Cerebral palsy (CP) has the highest lifetime economic cost and net cost of medical care compared to the most common birth defects. Reduced muscle strength affects functional abilities in children with CP. Volitional strength training in children with CP can produce substantial gains in force production with concomitant gains in function. Volitional strength training, however, may not be optimal for producing strength gains in this population due to reduced voluntary muscle activation. Neuromuscular electrical stimulation (NMES) activates a greater number of motor units and produces higher firing rates of the active motor units than can be obtained volitionally. Thus, NMES may offer advantages over volitional exercise programs for producing gains in force production. The proposed research is designed to develop improved rehabilitation techniques for children with CP. This study compares the effects of a 12-week NMES isometric strength-training program to 12 weeks of traditional isometric strength training in children with CP. A no-exercise control group with CP will also be studied. The specific aims are: 1) To compare the force generating, volitional activation, contractile and fatigue characteristics of the quadriceps femoris and triceps surae muscles in typically developing children and children with CP; 2) to assess the ability of NMES strength training to increase quadriceps femoris and triceps surae muscle force-generating ability as compared to volitional strength training and a no-exercise control group in children with CP; 3) to assess the mechanisms underlying improvements in force-generating ability of the quadriceps femoris and triceps surae after strength training in children with CP; and 4) to assess the ability of strength training to improve gross motor function and gait in children with CP. This work will help clinicians design rehabilitation strategies based on the physiologic differences in skeletal muscle and mechanisms for force production between children with CP and children of typical development. Increasing muscle force-generating ability of children with CP can improve a child's function during activities such as sitting, standing and walking.