Modification of Spastic Stretch Reflexes at the Elbow by Flexion Synergy Expression in Individuals With Chronic Hemiparetic Stroke

Abstract

Because flexion synergy expression modifies stretch reflexes in involved muscles, interventions that reduce flexion synergy expression may confer the added benefit of reducing spasticity during functional use of the arm.

Altered Neuromodulatory Drive May Contribute to Exaggerated Tonic Vibration Reflexes in Chronic Hemiparetic Stroke

“…However, some signs of an increased neuromodulatory influence appear to diminish when muscles are volitionally preactivated. For example, we and others have demonstrated that stretch reflex amplitude equilibrates between paretic and non-paretic muscles when the muscles are preactivated to the same extent prior to stretch (Lee et al, 1987 ; Burne et al, 2005 ; Mottram et al, 2009 , 2010 ; McPherson et al, 2017 ). Additionally, some estimates of PIC amplitude are indistinguishable between paretic and control muscles during volitional ramp contractions (Mottram et al, 2009 , 2010 ).…”

“…Focal ischemic stroke causes changes in descending neural drive that alter spinal motor excitability (Stinear et al, 2007 ; Heckman et al, 2009 ; Bradnam et al, 2012 ; McMorland et al, 2015 ; McPherson et al, 2017 , 2018 ; Owen et al, 2017 ). This can occur through direct effects on spinal neurons and/or by facilitating adaptive processes associated with shifts in the overall balance of spinal excitation and inhibition.…”

“…This can occur through direct effects on spinal neurons and/or by facilitating adaptive processes associated with shifts in the overall balance of spinal excitation and inhibition. In the chronic state post-stroke, spinal motor excitability generally increases in regions that control the paretic limbs; in regions innervating the non-paretic limbs, excitability remains at approximately pre-injury levels or is increased to a lesser degree (Lee et al, 1987 ; Powers et al, 1989 ; Thilmann et al, 1990 , 1991 ; Ibrahim et al, 1993 ; McPherson et al, 2011 , 2017 ; Hu et al, 2015 ). Increased spinal motor excitability could theoretically be related to alterations in ionotropic drive (i.e., signaling via ligand-gated channels that directly elicits excitatory or inhibitory post-synaptic potentials), neuromodulatory drive (i.e., signaling via metabotropic receptors to modify the intrinsic excitability of the cell), or both.…”

Biomechanical parameters of the elbow stretch reflex in chronic hemiparetic stroke

“…Building on recent work (Condliffe et al 2005; McPherson et al 2011; McPherson et al 2017), we chose to use larger displacement stretches (~90 degrees) than are traditionally used with ramp-and-hold (Powers et al 1989; Powers et al 1988; Thilmann et al 1991), pseudo-random binary (Ludvig et al 2011; Mirbagheri et al 2012), or multi-sine perturbations (Forbes et al 2011; Ludvig et al 2011; Mirbagheri et al 2012). The choice of large displacement ramp-and-hold stretches facilitates clinical translation because it most closely mirrors current clinical examinations in terms of the stretch profile (e.g., Ashworth, Tardieu), it does not require reprogramming of commercially available mechatronic devices, it does not require use of advanced system identification techniques or applied mathematics to compute reflex-related parameters (as do pseudo-random binary and multisine perturbations), and it ensures a sufficient angular range over which to exceed the threshold angle and estimate reflex stiffness.…”

“…The contralesional elbow flexors are implicated in a variety of functional impairments arising from changes in spinal motor excitability, including flexion synergy expression and spasticity (Ellis et al 2017; Jin and Zhao 2018; McPherson et al 2018a; McPherson et al 2018b; Sommerfeld et al 2012). We balance the competing needs of efficient implementation and high precision by using a clinically available mechatronic device and biomechanical measures of the stretch reflex response.…”