Full length articleAging effects on leg joint variability during walking with balance perturbations
Introduction
Balance control during walking depends on integrating reliable sensory feedback to plan and execute corrective motor responses [1]. These motor responses presumably manifest first at the individual muscle and joint levels and contribute to coordinated balance corrections at the whole-body level via adjustments in foot placement from one step to the next. Motivated by these ideas, we recently revealed that variability of foot placement (i.e., step width and step length) in older adults is more susceptible than in young adults to optical flow perturbations during walking, presumably governed by an age-related dependence on visual feedback for motor planning and execution [[2], [3], [4], [5], [6]]. However, foot placement variability is determined by step-to-step adjustments in the angles of the leg joints. Our understanding of which joints contribute more to foot placement variability and when during the gait cycle those joint-level adjustments occur, particularly in the presence of balance perturbations, is fundamentally incomplete.
Although average profiles of leg joint kinematics are ubiquitous in gait biomechanics research, there are surprisingly few studies describing gait phase-dependent changes in step-to-step variability at the joint level. However, it is highly intuitive that leg joint kinematic variability would exhibit gait phase-dependence, increasing during the gait cycle when step-to-step adjustments in hip, knee, and ankle joint kinematics are most prevalent. For example, regulating step length and step width during step-to-step transitions is critical for walking balance control, alluding to the potential for particularly large variability in leg joint kinematics [7]. Indeed, local dynamic stability, at least of sagittal plane knee joint kinematics, exhibits local maximums during heel strike and toe-off events in walking [8]. However, we currently lack empirical evidence for whether leg joint variability similarly exhibits phase-dependence across the gait cycle, even in healthy young adults.
Compared to young adults, older adults often walk with greater variability of foot placement (e.g., step width and step length) than young adults [9], and this variability can be retrospectively associated with falls history [10]. However, accompanying highly prevalent somatosensory changes with aging [[11], [12], [13], [14], [15], [16]], there is a well-documented dependence on vision for movement and balance control that are even more pronounced in older adults with a history of falls [[3], [4], [5], [6], [17]]. Accordingly, we have shown that optical flow perturbations applied during walking (i.e., the visual self-perception of walking imbalance) can elicit larger age-related decrements in foot placement variability than during unperturbed walking [[5], [18]]. These findings imply that optical flow perturbations could provide a unique opportunity to study how step-to-step adjustments in step width and step length are regulated via those in hip, knee, and ankle joint angles in the context of age-related changes in walking balance. Ultimately, understanding these joint-level adjustments and their relation to whole-body balance corrections via step width and step length could point to specific joint-level therapeutic targets to mitigate falls risk.
Therefore, this study aimed to investigate gait phase-dependence in and the effects of age on leg joint kinematic variability during walking with and without optical flow perturbations of different amplitudes. We hypothesized that leg joint kinematic variability would: 1) vary across the gait cycle (i.e., exhibit gait phase-dependence), and 2) increase in the presence of optical flow perturbations. We also hypothesized that 3) compared to young adults, the effects of optical flow perturbations on leg joint kinematic variability would be larger and more pervasive in older adults. Finally, the second goal of this study was to determine how step-to-step adjustments in hip, knee, and ankle joint angles contribute to those in step length and step width in old and young subjects − a question that informed a series of linear regressions between foot placement variability and leg joint variability.
Section snippets
Subjects
We recruited 11 healthy young (mean ± standard deviation, age: 24.8 ± 4.8 yrs, mass: 67.2 ± 8.83 kg; height: 172 ± 9 cm, 5 males, 6 females) and 11 healthy older adults (age: 75.3 ± 5.4 yrs, mass: 73.4 ± 16.1 kg, height: 175 ± 10 cm, 5 males, 6 females) to participate in this study. All subjects had normal or corrected to normal vision. Based on a health questionnaire, we excluded subjects based on the following: BMI ≥ 30, sedentary lifestyle, orthopedic or neurological condition, taking
Gait cycle phase and age group effects
During unperturbed walking, hip (Fig. 1, Fig. 2), knee (Fig. 3), and ankle (Fig. 4) joint angle variabilities differed significantly across different phases of the gait cycle (main effect, p’s < 0.001), and all reached their maximum following the instant of ipsilateral toe-off (i.e., ∼60–70% cycle). Compared with young adults, older adults walked normally with, on average, 34% larger coronal plane hip joint variability (p = 0.014, Fig. 2D), but no difference in sagittal plane hip (p = 0.074,
Discussion
We sought to investigate aging effects on leg joint kinematic variability during walking with and without balance perturbations and thereby elucidate the joint-level origins of whole-body walking balance control and deficits thereof in old age. Our data largely supported each of our hypotheses. In support of our first hypotheses, we found that leg joint variability in walking was gait phase-dependent, with step-to-step adjustments in hip, knee, and ankle joint kinematics occurring predominantly
Conclusions
We first conclude that, as hypothesized, leg joint variability does vary across the gait cycle, with step-to-step adjustments in hip, knee, and ankle joint kinematics occurring predominantly during push-off and early swing. Second, young adults responded to optical flow perturbations almost exclusively by increasing variability in coronal plane hip joint angle. Also as hypothesized, the same amplitude perturbations elicited larger and more pervasive increases in all joint variability outcome
Conflict of interest disclosure
The authors have no conflicts of interest to disclose.
Acknowledgements
We gratefully acknowledge Ms. Jessica Thompson and Ms. Heather Stokes for their help with data collection. This study was supported in part by grants from the National Center for Advancing Translational Sciences (UL1TR001111), the National Institutes of Health (R56 AG054797), and the University of Carolina at Chapel Hill and North Carolina State CLEAR Core.
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