VALIDATION OF ANKLE JOINT REACTION FORCES AND MOMENTS

  • N. Wrbaskic
  • J. Dowling

Abstract

INTRODUCTION Rigid link models of human limb segments and inverse dynamics have been used extensively in biomechanics to calculate net joint moments and joint reaction forces. One method of validation of these calculations is to compare the instantaneous segment power with the rate of change of energy (Robertson and Winter, 1980). A good match is often found with the exception of the foot segment during the early and late portions of the stance phase in walking. The purpose of this study was to determine if a match between the instantaneous power (SP) and the rate of change of energy (RCE) of the foot segment was a sufficient validation of ankle forces and moments. METHODS In order to validate any method, exact net joint moments and reaction forces need to be known. Since these values are never known exactly in human motion, computer simulation data available on the ISB website was used (Van den Bogert, 1996). This data included exact anthropometrics, kinematics, and kinetics and ground reaction forces of a rigid foot segment during the stance phase of running. The true segment power (SF) could be compared to a segment power achieved by relocating the ankle marker (SPr). The center of rotation of the ankle was relocated using polar coordinates and a 10 degree roution. This ensured that foot rigidity was maintained throughout the stance phase but different kinetics and kinematics would result. RESULTS AND CONCLUSIONS An excellent agreement was achieved between RCEr and SPr even though an incorrect ankle marker position was used. The comparison of SPr with the incorrect ankle position and the SPt showed errors up to 50W. The equating of RCE and SP is only a validation of the assumption of segment rigidity. It does not ensure that the reaction forces or moments are indeed correct. Furthermore, since the kinetics of the ankle are used in subsequent calculations of the kinetics at the knee and hip, a good match between RCE and SP of these segments does ensure accurate forces and moments at these joints either. REFERENCES Robertson, D.G. and Winter, D.A.J. Biomech. 13:845-854,1980. Van den Bogert, A.INVDYN. http://www.kin,ucalgary.ca/isb/data.