Objectives: To describe (1) physical strain of walking, (2) the proportion of participants walking above the anaerobic threshold, and (3) 4 phenotypes of physical strain of walking on the basis of deviations in aerobic capacity and walking energy cost (EC) in children and adolescents with cerebral palsy (CP). Design: Cohort study. Setting: Academic medical center. Participants: A sample (N=57) of participants (n=37; mean age, 13.5±4.0y) with CP (Gross Motor Function Classification System [GMFCS] levels I [n=13], II [n=17], and III [n=7]) and typically developing (TD) participants (n=20; mean age, 11.8±3.5y). Interventions: Not applicable. Main Outcome Measures: Oxygen consumption (Vo 2walk), speed, and EC were determined during walking at a comfortable speed. Peak oxygen consumption (Vo 2peak) and anaerobic threshold were measured during a maximal cycling exercise test. Aerobic capacity was reduce if lower than the 10th percentile, and EC was increased if higher than 3SD. Physical strain was defined as follows: (Vo 2walk/Vo 2peak)×100. Results: Participants with CP had a higher physical strain (GMFCS level I, 55%±12% GMFCS level II, 62%±17%; GMFCS level III, 78%±14%) than did TD participants (40%±11%) (P<.001). Forty-three percent of participants with CP showed a Vo 2walk at or above their anaerobic threshold as compared with 10% of TD participants (P=.007). Phenotypes showed that a reduced Vo 2peak (n=9) or an increased EC (n=9) lead to an 18% to 20% higher physical strain, whereas a combination (n=12) leads to a 40% increase. Conclusions: Children and adolescents with CP walk at a high physical strain, approximating intense exercise and a considerable proportion walks close to or above their anaerobic threshold, probably explaining fatigue and reduced walking distance. Both an increased EC and a reduced Vo 2peak contribute to high physical strain in children or adolescents with CP. The different causes of high physical strain in individuals with CP require different intervention strategies.