EMG-based co-contraction indexes for the assessment of motor impairments in persons post-stroke

Abstract

Neuromuscular coordination can be described by activations of agonist–antagonist (AA) muscle pairs and plays an important role in the performing of daily activities. It is an important mechanism used by the central nervous system to regulate joint stability and provide movement accuracy [1]. As a result of a stroke there is an alteration in these coordination patterns that results in motor deficits of the arm function [2]. In fact, persons post-stroke have an exaggerated AA muscle co-contraction [2] associated to a higher joint stiffness [1]. The amount of co-contraction can be quantified through the use of EMG- based co-contraction indexes (CCIs) [1,2]. Therefore, the purpose of the study was to evaluate whether such EMG-based CCIs are able to assess impairment in the motor control of the upper limb in persons post-stroke. Thirty-five persons post-stroke, in chronic or sub-acute stages, were recruited in this study. Both paretic and non-paretic arms were evaluated during the execution of functional tasks related to activities of daily living (i.e., reaching, object placing and forearm pronation). The upper limb kinematics and EMG data of three AA muscle pairs (anterior/posterior deltoids, triceps/biceps and pronator/supinator) were recorded through a movement analysis system. Ten healthy subjects (HS) provided the normative data. The Rudolph’s CCI [1] was computed as proposed by Don (range 0-100%, higher values indicate a greater AA muscle co-contraction) [3]. All participants post-stroke were clinically assessed through the Fugl-Meyer Motor Assessment of Upper Extremity (FM-UE). The differences of Rudolph’s CCI between HS and subjects post-stroke, considering both the more affected (MA) and the less affected (LA) arm, were tested by ANOVA. Bonferroni post-hoc analysis was also used to establish if there were statistically significant differences among the three groups. A deviation from normative data for the MA CCIs was found in almost all functional tasks and antagonist muscle pairs. The LA CCIs also showed a significant deviation from HS. Such pattern is reported in Fig. 1 for Rudolph’s CCI during the forearm pronation task. Higher values of the EMG-based CCI were found both in LA and MA arms in persons post-stroke, indicating alterations in the neuromotor coordination for both sides. The altered AA muscle activation of the LA arm (non-paretic, ipsilateral to the brain lesion) in persons post-stroke, despite its kinematics not being compromised, could be a consequence of the over-use of the non-paretic limb in performing functional tasks. Therefore, also the non-paretic upper limb may benefit from rehabilitation, as recently suggested [4]. Our results show that EMG-based CCI is a quantitative and objective measure able to detect motor control impairment, not detectable by clinical scales.