Modulation of ankle muscle postural reflexes in stroke: influence of weight-bearing load

Abstract

Objective-Given the known sensorimotor deficits and asymmetric weight-bearing posture in stroke, the aim of this study was to determine whether stroke affects the modulation of standing postural reflexes with varying weight-bearing load.Methods-Ten individuals with chronic stroke and 10 healthy older adult controls were exposed to unexpected forward and backward platform translations while standing. Three different stance conditions were imposed: increased weight-bearing load, decreased weight-bearing load, and selfselected stance. Surface EMG from bilateral ankle dorsiflexors (tibialis anterior) and extensors (gastrocnemius) were recorded and the magnitude of background muscle activity (prior to the platform translation) and postural reflex onset latency and magnitude (75 ms following reflex onset) were determined. Significance-The results provide some insight into standing postural reflexes following stroke. Results-Load

Altered timing of postural reflexes contributes to falling in persons with chronic stroke

“…Although others have reported delays in paretic limb onset latency and disturbed intralimb coupling in response to platform perturbations in persons with stroke (Badke and Duncan 1983;Berger et al 1988;Dietz and Berger 1984;Di Fabio and Badke 1988;Di Fabio et al 1986;Di Fabio 1987;Marigold et al 2004), none have attempted to relate these observations to falls. The results of this study suggest that upper motoneuron lesions affect the neuronal circuitry involved in the generation and organization of a postural reflex following a balance-threatening event during stance, which contributes to falls in this population.…”

“…The mean EMG signal for one second prior to the onset of platform movement was determined along with the standard deviation. Postural reflex muscle onset latency (see Figure 1) was defined as an increase in muscle activity that exceeded two standard deviations of the mean background activity for at least 30 ms following the onset of platform movement and was determined by a combination of visual inspection and computer algorithm via an interactive program (Marigold et al 2004). The person performing the visual inspection was blinded as to whether the person had fallen in response to the platform translation.…”

“…Postural reflex onset latencies of healthy older adults in response to similar platform translations range from 100-115 ms (Marigold et al 2004). Studies using transcranial magnetic stimulation have suggested that reflex responses after 80-90 ms may be partially mediated by a transcortical reflex loop (Nielsen et al 1997;Petersen et al 1998;van Doornik et al 2004).…”

“…The central nervous system must utilize and integrate the available sensory and environmental information to select an appropriate response strategy. During a stance perturbation, these postural reflexes are initiated with onset latencies between 100 and 120 ms in healthy older adults (Manchester et al 1989;Marigold et al 2004). In addition to the rapid timing of these responses, postural reflexes are coordinated into muscle synergies such that muscles act together as a functional unit through neuronal coupling (Shumway-Cook and Woollacott 2001).…”

“…Studies have shown that persons with stroke have delayed paretic limb muscle onset latencies following perturbations while standing on a moveable platform compared to their non-paretic limb and to healthy older adults (Berger et al 1988;Dietz and Berger 1984;Di Fabio and Badke 1988;Di Fabio et al 1986;Di Fabio 1987;Marigold et al 2004). In addition, the timing between the activation of the distal and proximal muscles (i.e.…”