The aim of this study was to develop quantitative analytical methods in the application of the pendulum test to both normal and spastic subjects. The lower leg was released by a torque motor from different starting positions. The resulting changes in the knee angle were fitted by means of a time-varying model. Stiffness and viscosity coefficients were derived for each half-cycle oscillation in both flexion and extension, and for all knee starting positions. This method was applied to the assessment of the effects of Botulinum toxin A (BTX) in progressive multiple sclerosis patients in a follow-up study. About half of the patients showed a significant decrement in stiffness and viscosity coefficients.