Foundations
Technique & Equipment
Each section is layered for Foundation, Trainee, and Clinician readers — set your reading level in the sidebar.
Showing5of 5 sections·Trainee
0:00The CDP system has three principal hardware components: a dual force plate, a movable surround, and a movable platform.
0:11The force plate measures centre of pressure — the point at which the vertical ground-reaction force acts. Sway is computed from the displacement of this point over time.
0:26Sway-referencing is the central technical innovation. The platform or surround rotates in real time to track the patient's body sway, holding the support or visual reference effectively stationary relative to the body.
0:44The patient cannot feel the rotation. From their perspective, the input has simply become uninformative — the floor moves with them, or the walls move with them, providing no orientation cue.
1:00The equilibrium score on each SOT trial is calculated from the peak anterior-posterior body sway, expressed relative to the theoretical 12.5 degree limit of stability.
1:17A score of 100 means the patient barely swayed at all. A score of zero means the patient exceeded the limit of stability — operationally, a fall.
1:32The Motor Control Test uses small, medium, and large rapid platform translations to evoke postural responses. The latency from translation onset to response is the key measurement.
1:49The Adaptation Test uses toes-up and toes-down platform rotations across five trials, measuring sway energy on each trial. The healthy nervous system rapidly damps the response — sway energy decreases trial-on-trial.
2:08Patient safety harnessing, calibration to the patient's height, and the same instructions on every trial — these procedural details matter as much as the equipment.
In this module
- The force plateFoundation · Trainee · Clinician
- Sway-referencing of surface and surroundFoundation · Trainee · Clinician
- Setup, calibration, and safety harnessFoundation · Trainee · Clinician
- Common artefacts and pitfallsTrainee · Clinician
- Normative databases and device differencesTrainee · Clinician
The force plate
The force plate is a rigid platform suspended on four strain-gauge transducers, one at each corner. From the four vertical forces, the system computes the centre of pressure — the single point at which the patient's vertical ground-reaction force can be considered to act.
As the patient sways, the centre of pressure shifts beneath them. Anterior-posterior shifts dominate quiet standing and are the principal CDP signal. Mediolateral sway is smaller in most conditions but matters in some disorders (cerebellar truncal ataxia in particular).
Commercial systems sample the force plate at 100 Hz or higher. The signal is filtered to remove cardiac and respiratory artefact and converted into degrees of body sway by assuming a single-segment inverted-pendulum model — a reasonable approximation for quiet stance though not for postural strategies that bend at the waist.
NeuroCom EquiTest booth
The reference CDP system — dual force plate, sway-referenced visual surround, software-controlled platform tilt, overhead safety harness.
- Three-walled enclosure isolates the patient from ambient visual flow.
- Dual force plates (one per foot) compute mediolateral and anteroposterior CoP.
- Servo motors tilt the platform in sync with measured AP sway.
- Visual surround sways under software control to deliver SOT 3 and 6.
Sway-referencing of surface and surround
Sway-referencing is the central technical innovation that makes CDP possible. The platform (or visual surround) rotates in real time about the ankle axis to track the patient's body sway, holding the support surface — from the patient's perspective — in a constant relationship to the body.
The patient cannot feel the rotation directly. What they experience is that the floor no longer provides reliable information about which way is down; if they lean forward, the platform rotates forward to match them, so the ankle joint angle doesn't change. Somatosensation now reports stationarity even as the body falls.
The same trick can be applied to the visual surround — rotating the walls in lock-step with body sway so that the visual flow field reports no movement. This is what produces conditions 3 and 6 of the SOT, where vision becomes uninformative.
Setup, calibration, and safety harness
Setup takes about five minutes. The patient stands on the force plate in stocking feet, with foot position marked using a height-based template — taller patients stand with their feet further apart, so that the centre of mass falls in a consistent location relative to the force-plate origin.
A safety harness, attached to an overhead frame, prevents falls without supporting the patient's weight during the trial. A correctly fitted harness is taut enough to catch but loose enough that the patient does not feel suspended; some systems include a tension sensor to flag trials in which the harness took load.
Calibration confirms that the force plate's centre is correctly registered and that the surround motors respond linearly. Daily zeroing is recommended; full calibration every few months or after any service event.
Common artefacts and pitfalls
Harness assist is the commonest artefact: a patient leans into the harness, transferring some weight to the overhead frame. Equilibrium scores rise spuriously. Modern systems flag this via tension sensors; older systems require operator vigilance and re-instruction.
Step responses end the trial early. Some scoring conventions credit a step as a fall (equilibrium score 0); others split the response between completed sway and the step itself. Know which convention your system uses before comparing scores across centres.
Foot placement drift across the six SOT conditions changes the effective base of support and can shift the equilibrium scores by several points. Re-mark foot position between conditions if the patient steps off and back on.
Anxiety stiffening reduces sway by recruiting co-contraction of agonist and antagonist muscles. The resulting scores are high but the strategy is non-physiological and shows up in strategy analysis (where it survives) as flat ankle traces with abnormally low sway frequencies.
Normative databases and device differences
Normative data are device-specific. NeuroCom/Natus, Bertec, and other manufacturers each publish their own age-stratified norms; cross-device comparison of raw scores is hazardous. Always interpret a patient's data against the norms supplied with the device that recorded them.
Age effects are real and substantial. Composite scores drop by roughly 0.4 points per year after age 60, with most of the drop on conditions 5 and 6. A composite of 65 in a 75-year-old may be within normal limits; the same score in a 30-year-old is clearly abnormal.
Paediatric norms are sparser but have improved. Rine and colleagues (2018) published age-stratified data for children 5–18, showing adult-like patterns by about age 7 with gradual maturation of conditions 5 and 6 through adolescence.
| Modality | Principle | What it measures | Best for | Caveat | Cost |
|---|---|---|---|---|---|
| Static posturographyBertec® BalanceCheck, Tetrax™ | Fixed force plate records CoP sway during quiet stance. | Sway area, sway velocity, path length, frequency content. | Population screening, longitudinal tracking, low-resource clinics. | Cannot localise the sensory or motor system at fault. | low |
| Dynamic posturographyResearch and modified clinical platforms | Force plate plus controlled platform perturbations (translation, tilt). | Reactive postural responses to mechanical disturbance. | Selected research populations and tertiary referral centres. | Standardisation between sites is variable; protocols often custom. | medium |
| Computerized Dynamic PosturographyNeuroCom® EquiTest, SMART EquiTest, Pro-Kin | Dual force plates + sway-referenced visual surround under software control. | SOT (sensory weighting), MCT (motor latencies), ADT (adaptation), LoS (voluntary control). | Vestibular labs — sensory dependence, central vs peripheral, rehab planning. | Cost, footprint and trained-operator requirement limit accessibility. | high |
| Wearable IMU systemsAPDM Opal, BioStamp, smartphone accelerometry | Miniature inertial sensors over trunk / lower limb measure sway, gait and turning. | ISway-style sway, turn velocity, gait variability — across real-world settings. | Community testing, tele-rehabilitation, longitudinal Parkinson tracking. | Lacks the controlled sensory-conflict manipulations of CDP. | low |
| VR / AR-augmented testingHeadset + force plate / IMU systems (research and emerging clinical) | Virtual or augmented visual environments graded for optic flow and complexity. | Sway under controlled, immersive visual perturbation. | Visual-dependency syndromes (PPPD), graded desensitisation, paediatric engagement. | Equipment cost and motion-sickness risk; clinical evidence still building. | medium |