Age-related differences in spatiotemporal markers of gait stability during dual task walking
Introduction
According to the Centers for Disease Control and Prevention, falls are the leading cause of unintentional injury deaths in the United States among people age 65 years and older, accounting for over 13,000 deaths in 2003, and non-fatal falls are among the leading causes of hospitalizations for older adults [1]. From a systems perspective, fall-related injuries are not inconsequential. Fall-related injuries in older adults resulted in US$ 19.5 billion in direct medical care costs in 2000 [2]. From an individual perspective, falls are not inconsequential either. Apart from injuries and subsequent morbidity that occur after a fall, psychological consequences occur as well. Many people who fall, whether they sustain injury or not, develop a fear of falling and subsequently limit their activity leading to reduced mobility, weakness and increased risk of future falls.
Risk factors for falls are described as being extrinsic or intrinsic. Extrinsic risk factors include environmental risks such as tripping hazards (e.g., throw rugs on floors), lack of stair railings, and poor lighting conditions, any of which can cause falls. Intrinsic risk factors generally include personal characteristics such as age, physiological impairments and medication use, among others, that can predispose an individual to falls. Modifying these risk factors when possible can reduce fall risk. One of the modifiable risk factors for falls is impaired gait stability. Gait instability is characterized by increased variability from stride to stride and is common in many older adults, even in absence of pathology [3]. Increased variability reflects inconsistent stepping patterns and reduced postural control during walking. In persons with neurological pathology, deficits in the central nervous system's ability to coordinate motor outputs are largely responsible for gait instability [4], [5]. In elderly persons without apparent neurological pathology, it is not entirely clear why gait instability occurs. The reasons are most likely multifactorial, including deficits in physiological function such as impaired joint range of motion and muscle performance and deficits in neuropsychological or cognitive status that can exacerbate the effects of impaired physiological capacity. Relatively few studies have examined age-related changes in gait stability [3], [6], [7], [8]. Nevertheless, initial investigations suggest that impaired gait stability may be a more powerful predictor of falling than static measures of balance [6].
Maintaining stability in walking was traditionally considered an automatic or reflex-controlled task requiring motor responses to sensory stimuli, but requiring minimal cognitive resources. Recent evidence, however, suggests that maintaining postural stability requires both cognitive and sensorimotor processes [9]. Dual task paradigms are used to examine the effects of cognitively challenging tasks on primary activities such as walking. In older persons compared to younger adults, postural stability in standing is significantly reduced during cognitively demanding tasks [10]. During walking, many falls in older persons occur not during normal walking, but rather when they are walking and performing secondary tasks such as talking [11]. The influence of cognitive activity on gait stability has been studied in patient populations and results consistently show decreased gait velocity and increased gait variability in dual task conditions [12], [13], [14], [15], [16]. To our knowledge, however, only three small studies have examined with cross-sectional designs whether age-related differences in gait variability during dual task walking exist in healthy persons [17], [18], [19]. In each, measures of gait variability were greater in older subjects than in younger subjects. Given the limited numbers of subjects in the aforementioned studies [17], [18], [19], whether there are age-related differences in gait variability during dual task walking, as compared to pathology-related differences, warrants further study.
The primary purpose of this cross-sectional study was to examine whether markers of gait instability, particularly reductions in gait velocity and increases in stride-to-stride variability in gait velocity, occur in older adults compared with younger and middle-aged adults during dual task walking. The secondary purpose of the study was to compare errors in the cognitive task during dual task walking between groups and to quantify the relationships between cognitive performance and walking performance.
Section snippets
Subjects
In a pilot study [18], stride-to-stride variability in gait velocity in older subjects increased from a coefficient of variation of 4.9% in a normal walking condition to a coefficient of variation of 16.4% in a dual task walking condition. To detect a comparable change in stride-to-stride variability among older subjects at α = 0.05 and to achieve a statistical power of 0.90, minimally 10 subjects per group were required to participate. We recruited 20 subjects per group, a conservative number of
Gait velocity
Gait velocity was slower in dual task walking than in normal walking (Fig. 1 has representative data; F1,57 = 55.061, p < 0.001). The difference in gait velocity was statistically significant within each group (Table 2). Additionally, gait velocity differed significantly between groups (F2,57 = 20.649, p < 0.001). Older adults walked more slowly than did younger and middle-aged adults in both walking conditions (Table 2). Gait velocity did not differ significantly between the younger and middle-aged
Effect of dual tasking on gait
Results of the study support our hypotheses that gait velocity would be reduced and stride-to-stride variability would be increased in dual task walking. Results also support our hypothesis that the gait changes would be most pronounced in older subjects. Gait velocity was lower in dual task walking than in normal walking in all three groups, but the difference in gait velocity between normal and dual task walking was greater in older adults (20% reduction) than in younger (8% reduction) and
Conclusion
Gait velocity decreased and stride-to-stride variability in gait velocity increased when people walked while performing a cognitively challenging task. The differences were most prominent among older adults. Gait variability observed in the dual task condition characterizes decreased gait stability and indicates that cognitively demanding tasks performed while walking have a destabilizing effect and may place older people at greater risk of falling. These findings may be relevant to the
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