Full length articleAging effect on step adjustments and stability control in visually perturbed gait initiation
Introduction
In daily walking, one often has to suppress preplanned step and adjust foot placements in response to environmental perturbations (uneven surface, obstacles, pets, etc.). The ability to make quick step adjustments is essential for maintaining locomotion stability, whereas limited capacity to adapt gait might contribute to tripping and falling [1]. In suddenly perturbed locomotion, the inhibition of original motor planning, the selection of alternative foot placement [2], and the postural stability control during rapid limb adjustments [3] all need to be executed in a timely manner.
With increased age, limited sensorimotor and cognitive function may lead to poor gait adaptability [1], [4]. More specifically, in response to sudden perturbations, the deficits of older adults in making rapid step adjustments may be caused by decline in musculoskeletal function to generate muscle power [5], reductions in joint range of motion [6], delayed perception of environmental changes [4], and impaired executive function [1].
Previous studies on step adjustments to sudden shifts in stepping targets have been primarily conducted in discrete step initiation [7], [8], [9], [10] and continuous walking scenarios [2], [3], [4], [11], [12], [13], but not in gait initiation task, which is also a challenge to the balance control system as the body moves from stable static balance to continuous unstable gait [14]. Gait initiation has also been proposed as a natural but deliberately destabilizing task that can be a valid instrument in assessing the balance ability of aging, frail or neurological impaired subjects [15], [16]. One hallmark of gait initiation is anticipatory postural adjustment (APA), which involves a sequence of weight shifting to propel the body mass forward and towards single stance limb before foot lift [17], [18], [19]. The APA, although named as anticipatory in nature, has also been associated with online postural control, as demonstrated by the correction of error during choice reaction stepping [19], [20]. Therefore, APA may be critical for making rapid step adjustments during perturbed gait initiation. Moreover, previous studies on step adjustments often used fixed delay intervals (ranges from 100 ms to 650 ms) [4], [9], [21] to trigger stepping adjustments, which might not be sensitive to individual differences in reaction time and motor execution time.
Given the goal of a successful adaptive gait is not only controlling the foot trajectory for alternative placement, but also maintaining balance during and after the step adjustment [3], [13], understanding how humans maintain and control stability when exposed to perturbations is crucial to determine the risk of falling [22]. In order to quantify the dynamic process of locomotion stability control, Hof et al. [23] introduced the measure of margin of dynamic stability (MDS), which was then widely applied in numerous locomotion research [3], [22], [24]. MDS was defined as the distance between the velocity extrapolated center of mass (XCoM) position and the edge of the base of support (BOS), and a stable gait was achieved as long as the XCoM remains within the BOS.
In the present study, we investigated the aging effect on step adjustments at gait initiation utilizing projected stepping-target perturbations. A novel method was used to trigger the stepping target relocation during gait initiation based on the subject’s APA weight shift pattern prior to foot lifting, so that the available response time was adjusted for each individual. The stepping accuracy, the foot landing rotation, as well as the MDS at and after the step adjustment were chosen as the primary outcome measures. We hypothesized that older adults would have decreased stepping accuracy as well as decreased stability, especially in more challenging condition (i.e., with increased time pressure).
Section snippets
Participants
Ten healthy college students (21.5 ± 1.9 years, 4 female) and ten healthy community dwelling older adults (68.0 ± 4.1 years, 6 female) participated in this study. None of the participants had a history of neurological, musculoskeletal disorder, or other conditions that limit mobility at the time of participation. All participants had normal or corrected to normal vision. The Mini-Mental Status Examination (MMSE) [25] and Timed Up and Go (TUG) test [26] were conducted for screening of cognitive and
Results
Table 1 listed the demographic characteristics of participants. No significant difference was found between groups except age.
Fig. 3A illustrates the group average stepping accuracy of each stepping condition. Overall, medial adjustment led to significantly decreased stepping accuracy, regardless of group, in intermediate and late adjustment conditions (80.1% in intermediate, 22.8% in late). The lateral stepping induced significant decreased stepping accuracy only in the late adjustment
Discussion
In the present study, the aging effect on step adjustments and stability control in perturbed gait initiation was evaluated. A novel method was used to trigger online stepping adjustments based on real-time analysis of weight distribution of gait initiation. Unlike previous studies using fixed delay intervals [4], [9], [21] to trigger stepping adjustments, we customized the trigger timing with individual differences in reaction time and execution timing of APA prior to foot lifting.
For step
Conclusion
In general, the present study showed that step adjustments in perturbed gait initiation were affected by the timing and direction of stepping target change, with the medial stepping being more challenging to execute compared with lateral stepping. The lateral adjustment task, however, yielded better separation between young and older subjects, which can be useful for future research on falls risk assessment and prevention. MDS measure also revealed that older subjects have deficits to restore
Conflict of interest statement
None of the authors have financial or other conflicts of interest in regards to this research.
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