Authors: Bahar Sharafi Gilles Hoffmann Andrew Q Tan Yasin Y Dhaher
Publish Date: 2016/08/05
Volume: 234, Issue: 12, Pages: 3497-3508
Abstract
The neuromuscular mechanisms that underlie poststroke impairment in reactive balance control during gait are not fully understood Previous research has described altered muscle activations in the paretic leg in response to postural perturbations from static positions Additionally attenuation of interlimb reflexes after stroke has been reported Our goal was to characterize poststroke changes to neuromuscular responses in the stance leg following a swing phase perturbation during gait We hypothesized that following a trip altered timing sequence and magnitudes of perturbationinduced activations would emerge in the paretic and nonparetic support legs of stroke survivors compared to healthy control subjects The swing foot was interrupted while subjects walked on a treadmill In healthy subjects a sequence of perturbationinduced activations emerged in the contralateral stance leg with mean onset latencies of 87–147 ms The earliest latencies occurred in the hamstrings and hip abductor and adductors The hamstrings the adductor magnus and the gastrocnemius dominated the relative balance of perturbationinduced activations The sequence and balance of activations were largely preserved after stroke However onset latencies were significantly delayed across most muscles in both paretic and nonparetic stance legs The shortest latencies observed suggest the involvement of interlimb reflexes with supraspinal pathways The preservation of the sequence and balance of activations may point to a centrally programmed postural response that is preserved after stroke while poststroke delays may suggest longer transmission times for interlimb reflexesStroke survivors have significant residual deficits related to balance control as evidenced by a high rate of falls Weerdesteyn et al 2008 Walking in the real world often requires corrective reactions to recover or maintain balance in response to unexpected perturbations Given that falls in stroke survivors most often occur while walking Hyndman et al 2002 reactive balance control during gait is critical to stroke survivors’ ability to walk independently Previous reports of poststroke neuromuscular responses to postural perturbation have focused on balance recovery from static postures eg Marigold and Eng 2006 Kirker et al 2000 While decreased stability in response to gait perturbations has been documented after stroke Krasovsky et al 2013 investigation of the neuromuscular responses during the functional task of balance recovery while walking is lacking in this population The goal of this work was to examine poststroke deficits in the functional neuromuscular responses that contribute to reactive balance control during gaitEmerging evidence suggests that chronic motor control impairments in poststroke individuals involve abnormal multisegmental behaviors For example abnormal coupling of stretch reflexes has been quantified under isometric conditions between hip adductor and knee extensor muscles in paretic legs of stroke survivors Finley et al 2008 We propose that given the complex biplanar nature of balance control during gait under destabilizing conditions aberrant multijoint neural constraints may impede the ability to generate appropriate neuromuscular responsesTo examine neuromuscular responses contributing to balance recovery during gait we employed an induced trip A trip involves interrupting the swing leg necessitating a stabilizing neuromuscular response in the support leg the stance leg contralateral to the perturbation Immediately following a trip prior to foot contact of the perturbed leg the role of the support leg is to stabilize the body by arresting the angular momentum resulting from the trip and to provide time and clearance for the placement of the perturbed foot Pijnappels et al 2004 The focus of this study was to characterize the neuromuscular responses to a trip in the support leg The initial single support phase of stabilization provides the opportunity to examine the stabilizing neuromuscular response in the paretic and nonparetic support legs in the absence of compensation from the opposite legPrevious studies have reported poststroke delays of muscle activations in the paretic leg following postural perturbations from standing position Marigold and Eng 2006 Kirker et al 2000 Dietz and Berger 1984 During gait bilateral delays were observed in an obstacle avoidance experimental paradigm van Swigchem et al 2013 Additionally bilateral attenuation of interlimb cutaneous reflexes has been reported during gait Zehr and Loadman 2012 While the origin of these poststroke changes is unclear examination under isolated singlejoint constraints pointed to strokemediated impairments in interlimb spinal pathways Stubbs et al 2012 In light of these findings and given the proposed role of interlimb reflexes in stumble correction during gait eg Haridas et al 2005 we hypothesized that altered timing sequence and magnitudes of perturbationinduced muscle activations would emerge in paretic and nonparetic support legs of stroke survivors compared to healthy control subjects
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