Optimal marker placement for calculating the instantaneous center of rotation.

A computer simulation with error propagation was performed to determine the optimal placement of marker points for calculating the instantaneous center of rotation (CRi). The authors assume that planar rigid body motion occurs between two positions, each defined by marker points. Noisy marker points were generated by perturbing their coordinates with random values from a normal population of errors. The effects of these errors on the range of errors in calculating CRi location were investigated. Parametric analysis determined that marker point placement had important effects on CRi error. Marker placement was optimal when the estimated CRi was located at the midpoint between the marker points. While increasing the distance between marker points increased accuracy, there is a critical distance above which no additional increase in accuracy was noted when using this placement. The farther the marker midpoint was from the CRi, the greater was the error. At these placements, increasing the distance between the marker points continually decreased CRi error. The methodologies presented here help to improve the accuracy with which the location of the CRi can be calculated. However, it is emphasized that the CRi remains sensitive to noise and investigations should apply this kinematic parameter knowingly.