 The study is about the effect of an exercise program using stationary bicycling for children with the spastic diplegic form of cerebral palsy. Spastic diplegia is a type of cerebral palsy that involves spasticity or tightness of the leg muscles. We hope to learn whether this type of exercise will allow the children to develop improved strength in the muscles that bend and straighten their knees, enhance their level of physical fitness, improve their ability to walk and improve their ability to perform other activities that are important to them. We hypothesize that children who participate in the stationary cycling intervention will gain strength in the muscles that bend and straighten their knees, will be able to complete a 600 yard walk run test a test of endurance more rapidly, and will improve their score on a test of function called the Gross Motor Function Measure a test designed specifically for children with cerebral palsy. 