To classify hypertonia, the resistance of a knee joint at different velocities in knee flexion and extension and the maximum activation of stretched and shortened muscles are measured with a hand-held force transducer, an electrogoniometer, and surface electrodes in 100 patients with CP. The strength of velocity-dependent hypertonia during passive stretch and position, and/or velocity thresholds will be calculated. The knee extension and flexion muscles will be classified bilaterally as normal, or as one of the four types based on the pattern of activation of stretched muscles. To determine the impact of other impairments on the function, the maximum isometric knee flexion and extension strength and the monosynaptic reflexes of the rectus femoris muscle will be measured.