Measure of stretch reflex spatial thresholds at the ankle in a patient with CP using the Anklebot

Abstract

Introduction: It iswell known that stretch reflex (SR) responses are increased in children affected by Cerebral Palsy (CP), but it is still unclear whether spasticity is better characterized by variation in SR gain or threshold. In this regard, Jobin and Levin [1] found that in children with CP the spatial threshold (ST) (i.e. the joint angle at which the stretch reflex begins to act) in the elbow flexor is more stable over time, than othermetrics based on stretch reflex activity. Furthermore, the same research group found that, at elbow joint, ST sensitivity to velocity can discriminate between neurological deficits of muscle tone [2]. The present work is focused on the integration of a robotic device for rehabilitation and an EMG system, in order to evaluate the relationship between ST and stretch velocity in a different joint, the ankle, in a patient with CP. Methods: A children (six years old) affected by spastic CP (Ashworth left ankle: 3, GMFCS: level IV, WeeFIM: 53.7%) was enrolled in this study. The SR was elicited in the left ankle (most affected side) by means of a robotic device (Pediatric Anklebot, IMT, USA [3]). The robot measured the maximum subject’s PROM and set automatically the range of the movement (95% of PROM). Five dorsiflexion and plantarflexion, with five different mean angular velocities (from 8◦/s to 160◦/s), in a random order, with 11±2 s between two perturbations were performed. The protocol was repeated three times. The joint anglewasmeasuredby theencoders embedded in the Anklebot (with zero corresponding to the neutral position, positive values to dorsiflexions, and negative values to plantarflexions). The EMG signals, synchronized with joint angle Fig. 1. Ankle plantarflexion (dashed line), EMG of anterior tibialis (solid line), and correspondent ST (filled circle).