Gait variability, that is, fluctuations in movement during walking, is an indicator of walking function and has been associated with various adverse outcomes such as falls. In this paper, current research concerning gait variability in persons with multiple sclerosis (MS) is discussed. It is well established that persons with MS have greater gait variability compared to age and gender matched controls without MS. The reasons for the increase in gait variability are not completely understood. Evidence indicates that disability level, assistive device use, attentional requirement, and fatigue are related to gait variability in persons with MS. Future research should address the time-evolving structure (i.e., temporal characteristics) of gait variability, the clinical importance of gait variability, and underlying mechanisms that drive gait variability in individuals with MS. 1. Gait Variability When one moves repeatedly, there are slight alterations in each individual movement. Traditionally, this variability in movement was viewed as random noise-providing minimal important information [1, 2]. With the introduction of chaos and complexity theory into the life sciences in the 1990s, this negative view of variability was challenged [3–5]. It is now maintained that movement variability is an important clinical phenomenon [6]. Gait (i.e., walking) is a complicated process involving coordination of multiple systems within the body (e.g., central nervous, musculoskeletal, and cardiovascular system) [7]. To walk, a person’s nervous system must send signals to control a large number of muscles while simultaneously processing sensory information in order to monitor and refine movements, all while maintaining an upright stance [7]. Given the multitude of muscles, and neural processes involved, gait variability likely arises from a combination of factors [8]. There is increasing evidence that gait variability is a quantifiable indicator of walking function [1, 2, 9]. In the past, gait variability was viewed as experimental artifact that should be filtered out to reveal average behavior [10]. However, more recently, evidence has demonstrated meaningful characteristics of gait variability, associated with neural control of walking [1, 2, 9, 10]. Increases in gait variability have been observed in individuals with advanced age [11–14] as well as various neurologically impaired populations, including spinal cord injury [15] and neurological conditions, such as Parkinson’s disease [16], dementia [17], and multiple sclerosis [18]. Gait variability has further been
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