An investigation of the accuracy in measuring the body center of pressure in a standing posture with a force plate.

It has been reported that accuracy in the measured point of force application depends on the position and magnitude of load. The aim of this study is to investigate the error in measuring the body center of pressure in a standing posture with a force plate and to improve the accuracy by proposing a correction algorithm. The measurement of the point of force application with a calibrated load demonstrated its co-ordinates, which were scaled down towards the zero co-ordinate and moved parallel relative to the true co-ordinates. This trend was exaggerated with any decrease in magnitude of the load. The root mean square (r.m.s.) and maximum of errors at light load of 10 kg were 10 and 18 mm in the X axis, and 6 and 12 mm in the Y axis, respectively. The error seems to result from such causes as (i) non-linearity of the load cell, (ii) deformation of the top plate due to load application and (iii) differences in characteristics among individual load cells, including amplifiers. A mathematical representation of the measured point of force application accounting for these causes of error has been made and an algorithm for estimating the true point at any magnitude of load with only one correction equation is proposed. Actual correction of the measured point demonstrated an expected improvement in the r.m.s. error to less than 1 mm at any magnitude of load greater than 10 kg and the validity of this algorithm was confirmed.