Abnormal muscle coactivation patterns during isometric torque generation at the elbow and shoulder in hemiparetic subjects

Abstract

To study abnormal spatial patterns of muscle activation in hemiparetic stroke, we compared EMG activity in paretic and contralateral elbow and shoulder muscles of 10 hemiparetic subjects during 1.5-s voluntary isometric contractions, against five to eight different loads. Isometric forces were generated in eight directions, referenced to a plane orthogonal to the long axis of the forearm, and were recorded by a three degrees of freedom load cell, mounted at the wrist. Surface and intramuscular EMGs of six elbow and six shoulder muscles were recorded from both impaired and contralateral upper extremities of each subject. The spatial characteristics of EMG activation of individual muscles were summarized using two measures. The first, called the 'net resultant EMG vector' is a new measure which calculated the vector sum of EMG magnitudes for each of the eight directions, and the second, index of EMG focus, is a measure of the range of EMG activation recorded for each load level. Use of these measures permitted us to describe spatial EMG characteristics quantitatively, which has not been done previously. We observed consistent and statistically significant shifts in the resultant EMG vector directions in the impaired limb, especially in shoulder and other proximal muscles. Significant increases in the angular range of EMG activity were also identified and were most evident at the elbow. Correlation analysis techniques were used to assess the degree of coactivation of different muscle pairs. There were consistent EMG coactivation patterns observed across all subjects (both normal and hemiparetic). However, in spasticparetic limbs, additional novel coactivational relationships were also recorded, especially between elbow flexors/shoulder abductors and elbow extensors/shoulder adductors. These novel coactivation patterns represent a reduction in the number of possible muscle combinations, or in the number of possible 'synergies' in the paretic limb of the stroke subject. This reduction in number of 'synergies' could result from a loss of descending command options; from an increased reliance on residual, descending brainstem pathways (such as the reticulospinal and vestibulospinal projections); from changes in spinal interneuronal excitability; or from a combination of several of these factors. The relative merits of these hypotheses are addressed.

Ipsilateral versus contralateral cortical motor projections to a shoulder adductor in chronic hemiparetic stroke: implications for the expression of arm synergies

“…This abnormal inter-joint coupling is an obligatory co-activation pattern that is observed in moderate-to-severe stroke (Brunnstrom 1970;Dewald and Beer 2001;Dewald et al 1995, Beer et al 2007Ellis et al 2007, Sukal et al 2007). There is also abnormal coupling between the shoulder abductors and elbow flexors (flexor synergy) in these stroke survivors.…”

“…After a hemiparetic stroke, the proximal limb is less affected than the distal limb (Colebatch and Gandevia 1989), nevertheless, normal function of the proximal upper arm is frequently severely impaired. Proximal upper limb impairment is characterized by weakness (Adams et al 1990;Bohannon and Smith 1987;Colebatch and Gandevia 1989;Lum et al 2003;Mercier and Bourbonnais 2004) and abnormal coordination between elbow and shoulder muscles expressed as flexor and extensor synergies (Twitchell 1951;Brunnstrom 1970;Beer et al 1999;Dewald and Beer 2001;Dewald et al 1995;). These impairments significantly affect reaching abilities in stroke survivors with poor recovery (Beer et al 2004(Beer et al , 2007Sukal et al 2007).…”

“…In spite of this, the loss of contralateral corticospinal input to the proximal limb, and subsequent expression of, or take-over by, ipsilateral pathways may explain the loss of independent joint control and abnormal coordination observed in the proximal limb after stroke. In moderate to severely impaired stroke survivors, this abnormal coordination is characterized by an obligatory co-activation between shoulder adductors and elbow extensors (extensor synergy) as well as between shoulder abductors and elbow flexors (flexor synergy) (Brunnstrom 1970;Dewald et al 1995;Dewald and Beer 2001;Ellis et al 2007). …”

Effects of Functional Electrical Stimulation with and without a Wrist-Hand Orthosis on Hand Opening in Individuals with Chronic Hemiparetic Stroke: A Pilot Study

“…Six months after the stroke, 33% to 60% still have a degree of UE hemiparesis [1,2]. Motor impairments strongly affect daily life activities [3,4]. For individuals with severely affected arms post stroke, the evidence that conventional therapy can restore meaningful hand function is not strong [4][5][6][7][8].…”

A method to capture six-degrees-of-freedom mechanical measurements of isometric shoulder and elbow torques during event-related fMRI

“…The purpose of this paper is to describe a methodology that addresses these two shortcomings in current experimental brain imaging technology. The method described is based on other methods used in isometric upper limb experiments outside the MR environment to study abnormal joint coupling following stroke induced brain injury (Beer et al, 1999;Dewald et al, 1995;Yao and Dewald, 2005). Portions of this paper have been presented to the International Society of Biomechanics and American Society of Biomechanics Joint Conference and the Society for Neuroscience Annual Meeting in abstract form (Krainak et al, 2005a(Krainak et al, ,2005b.…”