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A major movement related limitation for children with spastic cerebral palsy (CP) is the compromised gait pattern, which may explain their excessive energy cost of locomotion. The aims of this study were to determine differences in the O2 cost of locomotion between children with CP (7 males, 2 females; 12.7 ± 2.8 yr) and able-bodied controls (7 males, 1 female; 13.6 ± 2.1 yr) and to assess the contribution that cocontraction of agonist and antagonist muscles had upon the elevated O2 cost seen in children with CP versus able-bodied controls The treadmill submaximal walking protocol consisted of 2 × 4 min intermittent stages at 3 km·h-1 and 90% of the predetermined fastest walking speed (FWS) at 0% grade. Electromyographic data were collected during the final minute of each bout from vastus lateralis and hamstrings(thigh) and tibialis anterior and soleus (lower leg). Significant (P< 0.05) differences were noted at 3 km·h-1 for mass-relative˙VO2 (CP: 16.6 ± 6.5 vs control: 10.2 ± 1.2 ml·kg-1),%˙VO2max (CP: 53.5 ± 26.0 vs Control: 22.5 ± 4.93) and heart rate (CP: 143 ± 41 vs Control: 91± 14 beats·min-1). Thigh and lower leg muscle cocontraction accounted for 51.4% and 42.8%, respectively, of the variability in ˙VO2 for the subjects with CP at 3 km·h-1. These results suggest that cocontraction is a major factor responsible for the higher energy cost of walking seen in children with CP.