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TECHNICAL PAPERS

Enhancements in the Accuracy of the Center of Pressure (COP) Determined With Piezoelectric Force Plates Are Dependent on the Load Distribution

[+] Author and Article Information
Heinz-Bodo Schmiedmayer

Technische Universität Wien, Vienna, Austria

Josef Kastner

Forschungsinstitut für Orthopädietechnik, Vienna, Austria

J Biomech Eng 122(5), 523-527 (Apr 28, 2000) (5 pages) doi:10.1115/1.1289687 History: Received January 07, 1999; Revised April 28, 2000
Copyright © 2000 by ASME
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References

Kistler, 1993, Multicomponent Measuring Force Plate for Biomechanics Type 9287A (Operating Instructions), Kistler, Switzerland.
Nigg, B. M., and Herzog, W., eds., 1994, Biomechanics of the Musculo-Skeletal System, Wiley, New York.
Bobbert,  M. F., and Schamhardt,  H. C., 1990, “Accuracy of Determining the Point of Force Application With Piezoelectric Force Plates,” J. Biomech., 23, pp. 705–710.
Sommer, R., Kohler, D., and Calame, C., 1997, “Center of Pressure (COP) Enhancement in Piezoelectric Force Plates,” Proc. XVIth I.S.B. Congress, Tokyo, p. 18.
Winter, D. A., 1995, A.B.C. (Anatomy, Biomechanics and Control) of Balance During Standing and Walking, Waterloo Biomechanics, Waterloo.
Schmiedmayer,  H.-B., and Kastner,  J., 1999, “Parameters Influencing the Accuracy of the Point of Force Application Determined With Piezoelectric Force Plates,” J. Biomech., 32, pp. 1237–1242.
Praxmarer, N. L., 1996, “Simulation of the Plantar Pressure Distribution for Insole Design,” Ph.D. thesis, Technische Universität Wien, Vienna, Austria.
McCaw,  S. T., and DeVita,  P., 1995, “Errors in Alignment of Center of Pressure and Foot Coordinates Affect Predicted Lower Extremity Torques,” J. Biomech., 28, pp. 985–988.
Johnson, K. L., 1984, Contact Mechanics, Cambridge University Press, Cambridge.

Figures

Grahic Jump Location
Simulated errors for two point loads F1 and F2 located at r_F=[0,±yF]T. The errors ΔyCOP are given as functions of the “true” value yCOP for a narrow (yF=125 mm) and a wide (yF=250 mm) standing position as well as for a single point loading. For two point loading the uncorrected errors (before applying correction formulas) and the errors after correction are given.
Grahic Jump Location
Results for pressure distribution during slow walking. Pictures are as seen from above the plate where coordinates are according to the reference system given by Kistler 1. Left column of plots (a–c) shows results for t=0.07 s after heel strike whereas the right column (d–f ) shows results for t=0.378 s. The top row (a,d) shows the isopressure lines with a distance of Δp=2.5 N/cm2. The star gives the local COP for the given pressure distribution. The second row (b,e) shows the absolute values of the errors in mm of the measured COP dependent on the actual position of the true COP for the given pressure distribution. Only positions of the COP where the whole pressure area lies within the force plate are given. The last row (c,f ) gives the remaining error after application of the correction formula for single point loading.
Grahic Jump Location
Schematic diagram of a force plate with a single point load F_=[Fx,Fy,Fz]T and a couple MCOP,z applied: (a) geometric situation: (1[[ellipsis]]4) measurement posts with triaxial piezoelectric force transducers at coordinates [±ax,±ay,0];h absolute distance of the surface of the plate to the plane of measurement (i.e., the plane where the force components are measured by the piezoelectric transducers); P point of force application of a point load at coordinates [xCOP,yCOP,zCOP] with zCOP=−h; (b) free-body diagram of the force plate system: forces Fi,j and moments Mi,j(j=x,y,z) transmitted through the posts (i=1[[ellipsis]]4) in the plane of measurement (z=0).

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