8/14/2019 A Method for Estimating Center of Mass

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Proceedings of The First Joint BMEYEMBS Conference Serving Humanity, Advancing Technology O 1516 99 Atlanta GA USA

METHOD FOR ESTIM TING CENTER OF M SSFROM FORCEPL TE D T DURING QUIET ST NDINGRichmond B Chan, Ph.D.Rehabilitation Engineering Research Program

Northwestern University M edical School, Chicago,ILBSTR C T

A method is presented for obtaining the horizontal centerof mass COM) position from forceplate data during standing.This method uses double integration and moving averages.Experiments show that the d t obtained using this methodcorrelates well with d t obtained from motion analysis.INTRODUCTION

The center of mass COM ) and center of pressure COP )are important measurements in the study of standing balanceand posture. The COP can easily be obtained from force plated t Locating the COM, however, is a more complicatedproblem . Motion ana lysis system s have been used to gatherkinematic data for estimating COM [l] Some use kineticrelationships to estimate COM from force plate data [2,3].Breniere [4] used timdfi-equency domain transformations.Double integration from forceplate information to determineCOM has been used extensively in walking studies e.g. [l]).However, integration methods re sensitive to bound ryconditions and error from hese conditions may be significant,particularly for the case of standing, where the movement ofthe COM is usually small. This paper presents a newapproach for o btaining the COM from forceplate data.

METHODThe COM acceleration is directly related to the groundreaction force GRF). Thus the horizontal COM positionmay be obtained by double integrating the horizontal GRFHowever, s noted, improper boundary conditions, s well slow frequency drift and noise, can cause error.It ishypothesized that error can be reduced by assumingthat the moving average of COM and COP should more rless coincide. After obtaining s t), by double integration, thedifference between the moving average of s t) and the mov ingaverage of the CO P is subtracted from s t) to obtain r t). l hefollowing equations sum up the process of C OM e stimationin a sing le direction of the ho rizon tal plane:

a t) = rn t)v t) = [a t)-aJ dt

horizontal accelerationintegrated velocity

s t)=1 v t)-v,] dtr t) = s t) - [u t) - p t)]

integrated positionestimated COM

where f t) is the horizontal GRF, W is the body weight, g i sthe acceleration of gravity, a is the mean of a t), v, is themean of v t), u t) is the moving average of s t), p t) is themoving average of COP, and r t) is the estimated COM. Themoving averages are taken with a window of 4 seconds.EXPERIMENTS ND RESULTS

Six normal subjects 3 male and 3 female), ages 20-30years, participated in the d t collection experiments, afterinformed consent. They were instructed to stand normally on.a forceplate during the trials. Data was collected during threetrials, each t uty seconds in duration, for all subjects.Kinematic and kinetic data were collected simultaneously.Ground reaction forces were obtained with an MTI forceplateAdvanced Mechanical Technology, Inc. model WR6-5-1). ACODA-3 Motion Measurement System, located 5 metersdistant from the subject, detected the three-dimensionalposition of a retro-reflective marker on the subjects body.The marker was placed at the level of the hip. Its p ositio nwas used s an approximation of the position of thehorizontal movement of the COM he first 5 seconds ndthe last 5 seconds of the data were discarded. The computedCOM datawere compared with the COM data measured fromthe marker. An average correlation of 0.903 was found, witha st nd rd deviation of 0.091. This result suggests that themethod may permit simultaneous COP and COMmeasurements during standing, using forceplate data only.

REFERENCES[l ] M. Saini , D.C. K errigan, M.A. T hirunarayan , & M.Duff-Raffaele, The Vertical Displacement of the Centerof Mass During Walking: A Comparison of FourMeasu rement Methods. J. of Biomech. Engr vol. 120,pp. 133-139, Feb. 1998.[2] 0 Caron, B. F aure, Y Breniere. Estimating the centerof gravity of the body on the basis of the center ofpressure in standing posture. J Biomechanics vol. 30,nos. 11/12, pp. 1169-71, 1997.[3] T. Shimba, An estimation of center of gravity fTom forceplatform data. J Biomechanics vol. 17, no. 1, pp. 53-60, 1984.[4] Y. Breniere. Why we walk the way we do ? Journal oMotor Behavior vol. 28, pp. 291-298, 1996.

CKNOWLEDGMENTSupported by Award H133E980023 from the NationalInstitute on Disability and R ehabilitation Research NIDRR).

0-7803-5674-8/99/ 10.00 1999 IEEE 516