Subjects will have a diagnosis of spastic diplegic CP. Inclusion criteria will be: 1) age between 7 and 18 years; 2) ability to follow simple verbal directions; 3) good or fair selective motor control for at least one lower limb and 4) ability to walk independently indoors, with or without assistive devices (Levels I-III of the Gross Motor Function Classification System (GMFCS) [34]. Exclusion criteria will be: 1) orthopedic surgery, neurological surgery or baclofen pump implantation within the preceding 12 months; 2) botulinum toxin injections within the preceding three months; 3) serial casting or new orthotics within the preceding three months; 4) initiating oral medications that affect the neuromuscular system, e.g. baclofen, within the preceding three months; 5) onset of physical therapy, exercise, sport activity or change in assistive devices for walking within the preceding three months; 6) inability or unwillingness to maintain age appropriate behavior; 7) serious medical conditions such as cardiac disease, diabetes or uncontrolled seizures; 8) current participation in a fitness program that includes cardiorespiratory endurance exercise, at least one time per week and 9) significant hip, knee or ankle joint contractures preventing passive movement of the lower limbs through the pedaling cycle.