Force plates vs. balance boards when measuring center of pressure

[jdlamphrey]

When measuring center of pressure, what are the advantages and disadvantages of using force plates versus balance boards?

Thank you for your feedback,

Jess
Seton Hall University

[ccagran22]

Jess,

We using force-plates for quite a lot for CoP measurements but haven't used balance boards yet and I can therefore only tell you about my experiences with the FP. However, you might want to check the following article: Ross A. Clark, Adam L. Bryant, Yonghao Pua, Paul McCrory, Kim Bennell, Michael Hunt, Validity and reliability of the Nintendo Wii Balance Board for assessment of standing balance Original Research Article, Gait & Posture, Volume 31, Issue 3, March 2010, Pages 307-310. http://dx.doi.org/10.1016/j.gaitpost.2009.11.012

I would guess that the balance boards are a lot cheaper and lighter to carry around, but the FP to be more stable, versatile, with a larger range. I hope this information helps in any way.

Claus Cagran
Human Performance Research Graz, Austria

[sjones65]

Hi Jess,

I have some experience with both.

I think the greatest difference has to do with the way that centre of pressure is measured. Force plates include shear forces when calculating CoP. Balance boards do not. Balance boards have four vertical load sensors, one at each post, and do not measure any shear forces. Major differences in CoP measurements are going to depend on the application. If you are measuring fairly static loading scenarios with no significant shear forces, balance boards will probably give a pretty good estimate of CoP. If you have more dynamic situations, the difference between the measurements will be larger.

Wii balance boards can be pretty noisy at low weights (<30lbs or so). This is because of the noise in the sensors, and the equation they use to calculate CoP. If you need accurate CoP measurements at low weights, a Balance Board might not be the best choice.

The equations that the Balance Board uses to calculate CoP are:
TL = Top Right
BL = Bottom Right
TR = Top Right
BR = Bottom Right

long axis CoP = 21 * (TR + BR - (TL + BL) ) / (TL + TR + BL + BR)
short axis CoP = 12 * (TL + TR - (BL + BR) ) / (TL + TR + BL + BR)

As the weight decreases, the magnitude of the denominator decreases. Small changes in the numerator and denominator have a greater impact at lower weights.

Force plates can generally be sampled at higher and more accurate sampling rates, and are a lot more robust in this area. I have been able to record Balance Boards at up to 64Hz, but there tend to be dropped or repeated frames a few times each second.

With multiple Balance Boards, I have not been able to get them perfectly synchronized. Again, if the measurements are fairly static, this is not a big deal, but it can lead to errors if the movements are quicker.

I hope that this helps!

Simon Jones
Centre for Studies in Aging
Toronto, Canada

[jdlamphrey]

Hi Simon,

Thank you for your helpful response. Do you have a preference between piezo-electric (Kistler) and strain guage (AMTI) force platforms?

Thank you,
Jess

[rhinrichs97]

In response to Simon Jones' post about how center of pressure is calculated, I would like to know how shear forces are used to calculate the center of pressure using a force platform. I was always under the impression that the center of pressure was, by definition, the center of normal pressure and that the center of shear pressure potentially falls at a different spot on the force platform. To demonstrate my potential (mis)understanding consider this hypothetical (and simple) situation: Assume the force platform is horizontal so that normal forces are vertical and shear forces are horizontal. You push vertically downward with two discrete forces of equal magnitude. The center of normal pressure is half way between the two points of application of these forces on the plate. Now, keeping one force the same, you change the direction of the other force so that it has the same vertical component as before but you now add a horizontal component. In other words you push harder on the platform with one of the forces but at an angle such that the center of normal pressure remains half way between the points of application of the two forces. Where is the center of shear pressure? It is at the point of application of the angled force since that is the only shear force applied to the platform. Peter Cavanagh did some experiments a Kistler force platform many years ago at Penn State when I was a Ph.D. student. We were able to show that the calculated center of normal pressure did not change when a shear component was added to the second force. However we could not calculate the exact point of the center of shear pressure, only the line of action of the shear force relative to the center of the force plate. We did not publish these results since it seemed trivial at the time. Nonetheless it did point out the limitations of the term "center of pressure" as being synonymous with "center of normal pressure", at least when it came to a Kistler force platform. I currently use AMTI force platforms and I'm pretty sure the same limitation applies. If others have done more research on the center of shear pressure since Peter and I first looked at this 30 years ago I'd like to hear about it. Perhaps today's more modern force platforms have a different way of calculating center of pressure?

[sjones65]

Hi Rick,

Thanks for your comments. I've thought about what you wrote, and I hope that I can explain what I meant.

I wrote that the force plates use shear forces to determine COP because in the equations that I have used with both AMTI and Kistler, the shear forces are terms in the equation. Upon closer inspection, these terms only become important factors when the shear forces are applied at a different height than the centre of the force plate. Shear forces are used in the COP equations to move the centre of pressure from the horizontal plane at the centre of the force plate to the horizontal plane of the surface of the force plate. This would be why, in your experiments, adding a shear force did nothing to alter the location of the centre of pressure.

The Kistler COPy equation is identical to the Wii COPy equation except for the shear term fx*az0 (where az0 is the height of the force plate surface above the force plate centre):
COPy = -(a * (-fz1 + fz2 + fz3 - fz4) - fx*az0) / (fz1 + fz2 + fz3 + fz4)

If you were using a Wii Balance Board instead of a force plate and repeated the same experiment (adding a pure shear force to a normal-force-only scenario), you WOULD see a change in the centre of pressure - an unwanted change. I just tried this out and saw the Wii COP move when a shear force was added.

It's not force plates that have changed the way that they calculate COP - it is the Balance Boards that are calculating it differently.

Simon

[rhinrichs97]

Thanks, Simon. I agree with your assessment. Nonetheless I'd like to hear comments (from you and others listening) about my contention that the center of shear pressure can be in a totally different place on the platform than the center of normal pressure. In fact, the two may have no relationship to each other. What do you think?