Experimental analysis of planar motion was undertaken using the concept of instantaneous centre (ICM). Techniques for finding the ICM are highly sensitive to errors in locating points on the moving body. Assuming two-dimensional normally distributed errors in the data, a Monte-Carlo simulation for finding an ICM was performed. This revealed a shape of the ICM distribution unpredictable by current theories. A new analysis was based on the probability function for a normal distribution in two dimensions. Excellent agreement was obtained by the predictions of the theory and Monte Carlo Simulations. Excellent agreement was also obtained for analysing the motion of a device with known kinematics.

The theory provided means by which experiments of known accuracy may be designed to determine the ICM. To demonstrate this, motion was analysed for a human cadaveric knee undergoing testing on the knee analyser. This device cycled the tibia sideways in the mediolateral plane. The location of the ICM during inward motion of the tibia was 100 mm distal and 40mm lateral to the joint line. This suggested that sliding took place at the joint surfaces during this phase of testing. These results correlated with clinical and morphological evidence.