Relation between stretch and activation of the medial gastrocnemius muscle during gait in children with cerebral palsy compared to typically developing children

Stretch hyperreflexia is often a target for treatment to improve gait in children with spastic cerebral palsy (CP). However, the presence of stretch hyperreflexia during gait remains debated. Therefore, we assessed the relation between gastrocnemius medialis muscle-tendon stretch and muscle activation during gait in children with CP compared to typically developing (TD) children. 3D gait analysis including electromyography (EMG) and dynamic ultrasound was carried out to assess, respectively gastrocnemius medialis activation and fascicle, belly, and tendon stretch during treadmill walking. Musculotendon-unit stretch was also estimated using OpenSim. Ratios of EMG/peak lengthening velocities and accelerations were compared between CP and TD. Velocity and acceleration peaks prior to EMG peaks were qualitatively assessed. EMG/velocity and EMG/acceleration ratios were up to 500% higher for CP (n = 14) than TD (n = 15) for most structures. Increased late swing muscle activation in CP was often preceded by fascicle and musculotendon-unit peak lengthening velocity, and early stance muscle activation by peaks in multiple structures. Increased muscle activation in CP is associated with muscle-tendon stretch during gait. Concluding, late swing muscle activation in CP appears velocity-dependent, whereas early stance activation can be velocity- and acceleration-dependent. These insights into stretch reflex mechanisms during gait can assist development of targeted interventions.