A three-dimensional ultrasonic system for posture measurement.

A time-efficient, cost-effective, and accurate system has been developed for measuring static three-dimensional joint coordinates in the laboratory. This system uses a personal computer interface to determine the distance between transmitters positioned at body joints and receivers positioned near the subject by measuring the travel time of ultrasound. Distance data are then converted to spatial coordinates and joint angles. The system can determine the location of 14 joints at one time. An experiment using three distances and five orientations between a transmitter and a receiver was performed to investigate the significance of measurement errors for the new system. The results showed that the standard deviation of the distance measurement was about 0.2 cm for single orientation conditions and was about 1 cm for all conditions tested. A second experiment using 11 transmitters and four receivers was performed to investigate the significance of measurement errors when determining three-dimensional coordinates. The results showed no significant difference between actual and measured coordinates. The system was then used to study the posture of a subject's upper extremity. Eight postures representing a variety of typical reaching tasks were examined. The results showed that the system was suitable for three-dimensional posture measurement.