Reiner Beck (zebris)

09-03-2002, 05:45 PM

Dear List Members,

one week ago I posted following question:

I just learned that some people recently do use PDM (Pressure Distribution

Platforms) to perform balance and equilibrium tests on people. The persons

centre of gravity is calculated from the pressure distribution caused by the

person standing on the platform. The position of the centre of gravity is

recorded over time while the person stands on the platform eyes closed.

As a part of the evaluation a confidential ellipsis is approximated. This

ellipsis is containing the positions of the centre of gravity during the

test. Here comes my question:

How is this ellipsis interpreted ? Do the parameter of the ellipsis (length

major axis, length minor axis, ratio of this lengths, orientation, position

to feet) have a described indication ?

The following responses have been very helpful, many thanks to those who

contributed !!

Pax!

Perhaps this can be of some help to your query about the confidence

ellipse in posturology.

The force plate records the movements of the centre of pressure

(COP). The data consists of points with x-coordinates (medial-lateral

direction) and y-coordinates (posterior-anterior direction). In

calculating the confidence ellipse one first subtractes the averages

form x and y, then rotates the coordinate systems till the coordinates x'

and y' in the new coordinate system become uncorrelated, corr( x', y') =

0. (If we let x' and y' denote the vectors consisting of the dat points

x'[k] and y'[k], then the new coordinate axes are obtained as the

eigenvectors of the 2x2-matrix C with C[1][1] = x'*x', C[1][2] = C[2][1] =

x'*y', C[2][2] = y'*y' .) It's now supposed that x' and y' are statistically

indpendent. Next, if the standard deviation of x' and y' are stdx and

stdy, one assumes that the normalized variables x'' = x'/stdx, y'' =

y'/stdy are gaussian with zero mean and standard deviation = 1.

Finally, define the variable z = x''^2+y''2. According to our assumptions

of gaussian distribution this will have a chi-square distribution of

degree 2. For instance (looking up a table),

Prob(z < 5.995) = 0.95

which means that the 95% confidence ellipse will be a circle of radius

sqrt(5.995) in the x'',y''-system, and an ellipse with minor and major

axes of length sqrt(5.995)*stdx and sqrt(5.995)*stdy in the original

coordinate system. If the data points are approximately gaussian

distributed it means that ca 95% of the points are inside this ellipse.

The main parameters obtained from this ellipse is its area [units mm2]

and the inclination of the major axis. The area is e.g. used in

caluclating the Romberg quotient: QR = 100*S_ec/S_eo; where S_ec is

the area of the ellipse in case of eyes closed, S_eo the area in case of

eyes open. The 90% confidence ellipse is also commonly used

(replace 5.995 by 4.605 above).

One traditional test is to measure balance during quiet standing first

with eyes open, then with eyes closed, and compare the areas. So the

area is a basic parameter. Basically the tests consists of a

measurement under *normal* conditions, followed by a test with

altered conditions that might influence the balance control mechanism.

Also important is the spectrum of the x and y time series which may

reveal subtle changes in swaying. Odd pekas in the spectrum may

show some pathological condition. I have worked on an experimental

software for force plates and have thus become familiar with a number

of different posturological parameters in use and I am testing some

new ones. You might find this website useful (papers and documents):

http://perso.club-internet.fr/pmgagey/Index.htm

mantained by one of the masters in the field, P-M Gagey.

Regards

Frank B

===================================

Frank Borg

Chydenius Institute/ Jyväskylä Univ.

PB 567

Pitkänsillankatu 1-3

FIN-67701 Kokkola

Finland

frank.borg@chydenius.fi

====================================

Dear colleague,

as far as I know, different methods are used in the computation of the COP

during postural assesments. In the case you'll use the one mentioned in your

e-mail (which analyzes the ellipses), I think you should consider only the

90% of the total measured area. One of the most precise method I know is the

Principal Component Analysis (PCA), but I'm sorry I can't explain exactly

how it works here.

Good luck for your work.

Best wishes,

Federico Formenti

-------------------------------------

Federico Formenti

University of Verona

Department of Sport and Exercise Sciences

e-mail: fformenti@univr.it

off: 0039 045 895 26 33

fax: 0039 045 895 26 31

The clinical interpretation is complex. Some studies have

demonstrated clinical utility without trying to use it as an explicit

marker of a specific (uni-dimensional) biomechanical impairment.

From first principles the use of the ellipsoid is difficult to

interpret in isolation.

In my opinion the excursion of the FP (Force platform) data is not

independent of the level of compensation or control strategy the

subject uses. For example I did some work on sitting balance in

individuals with spinal cord injury. What I found was that

individuals with paraplegia had greater ellipsoid area (95%tile CI)

when compared to individuals with quadriplegia. This would seem

paradoxial since individuals with the higher lesion have poor sitting

balance.... however what we were seeing was a highly controlled

attempt by the individuals with poorer sitting balance. [This is

necessary because if they move away from their safe equilibrium

posture they have no capacity to save themselves]. The individuals

with paraplegia were much more comfortable to sit without support and

therefore did not need to have such a high frequency of control.

The sway paths lengths were the same and [as demonstrated by

frequency analysis] therefore the frequency content of the

Quadriplegic was significantly higher.

I have see this in individuals with brain injury and also in other

"stability" tests.

Cheers

GTA.

p.s Lord et al has used a modified excursion assessment as a factor

in prediction of falls in the elderly.

A long time ago I use the area of the ellipsoid, sway path length and

the frequency content. I chose these after a long look at the

literature. These may not be the best choices. [I have to do the

fourier transformation in excel on a computer which at the time had

1Mb RAM and was the best computer at the school:-)].

The area of the ellipsoid is the one option - of course how do you

fit the ellipsoid? I determined the 95%tile limits of the AP and ML

axes and then multiplied them - a scaling factors provides a real

translation to the area but statistically the data are the same. (4pi)

You may wish to translate the data around the axes to optimise the

fitting of the actual ellipsoid.

I suppose you have to decide if you use linear excursions in the ML

and AP directions or combine them for a 2D (planar estimate). The

difference probably depends on the foot position and control

strategies etc and your hypotheses.

Some people look at the frequency content and see if there is a lot

of power at the higher frequencies (peaks rather than median) -

suggesting a higher than average "frequency of control" evidence in

Brain injured individuals trying to maintain balance and people with

more specific neurological disorders such a Parkinson's Disease.

With the digital filtering capacities these days you could look at

combinations of the sway path length at frequencies higher than say

0.5Hz. and 1Hz , etc...

Comments from my side:

There is to differ between tests performed on force plates (FP, such as

AMTI) and pressure distribution measurement platforms (PDMs). The reason for

this is a big difference in the sampling rate between the systems (FP > 1000

Hz, PDM < 100 Hz). As a consequence things can only be compared and

transferred with great caution !

Many thanks !

Cheers

Reiner

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one week ago I posted following question:

I just learned that some people recently do use PDM (Pressure Distribution

Platforms) to perform balance and equilibrium tests on people. The persons

centre of gravity is calculated from the pressure distribution caused by the

person standing on the platform. The position of the centre of gravity is

recorded over time while the person stands on the platform eyes closed.

As a part of the evaluation a confidential ellipsis is approximated. This

ellipsis is containing the positions of the centre of gravity during the

test. Here comes my question:

How is this ellipsis interpreted ? Do the parameter of the ellipsis (length

major axis, length minor axis, ratio of this lengths, orientation, position

to feet) have a described indication ?

The following responses have been very helpful, many thanks to those who

contributed !!

Pax!

Perhaps this can be of some help to your query about the confidence

ellipse in posturology.

The force plate records the movements of the centre of pressure

(COP). The data consists of points with x-coordinates (medial-lateral

direction) and y-coordinates (posterior-anterior direction). In

calculating the confidence ellipse one first subtractes the averages

form x and y, then rotates the coordinate systems till the coordinates x'

and y' in the new coordinate system become uncorrelated, corr( x', y') =

0. (If we let x' and y' denote the vectors consisting of the dat points

x'[k] and y'[k], then the new coordinate axes are obtained as the

eigenvectors of the 2x2-matrix C with C[1][1] = x'*x', C[1][2] = C[2][1] =

x'*y', C[2][2] = y'*y' .) It's now supposed that x' and y' are statistically

indpendent. Next, if the standard deviation of x' and y' are stdx and

stdy, one assumes that the normalized variables x'' = x'/stdx, y'' =

y'/stdy are gaussian with zero mean and standard deviation = 1.

Finally, define the variable z = x''^2+y''2. According to our assumptions

of gaussian distribution this will have a chi-square distribution of

degree 2. For instance (looking up a table),

Prob(z < 5.995) = 0.95

which means that the 95% confidence ellipse will be a circle of radius

sqrt(5.995) in the x'',y''-system, and an ellipse with minor and major

axes of length sqrt(5.995)*stdx and sqrt(5.995)*stdy in the original

coordinate system. If the data points are approximately gaussian

distributed it means that ca 95% of the points are inside this ellipse.

The main parameters obtained from this ellipse is its area [units mm2]

and the inclination of the major axis. The area is e.g. used in

caluclating the Romberg quotient: QR = 100*S_ec/S_eo; where S_ec is

the area of the ellipse in case of eyes closed, S_eo the area in case of

eyes open. The 90% confidence ellipse is also commonly used

(replace 5.995 by 4.605 above).

One traditional test is to measure balance during quiet standing first

with eyes open, then with eyes closed, and compare the areas. So the

area is a basic parameter. Basically the tests consists of a

measurement under *normal* conditions, followed by a test with

altered conditions that might influence the balance control mechanism.

Also important is the spectrum of the x and y time series which may

reveal subtle changes in swaying. Odd pekas in the spectrum may

show some pathological condition. I have worked on an experimental

software for force plates and have thus become familiar with a number

of different posturological parameters in use and I am testing some

new ones. You might find this website useful (papers and documents):

http://perso.club-internet.fr/pmgagey/Index.htm

mantained by one of the masters in the field, P-M Gagey.

Regards

Frank B

===================================

Frank Borg

Chydenius Institute/ Jyväskylä Univ.

PB 567

Pitkänsillankatu 1-3

FIN-67701 Kokkola

Finland

frank.borg@chydenius.fi

====================================

Dear colleague,

as far as I know, different methods are used in the computation of the COP

during postural assesments. In the case you'll use the one mentioned in your

e-mail (which analyzes the ellipses), I think you should consider only the

90% of the total measured area. One of the most precise method I know is the

Principal Component Analysis (PCA), but I'm sorry I can't explain exactly

how it works here.

Good luck for your work.

Best wishes,

Federico Formenti

-------------------------------------

Federico Formenti

University of Verona

Department of Sport and Exercise Sciences

e-mail: fformenti@univr.it

off: 0039 045 895 26 33

fax: 0039 045 895 26 31

The clinical interpretation is complex. Some studies have

demonstrated clinical utility without trying to use it as an explicit

marker of a specific (uni-dimensional) biomechanical impairment.

From first principles the use of the ellipsoid is difficult to

interpret in isolation.

In my opinion the excursion of the FP (Force platform) data is not

independent of the level of compensation or control strategy the

subject uses. For example I did some work on sitting balance in

individuals with spinal cord injury. What I found was that

individuals with paraplegia had greater ellipsoid area (95%tile CI)

when compared to individuals with quadriplegia. This would seem

paradoxial since individuals with the higher lesion have poor sitting

balance.... however what we were seeing was a highly controlled

attempt by the individuals with poorer sitting balance. [This is

necessary because if they move away from their safe equilibrium

posture they have no capacity to save themselves]. The individuals

with paraplegia were much more comfortable to sit without support and

therefore did not need to have such a high frequency of control.

The sway paths lengths were the same and [as demonstrated by

frequency analysis] therefore the frequency content of the

Quadriplegic was significantly higher.

I have see this in individuals with brain injury and also in other

"stability" tests.

Cheers

GTA.

p.s Lord et al has used a modified excursion assessment as a factor

in prediction of falls in the elderly.

A long time ago I use the area of the ellipsoid, sway path length and

the frequency content. I chose these after a long look at the

literature. These may not be the best choices. [I have to do the

fourier transformation in excel on a computer which at the time had

1Mb RAM and was the best computer at the school:-)].

The area of the ellipsoid is the one option - of course how do you

fit the ellipsoid? I determined the 95%tile limits of the AP and ML

axes and then multiplied them - a scaling factors provides a real

translation to the area but statistically the data are the same. (4pi)

You may wish to translate the data around the axes to optimise the

fitting of the actual ellipsoid.

I suppose you have to decide if you use linear excursions in the ML

and AP directions or combine them for a 2D (planar estimate). The

difference probably depends on the foot position and control

strategies etc and your hypotheses.

Some people look at the frequency content and see if there is a lot

of power at the higher frequencies (peaks rather than median) -

suggesting a higher than average "frequency of control" evidence in

Brain injured individuals trying to maintain balance and people with

more specific neurological disorders such a Parkinson's Disease.

With the digital filtering capacities these days you could look at

combinations of the sway path length at frequencies higher than say

0.5Hz. and 1Hz , etc...

Comments from my side:

There is to differ between tests performed on force plates (FP, such as

AMTI) and pressure distribution measurement platforms (PDMs). The reason for

this is a big difference in the sampling rate between the systems (FP > 1000

Hz, PDM < 100 Hz). As a consequence things can only be compared and

transferred with great caution !

Many thanks !

Cheers

Reiner

---------------------------------------------------------------

To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl

For information and archives: http://isb.ri.ccf.org/biomch-l

---------------------------------------------------------------