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Recent ultrasound studies found increased passive muscle stiffness and no difference in tendon stiffness in highly impaired children and young adults with cerebral palsy. However, it is not known if muscle and tendon mechanical properties are already altered in highly functional children with cerebral palsy. Therefore, the purpose of this study was to compare the mechanical and material properties of the plantar flexors in highly functional children with cerebral palsy and typically developing children.Besides strength measurements, ultrasonography was used to assess gastrocnemius medialis and Achilles tendon elongation and stiffness, Achilles tendon stress, strain, and Young's modulus in twelve children with cerebral palsy (GMFCS levels I and II) and twelve typically developing peers during passive dorsiflexion rotations as well as maximum voluntary contractions.Despite no difference in ankle joint stiffness (P > 0.05) between groups, passive but not active Achilles tendon stiffness was significantly decreased (− 39%) and a tendency of increased passive muscle stiffness was observed even in highly functional children with cerebral palsy. However, material properties of the tendon were not altered. Maximum voluntary contraction showed reduced plantar flexor strength (− 48%) in the cerebral palsy group.Even in children with mild spastic cerebral palsy, muscle and tendon mechanical properties are altered. However, it appears that the Achilles tendon stiffness is different only when low forces act on the tendon during passive movements. Although maximum voluntary force is already decreased, forces acting on the Achilles tendon during activity appear to be sufficient to maintain typical material properties.Muscle-tendon properties are altered in highly functional cerebral palsy children.Despite altered tendon mechanical properties, material properties were the same.Muscle weakness is present in patients without clinically relevant contracture.Early treatment is necessary even in children with mild spastic cerebral palsy.