What is a "normal" centre of pressure pathway?

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Your answers to the above question are invited.

 

Dear Simon,
Tough question, I'd like to say the answer is 42

The path of the centre of pressure (CoP) during walking varies amongst individuals, by deviating to a greater or lesser extent towards the medial or lateral border of the foot. My co-workers and I recently looked at the influence of foot morphology on centre of pressure excursion during barefoot walking in 83 adults whose foot type had been classified as supinated, "normal" or pronated using the Foot Posture Index (FPI). Three CoP parameters were used: CoP Medial Excursion Area, CoP Lateral Excursion Area and CoP Total Excursion Area. Across the spectrum of foot types, we found that the more supinated a participant’s foot posture, the larger the area of lateral COP excursion, and conversely, the more pronated the foot posture, the smaller the area of lateral COP excursion. Furthermore, the supinated foot type had a relatively larger CoP total excursion area and the pronated foot type a relatively smaller CoP total excursion area.

Kind regards
Joshua Burns PhD, B App Sc (Pod) Hons
NHMRC Australian Clinical Research Fellow
Discipline of Paediatrics and Child Health
Faculty of Medicine, The University of Sydney, Australia

 

Hi Simon,

Some thoughts:

What whould you do with a normal value? I doubt that a finding of an abnormal pattern would be very predictive of pathology unless the pattern was from a foot that was visually abnormal. This the problem I've had with Root theory. The "normal" foot in root theory could still have pathology.

The center of pressure, at an instant of time, is an average. When data is averaged, meaningful information can be lost. For example a foot wtih calluses sub 1 and 5 could have the same pressure path was a foot with callus sub 2. The calluses would be more predictive of pathology than the center of pressure path.

Winter has postulated that people will alter their center of pressure path to fine tune medial to lateral balancing while walking. (The location of center of pressure is highly dependent on the relative activity of the extrinsici muscles of the foot. see Hicks) Craig Payne mentioned a study where people with foot pain had less variation in their center of pressure path. If you combine Winter's balance idea with pain avoidance I would prdict less variation in the center of pressure path. Some foot pain is dependent on foot position. If a certain position of the foot is avoided because of pain then there will be less variation in the CoP path. Balance, during walking, can also be achieved by changing the placement of the swing limb at contact. So the need to avoid pain overrules the bodies attempt to fine tune balance using center of pressure.

So, to answer your question: I don't think that establishing a normal value for center of pressure path location would be useful. You might be able to come up with an average value, but would it be useful for anything?

Cheers.

Eric

 

Simon, Josh, Eric and Others:

To continue along with Eric's comments, I also think the center of pressure (CoP) excursion or path is overated as an indicator of foot kinetics.

What are the widest ranges of CoP excursion that can occur in an individual's foot? The CoP path cannot be wider than the plantar surface of the foot. Therefore, CoP path is constrained by foot width. However, subtalar joint (STJ) axis location is not constrained by the width of the plantar surface of the foot and may, in fact, be often quite a distance medial to the forefoot. If we could measure the relatively large spatial movements or excursions of the STJ axis, and compared it to the relatively narrow excursions of the CoP, my bet would be that STJ axis excursion would be more predictive of foot and lower extremity mechanical pathology than CoP excursion.

Just some thoughts for you to chew on regarding the importance of "normal" CoP pathways.

 

Thank you for this post Simon!
Having just been given the use of both Pedar and Emed systems this topic is of great interest.
I have worked with good success without these types of systems- as have many others. I am now interested in finding out what clinically useful information they could give me that could not be obtained otherwise.
I have read Daryl Williams recent paper in JAPMA, which is helpful starting point.
Simon/ Kevin- have you done much more work with your STJAL?? The ability to track the STJ axis dynamically combined with a foot pressure analysis could be very interesting...
Also Josh,

Any chance you could elaborate on these parameters- are these standard Pedar measurments or are they customised? Do you think you could predict a person's foot posture by their COP readout?
Cheers,

 

Craig and colleagues,

Any chance you could elaborate on these parameters- are these standard Pedar measurments or are they customised?

The Emed (platform) parameters were selected to quantify CoP from the Novel AXIS Software Program (Novel gmbh, Munich, Germany).

Do you think you could predict a person's foot posture by their COP readout?

Not entirely. We can predict 20% of the variance in dynamic COP by a person's foot posture. Full details of study to be published in an upcoming issue of JAPMA.

Josh

 

Thanks Josh
I look forward to reading your paper.
Cheers,

 

The subtalar joint axis locator (STJAL) could be used in combination with a inshoe or pressure mat system to approximate the STJ moments at any instant during gait. However, it would be better to use the STJAL with a force plate (allows 3D force vector determination)which would then allow you to determine the medial-lateral and anterior-posterior shearing forces which are not otherwise detectable using pressure analysis systems. To my knowledge, the use of a STJAL with a force plate has not been done yet.

Simon gave me one STJAL to experiment with and I think he has one also. Other than that, we probably have the only two STJAL's that I know exist. When I originally thought up the device, probably over 10+ years ago, it seemed like it should work. Simon's diligence and mechanical ingenuity actually progressed the STJAL from an idea and drawing into a working apparatus that has been shown in our preliminary research to track the motions of the talus and this has been published in the literature (Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006). Thanks for that, Simon. Hopefully our reserach will get others interested in improving on the basic mechanical idea of such a device so that further experimental work may be performed with this type of device in cadaver and live subjects to see how well it tracks the STJ axis spatial location during non-weightbearing and weightbearing motions of the foot.

We have only scratched the surface of the vast potential that the STJ Axis Locator has in helping us to better understand the biomechanics of injury production within the human locomotor apparatus.

 

I don't have the reference at hand, but I believe Winter and Nigg, Bobbert have both done papers with markers on the foot and have determined the axis from the motion. The locater is a nice tool, but to look at moments about the STJ with force platform (or in shoe sensor) you would still have to digitize the location of the axis from the locator. Their methods would already give the digital location.

Cheers,

Eric

 

I am not aware of this research. Maybe you can dig this one up for me, Eric. I don't remember any research that tracked STJ axis spatial location from markers since the very limited excursions of the talo-calcaneal joint during gait would likely preclude accurate determination of STJ axis location by this type of method.

 

van den Bogert AJ, Smith GD, Nigg BM. In vivo determination of the anatomical axes of the ankle joint complex: an optimization approach.
J Biomech. 1994 Dec;27(12):1477-88.

Scott SH, Winter DA. Talocrural and talocalcaneal joint kinematics and kinetics during the stance phase of walking.
J Biomech. 1991;24(8):743-52.

 

If you replaced the current markers on the locator with "active markers" or markers that could be picked up by motion analysis, this would not be a problem. Still waiting for Dave Smith to come up with his protocol for this study

 

Eric:

Van Den Bogert's paper, if I remember correctly, used a mathematical optimization approach for attempting to determine the STJ axis which was done in a non-weightbearing fashion and has never been duplicated by other researchers to my knowledge. This method would be highly impractical for us to perform, if it could be performed. Steve Piazza and his crew at Penn State tried to duplicate their research and found their results to be too variable to be practical.

Scott and Winter's research did not, to my knowledge, measure rotations and translations of the STJ axis in space. Didn't they also use bone pins for this study??

Please correct me if I'm wrong.

 

Pierrynowski MR, Finstad E, Kemecsey M, Simpson J.
Relationship between the subtalar joint inclination angle and the location of lower-extremity injuries.
J Am Podiatr Med Assoc. 2003 Nov-Dec;93(6):481-4.

This study used Van Den Bogert's 'optimisation code' to determine STJ inclination angle- they tracked the position of the foot in relationship to the leg using markers to find this angle. They didn't actually dynamically track the axis- only determine it's position in the sagittal plane. It was not examined dynamically with respect to during gait, so not much help here.

I find the idea about tracking the axis, and examining the forces to estimate moments throughout the gait cycle very interesting. I probably have the resources to do it- minus the STJAL...
Simon and Kevin, I might send you a PM later about this as it is the weekend already- it's Friday here which is like Sunday... Saturday is like Saturday, but Sunday is like Monday... takes some getting used to...

Cheers,

 

Selection of useful parameters to evaluate center-of-foot pressure movement.
Percept Mot Skills. 2006 Dec;103(3):959-73Demura S, Kitabayashi T, Noda M

 

This study aimed to objectively summarize the parameters for evaluating the center-of-foot pressure and to select useful parameters with high reliability and validity with 220 healthy university students. In addition, 50 healthy university students were selected to examine cross validity. The measurement of center-of-foot pressure was carried out 3 times with a 1-min. rest and the mean of Trials 2 and 3 was used for the analysis. The data sampling frequency was 20 Hz. 34 parameters were selected from 6 domains: distance, distribution, area, velocity, spectrum, and vector. As a result of factor analysis the following four factors were interpreted: unit time sway, front-back sway, left-right sway and high frequency band sway. Their intraclass correlation coefficients were very high (ICC=.89-.95). Five parameters representing each factor were selected using stepwise selection by regression analysis. There were no significant differences between the mean factor scores estimated by the above five parameters in the original and in cross-validity groups, and relationships between factors in both groups were very high. The center-of-foot pressure may be predicted and systematically evaluated by the above four factors. Five parameters selected from each factor are considered to be useful ones with high validity and practicability.

Am I missing something? Did this study just show that you can predict center of pressure path by looking at center of pressure path?

Reliability and Validity. I can see that they may be looking at reliability. But, doesn't validity mean that results mean what you think it means. The abstract certainly does not address validity.
Cheers,
Eric

 

I'm with you on that one Eric. Seems they examined predictors of centre of pressure with centre of pressure. No attempt at validity. Reminds me of a paper by a researcher who's first publication had a title which fell through the net, claiming to be a study of reliability and validity when only validity was examined....... Guilty as charged. Begs the question though of reviewers credibility.

Mind you, I once read a paper which could have been used to predict fat pad expansion of the heel from non-weightbearing to weightbearing but the author got the dependent and independent variables the wrong way around. y=mx + c