Measurement of posture by triangulation using potentiometers

To measure the posture of a paraplegic while standing up, an economical method has been developed. Markers, glued over the joints, hook onto strings which run to rotary potentiometers mounted on a fixed frame. Springs maintain a near-constant small tension in the strings, and the potentiometers rotate as the strings wind onto pulleys. The positions are calculated by triangulation, using two potentiometers per joint, assuming that body segment lengths are constant, or three potentiometers without this assumption. Using a second-order polynomial fit, the random error in length measurement for each potentiometer is less than +/-2 mm for the string length from 0 mm to 1100 mm, or less than +/-1 mm in actual range from 600 mm to 1000 mm. With two potentiometers per joint, using a second-order polynomial fit and assuming the ankle position is known exactly, an estimate of the resulting errors in the knee and hip marker positions are 4 and 8.5 mm, respectively.