Abstract
Stretch reflex hyperactivity in the gastrocnemius of children with spastic cerebral palsy (CP) is commonly evaluated by passively rotating the ankle joint into dorsiflexion at different velocities, such as applied in conventional clinical spasticity assessments. However, surface electromyography (sEMG) collected from the medial gastrocnemius (MG) during such examination reveals unexplained heterogeneity in muscle activation between patients. Recent literature also highlights altered muscle tensile behavior in children with spastic CP. We aimed to document MG muscle and tendon lengthening during passive ankle motion at slow and fast velocity and explore its interdependence with the elicited hyperactive stretch reflex. The ankle of 15 children with CP (11 ± 3 years, GMFCS 9I 6II, 8 bilateral, 7 unilateral) and 16 typically developing children (TDC) was passively rotated over its full range of motion at slow and fast velocity. Ultrasound, synchronized with motion-analysis, was used to track the movement of the MG muscle-tendon junction and extract the relative lengthening of muscle and tendon during joint rotation. Simultaneously, MG sEMG was measured. Outcome parameters included the angular and muscle lengthening velocities 30 ms before EMG onset and the gain in root mean square EMG during stretch, as a measure of stretch reflex activity. Compared to slow rotation, the muscle lengthened less and stretch reflex activity was higher during fast rotation. These velocity-induced changes were more marked in CP compared to TDC. In the CP group, muscle-lengthening velocity had higher correlation coefficients with stretch reflex hyperactivity than joint angular velocity. Muscles with greater relative muscle lengthening during slow rotation had earlier and stronger stretch reflexes during fast rotation. These initial results suggest that ankle angular velocity is not representative of MG muscle lengthening velocity and is less related to stretch reflex hyperactivity than MG muscle lengthening. In addition, muscles that lengthened more during slow joint rotation were more likely to show a velocity-dependent stretch reflex. This interdependence of muscle lengthening and stretch reflexes may be important to consider when administering treatment. However, muscle and tendon lengthening properties alone could not fully explain the variability in stretch reflexes, indicating that other factors should also be investigated.
Highlights
Cerebral palsy (CP), the most common childhood disability, is an umbrella diagnosis attributed to a lesion in the developing fetal or infant brain [1]
Of the 38 children who participated in the study, children with CP and typically developing children (TDC) had full data sets and were further analyzed (Table 1)
Data were excluded from further analysis in two TDC due to incorrect synchronization of signals, in one child with CP due to poor US image quality, in one child with CP due to missing a technical cluster during calibration file, and in two children with CP and 1 TD child due to artifacts in the EMG signal
Summary
Cerebral palsy (CP), the most common childhood disability, is an umbrella diagnosis attributed to a lesion in the developing fetal or infant brain [1]. Spasticity, defined as velocity-dependent stretch reflex hyperactivity [2] is diagnosed in 80% of the cases and has been indicated to contribute to gait deviations including limited dorsiflexion during swing and at initial contact, decreased ankle dorsiflexion during mid-stance and limited push off power at terminal stance [3]. It is long thought that stretch reflex hyperactivity contributes to progressively worsening secondary musculoskeletal impairments including increased muscle stiffness, contracture and eventually deformities of immature skeletal bones [4]. Based on this assumption, early aggressive treatment of stretch reflex hyperactivity is commonly indicated to delay and reduce the need for orthopedic surgery [5]. Proper diagnosis of stretch reflex hyperactivity is clinically relevant and a deeper understanding of its relationship with muscle dynamics is currently lacking
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