Motor Overflow and Spasticity in Chronic Stroke Share a Common Pathophysiological Process: Analysis of Within-Limb and Between-Limb EMG-EMG Coherence

Abstract

The phenomenon of exaggerated motor overflow is well documented in stroke survivors with spasticity. However, the mechanism underlying the abnormal motor overflow remains unclear. In this study, we aimed to investigate the possible mechanisms behind abnormal motor overflow and its possible relations with post-stroke spasticity. 11 stroke patients (63.6 ± 6.4 yrs; 4 women) and 11 healthy subjects (31.18 ± 6.18 yrs; 2 women) were recruited. All of them were asked to perform unilateral isometric elbow flexion at submaximal levels (10, 30, and 60% of maximum voluntary contraction). Electromyogram (EMG) was measured from the contracting biceps (iBiceps) muscle and resting contralateral biceps (cBiceps), ipsilateral flexor digitorum superficialis (iFDS), and contralateral FDS (cFDS) muscles. Motor overflow was quantified as the normalized EMG of the resting muscles. The severity of motor impairment was quantified through reflex torque (spasticity) and weakness. EMG-EMG coherence was calculated between the contracting muscle and each of the resting muscles. During elbow flexion on the impaired side, stroke subjects exhibited significant higher motor overflow to the iFDS muscle compared with healthy subjects (ipsilateral or intralimb motor overflow). Stroke subjects exhibited significantly higher motor overflow to the contralateral spastic muscles (cBiceps and cFDS) during elbow flexion on the non-impaired side (contralateral or interlimb motor overflow), compared with healthy subjects. Moreover, there was significantly high EMG-EMG coherence in the alpha band (6–12 Hz) between the contracting muscle and all other resting muscles during elbow flexion on the non-impaired side. Our results of diffuse ipsilateral and contralateral motor overflow with EMG-EMG coherence in the alpha band suggest subcortical origins of motor overflow. Furthermore, correlation between contralateral motor overflow to contralateral spastic elbow and finger flexors and their spasticity was consistently at moderate to high levels. A high correlation suggests that diffuse motor overflow to the impaired side and spasticity likely share a common pathophysiological process. Possible mechanisms are discussed.

The Effects of Botulinum Toxin Injections on Spasticity and Motor Performance in Chronic Stroke with Spastic Hemiplegia

“…It is likely that the clinical scales, such as MAS, lacks the sensitivity to quantify the abnormal tone with common definitions of stretch-reflex mediated spasticity [ 43 ]. In this study, we used an established biomechanical assessment method to quantify and differentiate reflex and non-reflex components of spastic hypertonia [ 10 , 11 , 44 ]. The results demonstrated significant decrease in the reflex component of spasticity, while the non-reflex muscular component remained unchanged after BoNT injections.…”

“…Spasticity is attributed to unopposed descending excitatory input to spinal reflex circuits from brainstem, and is related to disordered motor control [ 49 ]. In a recent study, it was found that reflex torque was correlated with spontaneous activity of spastic muscles [ 11 ]. Therefore, reduction of reflex torque after BoNT injections could indicate suppression of spontaneous activity in spastic muscles, and thus less “noise” during voluntary contraction of weakened spasticity muscles after BoNT.…”

“…However, it is important to point out that not all hypertonia (increased muscle resistance) is stretch reflex mediated spasticity [ 7 ]. Quantification of resistance torques in response to motorized stretch in laboratory settings is often able to differentiate neural (i.e., stretch reflex) and non-neural components of hypertonia [ 8 , 9 , 10 , 11 ]. Spasticity can cause problems directly, such as abnormal joint position and posture, pain, and hygiene difficulties.…”

Validity and reliability of an accelerometer-based assessgame to quantify upper limb selective voluntary motor control

“…It has been shown that children and adolescents affected by CP exhibit more mirror movements in their less affected hand, when performing movements with their more affected hand [ 30 , 33 ]. Even though patients with stroke did not show increased EMG activation on the contralateral side, they did exhibit increased ipsilateral muscle activation [ 34 ]. In line with those results, Sukal et al [ 35 ] found increased ipsilateral joint coupling for patients with hemiplegic CP.…”

Parkinson's Disease Exhibits Amplified Intermuscular Coherence During Dynamic Voluntary Action

“…The potential association between acoustic startle responses and intermuscular coherence during voluntary tasks has never been fully explored, and current evidence is indirect. For example, spasticity in chronic stroke is thought to relate to overdriven reticulospinal drive to muscles (74,75), and both alpha-band intermuscular coherence (76,77) and responses to acoustic startle are increased in spastic muscles of stroke survivors (78,79). However, if PD amplifies reticulospinal output, startle responses should be increased in magnitude or consistency relative to controls, but they are not (80).…”