Test Protocols

Motor Control Test (MCT)

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In this module

  1. Overview of the MCTFoundation · Trainee · Clinician
  2. Translations: amplitudes and directionsFoundation · Trainee · Clinician
  3. LatencyFoundation · Trainee · Clinician
  4. Amplitude scalingFoundation · Trainee · Clinician
  5. Weight symmetryTrainee · Clinician
  6. Pattern interpretationTrainee · Clinician

Overview of the MCT

The Motor Control Test measures the automatic postural response to a sudden horizontal translation of the platform. It is the protocol most sensitive to central involvement, because the response transit time — the latency — reflects long-loop brainstem-spinal pathways.

The test takes about three minutes. The patient stands quietly while the platform delivers six perturbations: small, medium, and large translations in both forward and backward directions, with three trials at each amplitude-direction combination.

Three measurements come out of each trial: latency (in milliseconds), amplitude (in some systems expressed as response strength relative to platform displacement), and weight symmetry (the relative load on each leg during the response).

Translations: amplitudes and directions

Translation amplitudes are scaled to patient height. A small translation moves the platform about 1.5 cm in a young adult; large translations are about 5.5 cm. The translation completes in roughly 400 ms, with peak velocity in the middle of that window.

When the platform translates backward, the patient's centre of mass lags behind — they begin to fall forward. The corrective response is a backward sway driven by anterior calf muscles. Forward platform translations produce the opposite response.

The amplitude scaling lets the test distinguish weak responses (small amplitude on a large perturbation) from absent responses (no response at any amplitude). It also reveals exaggerated responses — over-scaling — which suggests anxious hypersensitivity rather than organic disease.

Latency

Latency is measured from platform-translation onset to the first deflection of the patient's centre of pressure in the corrective direction. Healthy adults respond at 120–150 ms; the lower limit of normal is around 110 ms and the upper limit around 165 ms.

Latencies above 170 ms in a patient with intact peripheral nerves implicate the long-loop pathway. The most informative single lesion is a brainstem stroke involving the descending vestibulospinal or reticulospinal tracts; less dramatically, cerebellar disease and demyelination can also prolong latency.

Peripheral neuropathy can prolong latency by slowing the afferent or efferent limb, but more often reduces amplitude. A combination of prolonged latency and reduced amplitude with confirmed neuropathy is consistent with the peripheral diagnosis. The same combination in a patient with intact peripheral nerves is a flag for central pathology.

Amplitude scaling

Amplitude scaling describes how the response size varies with perturbation size. A healthy adult produces a small response to a small perturbation and a large response to a large one, scaled approximately linearly.

Under-scaling — responses too small for the perturbation — occurs in peripheral neuropathy, in late-stage central disorders, and in some patients with chronic deconditioning. Functionally, it represents an inadequate corrective force.

Over-scaling — responses too large for the perturbation — is more characteristic of anxious patients, including those with PPPD. The brain over-corrects, sometimes producing oscillation between the initial response and a counter-response. Over-scaling is one of the supportive CDP features of PPPD listed in the Bárány Society criteria.

Weight symmetry

Weight symmetry compares the load on the two legs during the corrective response. The convention varies between systems; one common form expresses asymmetry as (R − L) / (R + L), with healthy values near zero (perfect symmetry).

Asymmetric responses can reflect lower-limb pathology — pain, weakness, recent surgery — or hemiparesis. They are less specific than latency or amplitude findings and need clinical correlation.

Persistent asymmetry after a unilateral vestibular lesion is unusual on the MCT, because the perturbation drives a bilateral postural response that doesn't depend on labyrinthine input. If asymmetry is present in this setting, look for an additional lower-limb cause.

Pattern interpretation

Three latency-based patterns matter most. Normal latency, normal amplitude: the long-loop pathway is intact; central pathology is unlikely. Prolonged latency, any amplitude: long-loop involvement; central or peripheral nerve cause; correlate clinically. Normal latency, reduced amplitude: peripheral or musculoskeletal cause more likely.

Amplitude scaling adds nuance. Linear under-scaling across amplitudes suggests neuropathy or deconditioning; non-linear over-scaling at small amplitudes with normal scaling at large amplitudes is a hint toward anxious hypersensitivity.

The MCT alone rarely makes a diagnosis. Its value lies in confirming or refuting central involvement when the SOT shows a peripheral pattern, and in objectively documenting the hypersensitivity profile that may underlie PPPD.