Variability of step kinematics in young and older adults
Introduction
Fall-related injuries are the most common and serious medical problems facing older adults [1]. Twenty percent of the eight million fall-related emergency department visits in 1994 involved adults who were 65 years of age or older [2]. Falls and fall-related injuries are the second leading cause of injury death among people between the ages of 75 and 84 years, and the leading cause of injury death among people 85 years of age and older [2].
Fewer than 2% of falls by older adults result in a hip fracture. In contrast, more than 90% of hip fractures in older adults result from a fall [1] and most of these fractures result from a fall occurring during locomotion. This underscores the complex relationships between fall initiation, descent, impact, and the biomechanics of the proximal femur. Because most hip fractures involve an impact on or around the greater trochanter, falls to the side are associated with a particularly high risk of injury. Indeed, the risk for a hip fracture is increased by six-fold when the fall occurs to the side [3]. This highlights the potential value of determining specific gait variables that increase the risk for falls to the side, that is, falls that expose the hip to impact forces. One possible variable is the control of step width.
During locomotion, the lateral placement of the foot, which dictates step width, is the dominant contributor to medial–lateral acceleration of the trunk [4]. The lateral placement of the foot is controlled during the preceding swing phase by the hip abductors and adductors, which are also responsible for medial–lateral trunk stability during the stance phase. The importance of step width control raises questions about the relationships between step width control errors, medial–lateral stability, and falls.
A common method to analyze motor control errors is movement variability. Two studies have reported on relationships between step kinematic variability and falling behavior in older adults [5], [6]. Decreased step width variability, in conjunction with increased step width, prospectively discriminated older adults who fell from those older adults who did not fall [5]. In contrast, increased variability of step time prospectively discriminated older adults who had fallen from older adults who had not fallen [6]. This apparent disparity reflects that the directional importance of altered variability cannot be generalized but rather is likely specific to the outcome measurement [7]. Nevertheless, the differences in technologies used and experimental methods used by Maki [5] and Hausdorff et al. [6] preclude resolution via direct comparison.
In the present study, we used an instrumented treadmill that allowed simultaneous quantification of both temporal and spatial step kinematics during sustained locomotion, that is, for hundreds of continuous steps. This conferred upon the present study those measurement advantages occurring in the studies of Maki [5] and Hausdorff et al. [6] The purposes of the study were to determine the extent to which age influences the variability of step kinematics and if step kinematic variability is influenced by walking velocity and the use of handrails during treadmill gait.
Section snippets
Instrumentation
The instrumented treadmill and analysis used in this study has been described previously in greater detail [8], [9]. The treadmill consists of a Tredex treadmill (Universal Gym Equipment, Cedar Rapids, IA) equipped with two AMTI forceplates (AMTI Inc., Newton, MA). The forceplates are aligned in an anterior–posterior fashion directly beneath the treadmill belt. Ground reaction forces and moments were measured from both forceplates, digitized at 100 Hz, and stored for post-processing. The
Results
To place the results with respect to variability in context, descriptive statistics for step length, step width, and step time were calculated. These values were computed independently for the effect of age (Table 1), the effect of walking velocity (Table 2), and the effect of handrail (Table 3).
The differences between the values measured for the left and right legs were not significant for the variability of step length and step time (P=0.157 and 0.851, respectively). Thus, all reported values
Discussion
A prominent finding of the present work is that step width variability increased by nearly 20% in the older subjects compared to the young subjects. This finding is consistent with the findings of Grabiner et al. [11], who reported increased step width variability of 35% in older adults during walking over an instrumented surface.
Our finding of an absence of left-right differences in step kinematic parameters in healthy older adults has not been previously reported. This should not be assumed
Acknowledgements
We gratefully acknowledge Darryl Barnes, MD, Barbara Messinger-Rapport, MD, and Susan Shutte who assisted in the data collection and analysis.
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