenderness.

What a Positive Means

An active extension deficit with a full passive range is an extension lag, indicating a quadriceps problem rather than a joint one. 4.7.8 covers it.

An equal active and passive deficit indicates a joint or soft tissue restriction, and the end feel says which. 4.7.5 covers end feel.

An inability to hold the knee straight against gravity indicates an extensor mechanism problem, which includes a tendon rupture and is urgent. 4.6.6 covers it.

A reduced active flexion with a full passive flexion indicates pain, weakness or reluctance, and asking the patient which is usually productive. 4.7.4 covers flexion.

A range that is reduced in both knees raises a generalised cause rather than a knee injury. 4.2.14 covers inflammatory pain.

MRCS Level

How It Is Performed

In a viva measuring active before passive is the expected order, and recording both is what distinguishes a measurement from an impression.

The expected points are the supine position, the active extension and flexion measured and stated, both sides, and the straight leg hold against gravity.

Say that you would measure with a goniometer and state the landmarks, since an unqualified figure cannot be compared later. 4.1.8 covers the record.

Say that you would measure active before passive, because moving the knee passively first alters what the patient will then attempt.

Say that you would assess the effusion first, since an effusion inhibits the quadriceps and reduces active motion without any structural cause. 4.8.2 covers the mechanism.

What a Positive Means

The reasoning expected is that active motion requires a muscle capable of generating force, a tendon capable of transmitting it and a joint capable of moving, so a deficit localises to one of the three. 2.4.3 covers patellar biomechanics.

Terminal extension is the part of the arc where the quadriceps works at its greatest mechanical disadvantage, which is why an extension lag is the commonest active deficit. 4.7.8 covers it.

Reflex inhibition from an effusion reduces quadriceps output without any weakness, which is why an effused knee has an active deficit that resolves. 4.8.2 covers it.

The screw home mechanism requires rotation in terminal extension, so a knee that cannot rotate has an active extension deficit for a reason unrelated to the muscle. 3.3.5 covers that mechanism.

How Good It Is

Active range is reproducible when the method is stated and poorly reproducible when it is not, which puts the record rather than the examiner in the critical path.

It is the measurement that corresponds to function, which makes it the more useful of the two ranges to follow over time. 4.7.9 covers functional range.

It depends on the patient, so it fails in a guarding or uncooperative patient, and in those cases the passive range is the more interpretable measurement. 4.7.2 covers it.

It cannot distinguish weakness from inhibition from reluctance, which the effusion assessment and the resisted tests do. 4.8.3 covers the effusion test.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the active range contributes to timing, to feasibility and to one urgent diagnosis.

The urgent diagnosis is an extensor mechanism rupture, identified by the inability to hold the knee straight against gravity, and it is a clinical diagnosis with an operative answer. 4.6.6 covers the tendons.

The timing contribution is that a knee with a poor active range is not ready for a ligament reconstruction, because operating on it produces a stiff result. 4.7.7 covers contractures.

The feasibility contribution is that the active range sets what a rehabilitation programme starts from, and a programme planned against a passive figure overestimates the starting point. 4.2.34 covers previous rehabilitation.

Following the active range after surgery is more informative than following the passive one, because it reflects the muscle as well as the joint. 4.7.10 covers post-operative range.

Where It Misleads

The commonest error is recording one range rather than both.

The second is measuring without a stated method.

The third is reading an active deficit in an effused knee as a structural problem. 4.8.2 covers it.

The fourth is not testing the straight leg hold, which is how an extensor mechanism rupture is missed.

The fifth is measuring with a pillow under the knee, which hides an extension deficit.

Fellowship Level

Where It Misleads

In referred practice the active range is the measurement that explains a dissatisfied patient with a technically satisfactory knee.

A patient with a full passive range, a persistent active extension deficit and a competent reconstruction has a quadriceps problem, and surgery does not treat it. 4.3.9 covers wasting.

A patient whose active range has deteriorated while the passive range has not has developed inhibition, pain or weakness rather than stiffness, and those have different treatments. 4.7.2 covers passive motion.

A patient with an active deficit years after a reconstruction may have a patellofemoral problem driving the inhibition, and the sitting examination is where it appears. 4.3.4 covers that assessment.

In a knee with a previous extensor mechanism repair the active range may be the best achievable, and setting that expectation is more useful than pursuing a number. 4.6.6 covers the tendons.

In medicolegal work the active range is the figure that corresponds to the claimed disability, and its absence from a record is the commonest reason such a claim cannot be assessed. 4.23.7 covers that setting.