The influence of a concurrent cognitive task on the compensatory stepping response to a perturbation in balance-impaired and healthy elders

Abstract

This study investigated the influence of a concurrent cognitive task on the compensatory stepping response in balance-impaired elders and the attentional demand of the stepping response. Kinetic, kinematic and neuromuscular measures of a forward recovery step were investigated in 15 young adults, 15 healthy elders and 13 balance-impaired elders in a single task (postural recovery only) and dual task (postural recovery and vocal reaction time task) situation. Results revealed that reaction times were longer in all subjects when performed concurrently with a compensatory step, they were longer for a step than an in-place response and longer for balance-impaired older adults compared with young adults. An interesting finding was that the latter group difference may be related to prioritization between the two tasks rather than attentional demand, as the older adults completed the step before the reaction time, whereas the young adults could perform both concurrently. Few differences in step characteristics were found between tasks, with the most notable being a delayed latency and reduced magnitude of the early automatic postural response in healthy and balance-impaired elders with a concurrent task.

Introduction

The simultaneous performance of a postural and cognitive task can have a deleterious effect on balance control particularly in elders, due to a reduction, or misallocation of attentional resources. This inability to successfully perform a postural task simultaneously with a cognitive task has been suggested as a potential contributor to falls in elders with a clinical balance impairment [1].

Dual task paradigms have been used to examine the relative attentional demands associated with different types of postural tasks. As postural demands increase, the demand for attentional resources increases as well [2], [3]. Using a secondary cognitive task, Brown et al. (1999) reported that step and in-place responses to a platform perturbation require similar attentional resources in young adults. However, in healthy elders, they reported a trend for the step strategy to have a greater attentional demand (longer cognitive response times). If a step response was attentionally demanding in balance-impaired elders, this could explain postural instability occurring when performed in a dual task situation. Thus one purpose of this study was to examine the attentional demands of a step response in balance-impaired elders compared with young and healthy elders, and to determine whether a compensatory step response was more attentionally-demanding than an in-place response. We hypothesized that the attentional demands associated with stepping would be greatest for balance-impaired elders, compared with young and healthy elders.

Dual task paradigms have also been used to demonstrate the deleterious effects of a secondary task on postural control in young and healthy elders. In a dual task situation, healthy elders make more frequent obstacle contacts in gait [4], take a longer time to recover from instability [5], and increase sway in quiet stance [6] in comparison to a single task situation. Furthermore, Rankin et al. (2000) investigated the in-place postural response to a platform perturbation in young and healthy elders in a single task (recovery only) and dual task (while concurrently performing a counting task) situation. They reported a reduction in the amplitude of the late neuromuscular response of the gastrocnemius (350–500 ms post perturbation) to be greater in healthy elders in a dual task situation, than in young adults.

Previously, we have found that when responding to a platform perturbation while concurrently performing a cognitive task, balance-impaired elders take a longer time to regain a stable position than when responding to the postural task alone [7]. Other studies have reported that balance-impaired elders show a decrement in postural task performance in quiet stance when simultaneously performed with a cognitive task [1], [2], [3], [4], [5], [6], [7], [8]. This decrement in postural ability while performing a simultaneous task is also prevalent in gait, a situation of high risk for falls. One study reported that elders take a longer time to perform the Timed Up and Go Test with the addition of a simultaneous cognitive or motor task [9], while another reported that elders stop walking when talking [10]. This inability to walk and talk simultaneously was associated with a greater risk of falls in the succeeding 6 month period [11].

In elders, most falls occur as a result of an inability to react appropriately and produce an effective compensatory response. The most common recovery strategy used by elders in response to a challenging perturbation is to step [12], [13]. One factor potentially limiting an effective stepping response in elders could be the simultaneous performance of a secondary cognitive task. In healthy young and elders, the compensatory stepping response is altered with the performance of a simultaneous cognitive task [14]. It is not known what the effect of a secondary task is on the ability to recover postural stability in balance-impaired elders. Thus, a second purpose of this study was to investigate the influence of the performance of a cognitive task on the compensatory recovery effort of balance-impaired elders. We hypothesized that in a dual task situation, balance-impaired elders would step more frequently, take more multiple steps, and the characteristics of the steps would reflect a more unstable recovery than in a single task situation.