Upper Limbs Muscle Co-contraction Changes Correlated With the Impairment of the Corticospinal Tract in Stroke Survivors: Preliminary Evidence From Electromyography and Motor-Evoked Potential

Abstract

Objective: Increased muscle co-contraction of the agonist and antagonist muscles during voluntary movement is commonly observed in the upper limbs of stroke survivors. Much remain to be understood about the underlying mechanism. The aim of the study is to investigate the correlation between increased muscle co-contraction and the function of the corticospinal tract (CST).

Methods: Nine stroke survivors and nine age-matched healthy individuals were recruited. All the participants were instructed to perform isometric maximal voluntary contraction (MVC) and horizontal task which consist of sponge grasp, horizontal transportation, and sponge release. We recorded electromyography (EMG) activities from four muscle groups during the MVC test and horizontal task in the upper limbs of stroke survivors. The muscle groups consist of extensor digitorum (ED), flexor digitorum (FD), triceps brachii (TRI), and biceps brachii (BIC). The root mean square (RMS) of EMG was applied to assess the muscle activation during horizontal task. We adopted a co-contraction index (CI) to evaluate the degree of muscle co-contraction. CST function was evaluated by the motor-evoked potential (MEP) parameters, including resting motor threshold, amplitude, latency, and central motor conduction time. We employed correlation analysis to probe the association between CI and MEP parameters.

Results: The RMS, CI, and MEP parameters on the affected side showed significant difference compared with the unaffected side of stroke survivors and the healthy group. The result of correlation analysis showed that CI was significantly correlated with MEP parameters in stroke survivors.

Conclusion: There existed increased muscle co-contraction and impairment in CST functionality on the affected side of stroke survivors. The increased muscle co-contraction was correlated with the impairment of the CST. Intervention that could improve the excitability of the CST may contribute to the recovery of muscle discoordination in the upper limbs of stroke survivors.

Keywords: correlation analyses; corticospinal tract; motor-evoked potential; muscle co-contraction; stroke.

FIGURE 1

The experimental setup. (A) The electromyography (EMG) evaluation during horizontal task. (B) The experimental setup for motor-evoked potentials (MEP) evaluation.

FIGURE 2

The EMG signals of a typical trial from one stroke survivor captured during horizontal task. The EMG signal of flexor digitorum (FD) and extensor digitorum (ED) are shown together. (A) Affected side; (B) unaffected side.

FIGURE 3

The mean and standard deviation of root mean square of EMG data of all participants during horizontal task involved in the study. The significant difference between the groups is indicated by (*p < 0.05 with one-way ANOVA). ED, extensor digitorum; FD, flexor digitorum; BIC, biceps brachii; TRI, the triceps brachii.

FIGURE 4

Scatter diagrams of clinical scales and MEP parameters on the affected side of stroke survivors. (A) MEP latency versus Fugl-Meyer assessment upper extremity scales. (B) CMCT of MEP versus Fugl-Meyer assessment upper extremity scales. (C) MEP latency versus ARAT. (D) CMCT of MEP versus ARAT. ARAT, Action Research Arm Test; CMCT, central motor conduction time.

FIGURE 5

Scatter diagrams of the co-contraction index of flexor digitorum and extensor digitorum during horizontal task versus (A) MEP latency, (B) amplitude of MEP, (C) resting motor threshold of MEP, and (D) central motor conduction time of MEP.