A computer-based system has been developed in order to quantify simultaneous passive resistance to motion of the hip and knee joints of spastic subjects. The system provides a three-dimensional analysis of joint torques and reaction forces as well as the energies of passive resistance. It is utilized clinically with both ambulatory and nonambulatory subjects to monitor surgical treatment of hypertonicity. The system is comprised of a motorized dynamometer, adjustable chair, electronics for signal amplification and processing, and a microcomputer for data acquisition and analysis. Mechanical cross-talk is minimized by the dynamometer design which incorporates five strain-gage bridges. EMG data from the quadricep, hamstring, anterior tibialis and gastrocnemius muscle groups is simultaneously acquired with surface electrodes during subject tests. The device has been used clinically to test children with spastic cerebral palsy who have received surgical treatment. Pre- and post-operative evaluations as well as longer term follow-up studies have been conducted.