Position-dependent torque coupling and associated muscle activation in the hemiparetic upper extremity

Abstract

Previous studies have demonstrated abnormal joint torque coupling and associated muscle coactivations of the upper extremity in individuals with unilateral stroke. We investigated the effect of upper limb configuration on the expression of the well-documented patterns of shoulder abduction/ elbow flexion and shoulder adduction/elbow extension. Maximal isometric shoulder and elbow torques were measured in stroke subjects in four different arm configurations. Additionally, an isometric combined torque task was completed where subjects were required to maintain various levels of shoulder abduction/adduction torque while attempting to maximize elbow flexion or extension torque. The dominant abduction/elbow flexion pattern was insensitive to changes in limb configuration while the elbow extension component of the adduction/extension pattern changed to elbow flexion at smaller shoulder abduction angles. This effect was not present in control subjects without stroke. The reversal of the torque-coupling pattern could not be explained by mechanical factors such as muscle length changes or muscle strength imbalances across the elbow joint. Potential neural mechanisms underlying the sensitivity of the adduction/elbow extension pattern to different somatosensory input resultant from changes in limb configuration are discussed along with the implications for future research.

Shoulder abduction-induced reductions in reaching work area following hemiparetic stroke: neuroscientific implications

“…Conversely, if the impairment is the result of a different mechanism such as shoulder weakness/paresis, full reaching work area would be expected to remain relatively constant until an abduction level is reached where subjects are unable to produce enough torque with the shoulder abductors to lift the paretic arm. Results of this study support the hypothesis that reaching reductions in individuals with stroke are caused by abnormal shoulder abductor/elbow flexor coupling and may be explained by an increased contribution of bulbospinal systems as reported previously (Beer et al 2007;Ellis et al 2007). Portions of this work have been published in abstract form (Sukal et al 2005).…”

“…This coactivation is implicated in stereotyped movement patterns that were first described as a "flexion synergy" (Twitchell 1951;Brunnstrom 1970). It is well documented that abnormal muscle coactivation is present in individuals with moderate to severe stroke (Dewald et al 1995Beer et al 2007;Ellis et al 2007) and that these coactivations result in a reduced active range of motion when reaching against gravity (Beer et al 1999(Beer et al , 2000(Beer et al , 2004. Although Beer and colleagues were able to elegantly quantify reaching kinematics as a function of supported versus unsupported reaching, the methodologies were limited to only two reaching conditions, and a center-out reaching task that did not represent the entire range of motion or work area of the subject.…”

“…This may be attributed to an incremental substitution of corticospinal by corticobulbospinal motor pathways (Beer et al 1999(Beer et al , 2007Ellis et al 2007). To fully characterize the nature of this substitution, it was necessary to test an array of abduction requirements ranging from cases where the subject's arm was fully supported (i.e.…”

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.…”

“…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). …”