The CoP zero point

IN ONE LINE

• • Two feet: zero is the line where the two devices meet.
• • One foot: zero is the centre of that device.

THE FOUR CASES

WHICH WAY IS POSITIVE

CASE 1 OF 4: TWO FEET ON PLATES

Two zero points exist at the same time: one for the overall CoP, one for each foot.

HOW TO READ IT

• The overall CoP is measured from the line where the two plates touch. Standing evenly, it reads close to 0, 0.
• Each foot also gets its own reading, measured from the centre of its own plate so a foot standing dead centre reads 0, 0 even if the person is leaning hard to one side.
• The dashed inner rectangle is where the sensors sit. It is the safe standing area for the patient. It is not a hard limit: a reading can go a little past the dashed line. The plate has its own weight, so when the load moves past the sensors, the sensors on the other side start to unload. When this happens, the plate is close to tipping.

CLASSIC PLATE 165 × 320 MM READINGS

These values are the sensor limits (the dashed line), not hard limits. Other plate sizes reach further (see the Sensor Limits table in the Amplitudes section).

CASE 2 OF 4: TWO FEET ON DELTAS

Exactly the same rule as plates — the devices are just much larger, so a reading can reach much bigger numbers.

HOW TO READ IT

• Same two zero points as plates: one at the meeting line for the overall CoP, one at each device centre for the per-foot values.
• The dashed shape is a trapezoid, not a rectangle — the long sensor side of each delta faces inward, toward the other foot.
• Bigger device, not bigger movement. A delta is nearly three times wider than a plate, so its limits are much larger. But the same body movement does not give a number three times larger. The CoP follows where the feet really stand, and the person's stance sets that, not the size of the device. On deltas, people usually stand near the inner edges. So the per-foot left-right value is usually positive on the left foot and negative on the right foot. On plates it is usually close to 0 on both feet.
• Sway values (path length, velocity, ellipse) still read higher on deltas because of sensor noise — see "Before comparing two results".

CLASSIC DELTA 460 × 804 MM READINGS

3D Deltas has its own section next. Delta Mini follows the same rule with its own dimensions (see the Sensor Limits table in the Amplitudes section).

BOTH FEET: 3D DELTAS

Three sensors per device instead of four, and the widest reach of any device — but the zero points sit in exactly the same two places.

HOW TO READ IT

• Same two zero points again: the meeting line for the overall CoP, each device centre for the per-foot values.
• Each device has three sensors, not four: two on the inner edge (top and bottom), and one alone on the outer edge at mid-height. That triangle is where the sensors sit, and it is the safe standing area. A reading can go a little past it, as on plates.
• This is the widest setup Kinvent makes. The overall value can reach ±500 mm (more than three times a classic plate pair). That is the size of the area, not the size of the sway: the same body movement gives a similar movement in millimetres on every device (apart from sensor noise — see "Before comparing two results").

3D DELTA 570 × 640 MM READINGS

CASES 3 AND 4 OF 4: Standing on one leg

There is no meeting line to measure from, so zero is simply the centre of the device being stood on.

WHAT THIS MEANS IN PRACTICE

• A single-leg reading of 0, 0 means the person is balanced over the middle of the device — nothing more.
• Two single-leg tests are shown side by side so left and right can be compared, but each side is still measured from its own centre.

ONE THING TO WATCH

In the side-by-side chart the bold vertical line in the middle looks like a zero line, but it is only a grid line. Each foot's zero is the crosshair at the middle of its own half. In the two-feet chart the same bold line is the zero — that is the only chart where it is.

SIGNS: What plus and minus mean

The chart is a view from above, with the person facing the top of the page.

READING A VALUE OUT LOUD

COP IN THE CSV EXPORT

The parameters are in millimetres. The raw CoP columns are not.

• Parameters, such as AP Mean Position and ML Mean Position, are in millimetres. They use the zero points and the signs in this guide: negative is toward the left and toward the heels.
• Raw CoP columns. In the "Raw Channel Data" part of a two-feet test, columns L to S hold AP COP (y) and ML COP (x) for the left device, the right device and the total. These are not millimetres. They are a position on a 0 to 1 scale across the two devices.

So in the total columns, the centre between the two devices is 0.5, 0.5 - not 0, 0.

Single-leg test: the 0 to 1 scale covers only the one device in use, so its centre is 0.5, 0.5.

HOW THE 0 TO 1 VALUE IS CALCULATED

The software takes the same CoP in millimetres that the parameters use (X and Y in this guide), and converts it. W is the device width and L is the device length.

Any result below 0 is written as 0, and any result above 1 is written as 1.

WHERE THE DASHED SENSOR LINES FALL ON THE 0 TO 1 SCALE

* Deltas: the AP values are for the long sensor side, which faces inward. On the outer side the sensors are closer together (classic delta: 0.294-0.706). The two-feet total columns use the same values as the left and right device columns. A reading between a dashed line and 0 or 1 is possible: the patient is outside the safe standing area and the device is close to tipping.

AMPLITUDES: How big is a big number

A value only means something next to the size of the device it came from.

SENSOR LIMITS OF EVERY DEVICE

All figures in millimetres. These limits are where the sensors sit inside each device. They mark the safe standing area for the patient. They are not a hardware limit: a reading can go a little past them, because the device has its own weight and the sensors on the other side start to unload. A reading close to a limit means the person is at the edge of the safe area, and the device is close to tipping.

A WORKED EXAMPLE: SHIFTING WEIGHT SIDEWAYS

The overall left-right value depends on two things: how the weight is shared between the feet, and how far apart the feet are. The table shows a weight shift with each foot at the centre of its own device. On plates this puts the feet 165 mm apart. On deltas it puts them 460 mm apart.

The deltas numbers are bigger because the feet are much further apart. The body is doing something different, not the device. With the feet the same distance apart, the same weight shift moves the value by the same amount on plates and on deltas.

In normal use, people on deltas do not stand at the device centres. The feet sit where the person's stance puts them, usually near the inner edges. So use this table to understand the maths, not as typical values.