(D-131) Older Adults Do Not Show Greater Vulnerability to Walking Balance Challenges Compared to Young Adults

Falls in our older adult population (age ≥ 65 years) are a significant public health concern, with one in four older adults falling each year [1]. This presents a costly burden to both the individual and society; injuries from falls can reduce quality of life and independence and direct medical costs to treat fall-related injuries is $50 billion annually [2]. Despite significant scientific and societal efforts to reduce the number of falls in older adults, the rate of injurious falls is accelerating. In response to unanticipated perturbations, reactive responses, which are compensatory in nature, disproportionately depend on vigorous contractions from lower extremity muscles [3, 4]. Conversely, if a threat to balance is anticipated, proactive adjustments based on voluntary predictions can alleviate the demands for reactive muscular actions [5-9]. Reactive balance responses can be compromised by age-related declines in muscle strength, reaction time, and sensory processing, thus making older adults more vulnerable to unanticipated balance challenges [10-13]. Therefore, the purpose of this ongoing study is to investigate the modifying influence of age (older vs. younger adults) on the effects of anticipation on vulnerability to walking balance challenges. We hypothesized that, compared to anticipated perturbations, unanticipated perturbations will elicit larger reactive balance responses, evidenced by greater changes in whole-body angular momentum. We also hypothesized that this effect of anticipation would be larger for older than for younger adults.

Twenty younger adults (age: 22.75±3.31 years; preferred walking speed 1.39±0.14 m/s ) and twenty healthy older adults participated in this single-visit study (age, 71.22 ± 4.63 yrs; preferred walking speed 1.28 ± 0.19 m/s). We collected motion capture data while participants walked on a force-sensing, dual-belt treadmill (Bertec Corp., Columbus, Ohio, USA) for two minutes at their preferred walking speed for acclimation. Participants then completed a block-randomized series of walking trials with the addition of two contexts of anterior and posterior perturbations. In one trial, participants responded to 6% body weight, 200 ms waist-pull perturbations delivered at the instant of heel strike in either the anterior or posterior direction via a motor-driven cable system. The components of this system have been described in detail previously [44]. In another trial, participants responded to 200 ms, 6 m/s² treadmill belt accelerations or decelerations delivered at the instant of heel strike using a custom Matlab script (MathWorks, Natick, MA, USA). For both contexts and directions, we delivered perturbations either unexpectedly (i.e., unanticipated) or at the end of a three second verbal countdown (i.e., anticipated). Eight combinations of perturbation side, direction, and anticipation were repeated twice for a total of 16 perturbations in each trial. We defined the direction of each perturbation as that in which the perturbation acted. In other words, anterior perturbations included forward waist-pulls and treadmill decelerations, while posterior perturbations included backward waist-pulls and treadmill accelerations. Total walking duration averaged approximately 8 min.

Thus far, our results do not support our hypothesis that, compared to anticipated perturbations, unanticipated perturbations would elicit larger reactive balance responses, evidenced by greater changes in whole-body angular momentum. Regardless of perturbation direction and context, anticipated and unanticipated perturbations did not elicit significantly different ranges of whole-body angular momentum. Additionally, our current results do not support our hypothesis that this effect of anticipation would be larger for older than for younger adults. Within each perturbation condition, WBAMrange did not differ greatly between older and younger adults.

Indeed, our results show early evidence that perturbation direction and context matters. Perturbations that compel a backwards fall (i.e. treadmill belt decelerations and backward waist pulls) elicited greater WBAMrange than perturbations that compel a forwards fall (i.e. treadmill belt accelerations and forward waist pulls) (Fig 1A-B). In the sagittal plane (i.e., the direction acted upon by each perturbation), we found that, qualitatively, both older and younger adults exhibited greater positive (i.e., backward) momentum following a treadmill belt deceleration compared to unperturbed walking. However, younger adults used more negative (i.e., forward) momentum to recover from treadmill belt decelerations than older adults, thus suggesting a different recovery strategy. Compared to unperturbed walking, both older and younger adults had greater WBAMrange in response to treadmill belt decelerations, regardless of anticipation. We did not find significant differences between forward and backward waist-pulls. Thus, our results suggest perturbations applied at the base of support increase vulnerability compared to those applied at the center of mass.

Our results thus far suggest that regardless of perturbation context, direction, and anticipation, older adults are not more vulnerable to balance perturbations than younger adults. Indeed, our suggest that direction, rather than anticipation, show greater effects on vulnerability to walking perturbations. Thus far, our results suggest that older adults’ vulnerability to walking balance challenges increases in response to perturbations that compel a backward fall compared to a forward fall, regardless of perturbation context. Our immediate next step is to combine these outcomes with measured muscle neuromechanics to explore causal relations underlying these differences in susceptibility.

1.            WISQARS (Web-based Injury Statistics Query and Reporting System)|Injury Center|CDC.

2.            Tromp, A.M., et al., Fall-risk screening test. Journal of Clinical Epidemiology, 2001. 54(8): p. 837-844.

3.            Tang, P.F., Woollacott, M.H., Chong, R.K.Y., Control of reactive balance adjustments in perturbed human walking: roles of proximal and distal postural muscle activity. 1998, Exp Brain Res. p. 141-152.

4.            Chvatal, S.A. and L.H. Ting, Voluntary and reactive recruitment of locomotor muscle synergies during perturbed walking. J Neurosci, 2012. 32(35): p. 12237-50.

5.            Marigold, D.S., Patla, A.E., Strategies for dynamic stability during locomotion on a slippery surface: Effects of prior experience and knowledge. 2002, American Physiological Society. p. 339-353.

6.            Pavol, M.J., E.F. Runtz, and Y.-C. Pai, Young and Older Adults Exhibit Proactive and Reactive Adaptations to Repeated Slip Exposure. The Journals of Gerontology: Series A, 2004. 59(5): p. M494-M502.

7.            Bouisset, S., & Zattara, M., Biomechanical study of the programming of anticipatory postural adjustments associated with voluntary movement. Journal of biomechanics, 1987. 20(8): p. 735–742.

8.            Massion, J., Movement, posture and equilibrium: interaction and coordination. Progress in neurobiology, 1992. 38(1): p. 35-36.

9.            Lyon, I.N., Day, B. L., Control of frontal plane body motion in human stepping. Experimental brain research, 1997. 115(2): p. 345-356.

10.         Deshpande, N., et al., Ankle proprioceptive acuity is associated with objective as well as self-report measures of balance, mobility, and physical function. Age (Dordr), 2016. 38(3): p. 53.

11.         Lord, S.R., Ward, J. A., Williams, P., & Anstey, K. J., Physiological factors associated with falls in older community-dwelling women. Journal of the American Geriatrics Society, 1994. 42(10): p. 1110-1117.

12.         Thompson, J., P. Plummer, and J.R. Franz, Age and falls history effects on antagonist leg muscle coactivation during walking with optical flow perturbations. Clin Biomech, 2018. 59: p. 94-100.

13.         Mau-Moeller, A., et al., The Relationship between Lean Mass and Contractile Properties of the Quadriceps in Elderly and Young Adults. Gerontology, 2015. 61(4): p. 350-4.