Aim: We compared the outcomes of manual and motorized instrumented ankle spasticity assessments in children with cerebral palsy (CP). Method: Ten children with spastic CP (three males, seven females; mean age 11y [standard deviation 3y], range 6–14y; Gross Motor Function Classification System levels I–III) were included. During motorized assessments, fast (100°/s) rotations were imposed around the ankle joint by a motor-driven footplate; during manual assessments, rotations of comparable speed were applied by a therapist using a foot orthotic. Angular range of motion, maximum velocity, acceleration, work, and muscle activity (electromyography [EMG]) of the triceps surae and tibialis anterior were compared during passive muscle stretch between motorized and manual assessments. Both movement profiles were also compared to CP gait ankle movement profile. Results: The imposed movement profile differed between methods, with the motorized assessment reaching higher maximum acceleration. Despite equal maximum velocity, the triceps surae were more often activated in motorized assessments, with low agreement of 44% to 72% (κ≤0) for EMG onset occurrence between methods. The manually applied ankle velocity profile matched more closely with the gait profile. Interpretation: The differences in acceleration possibly account for the different muscle responses, which may suggest acceleration, rather than velocity-dependency of the stretch reflex. Future prototypes of instrumented spasticity assessments should standardize movement profiles, preferably by developing profiles that mimic functional tasks such as walking.