Muscle activities used by young and old adults when stepping to regain balance during a forward fall

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Abstract

The current study was undertaken to determine if age-related differences in muscle activities might relate to older adults being significantly less able than young adults to recover balance during a forward fall. Fourteen young and twelve older healthy males were released from forward leans of various magnitudes and asked to regain standing balance by taking a single forward step. Myoelectric signals were recorded from 12 lower extremity muscles and processed to compare the muscle activation patterns of young and older adults. Young adults successfully recovered from significantly larger leans than older adults using a single step (32.2° vs. 23.5°). Muscular latency times, the time between release and activity onset, ranged from 73 to 114 ms with no significant age-related differences in the shortest muscular latency times. The overall response muscular activation patterns were similar for young and older adults. However older adults were slower to deactivate three stance leg muscles and also demonstrated delays in activating the step leg hip flexors and knee extensors prior to and during the swing phase. In the forward fall paradigm studied, age-differences in balance recovery performance do not seem due to slowness in response onset but may relate to differences in muscle activation timing during the stepping movement.

Introduction

Falls among older adults are associated with high rates of serious injury. These injuries may in part result from age-related declines in the ability to regain stable balance after a disturbance such as a trip [3]. The recovery of balance after a trip requires the sensation of a loss of balance, the planning of a recovery strategy and the execution of compensatory stepping by the musculoskeletal system. Therefore age-related changes in sensory performance, motor control, muscular strength and flexibility could all affect abilities to regain balance. Further research is necessary to ascertain how these various mechanical and neurological factors may contribute to fall injuries in older adults [15].

Significant age-related differences in the use of stepping to recover balance have been found [11], [13], [17], [19]. For example, it was shown that the maximum forward lean angle from which balance could be regained with a single step was significantly smaller for healthy older adults than for young adults [17], [19]. In those tests, maximum lean angles were highly correlated with forward step velocity. The current study expands on these results by determining if age differences in neural factors, such as muscular activation latencies and timing, could contribute to difficulties the elderly have in recovering balance by taking a rapid step upon release from a forward lean.

Age effects on muscle activities have been studied in response to platform perturbations of standing balance. It has been shown that the elderly are only slightly slower than the young in initiating active muscle responses following a perturbation [20], [14]. In addition, older adults demonstrate significantly lower cross-correlations of activity amplitudes between synergistic muscles, and greater amounts of activity in antagonistic muscles than do young adults when regaining balance with a sway response [12], [21]. The differences in antagonism may reflect a strategy of older adults relying on greater amounts of joint stiffness than young adults [21].

Age related losses in strength would likely impact abilities of older adults to step rapidly to recover balance. Older adults of ∼70 years demonstrate ∼20–40% losses in absolute strength (as reviewed by Doherty et al. [8]). Additionally the ability of older adults to generate joint torques rapidly is similarly diminished [16]. Thus to execute a step as rapidly as young adults, it seems that the elderly would have to adapt their muscle activation patterns, possibly using greater and longer muscle activations, to compensate for changes in muscle contraction capabilities. Based on observations of age-effects in postural perturbations and strength development, the following hypotheses were tested:

  • Age does not significantly increase the latency between release from a lean and onset of muscular activities.

  • Older adults will employ significantly greater muscle activities than young adults do when stepping following release from a fixed lean angle.

  • The timing of muscle activities during the stepping movement will differ between young and older adults.

Section snippets

Subjects

Fourteen Young (YM: mean age 24 years, range 19 to 29 years) and twelve Old (OM: mean age 72 years, range 66 to 80 years) healthy male subjects participated. The mean (SD) heights of the YM and OM were 176.9 (7.7) and 172.6 (4.9) cm, while the mean body masses were 74.9 (11.0) and 76.1 (7.2) kg, respectively. Young subjects were recruited among University staff and students. Old were independent community-dwelling members. Potential subjects were asked to identify their dominant foot by

Maximum leans

Young adults were able to recover balance with a single step from significantly larger leans than old adults (P<0.0001). Maximum lean angles were 32.2° (standard deviation=3.0°) for the young adults, compared to 23.5° (2.6°) for the old adults.

Step timing and step lengths

Step liftoff and landing times each significantly decreased with increasing lean angle (Table 1). At small lean magnitudes (lean loads less than or equal to 25% of body weight), step timing and lengths were not significantly different between the young

Discussion

The current study was undertaken to determine if age-related differences in the onset, magnitude and timing of muscle activities relate to decreased abilities of old adults to recover balance by taking a single rapid step during a forward fall. The results of this study demonstrate that latency times of young and old adults are quite similar, and that the latencies are unrelated to the maximum lean angle from which a subject can recover balance. However old adults were slower in deactivating

Acknowledgements

The financial support of the American Federation for Aging Research and the National Institute of Aging is gratefully acknowledged. We also thank Murrie Green, Janet Grenier, Julie Grunawalt and Rhonda Keller for their assistance.

Darryl G. Thelen PhD, received a BS degree in mechanical engineering from Michigan State University in 1987 and MSE and PhD degrees in mechanical engineering from the University of Michigan in 1988 and 1992 respectively. From 1992 to 1994 he was a postdoctoral fellow with the Institute of Gerontology at the University of Michigan, and since 1994, has been an Assistant Professor of Engineering at Hope College in Holland, MI. Dr Thelan's research interests include biomechanics of mobility

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  • Cited by (82)

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    Darryl G. Thelen PhD, received a BS degree in mechanical engineering from Michigan State University in 1987 and MSE and PhD degrees in mechanical engineering from the University of Michigan in 1988 and 1992 respectively. From 1992 to 1994 he was a postdoctoral fellow with the Institute of Gerontology at the University of Michigan, and since 1994, has been an Assistant Professor of Engineering at Hope College in Holland, MI. Dr Thelan's research interests include biomechanics of mobility impairments in older adults, computer simulation of human movement and quantitative electromyography. He has received research support from the National Institute on Aging, the National Science Foundation and the American Federation for Aging Research.

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    Albert B. Schultz PhD, is the Catherine and Ame Vennema Professor of Mechanical Engineering and Applied Mathematics in the Department of Mechanical Engineering and Applied Mechanics, and a Research Scientist in the Institute of Gerontology at the University of Michigan, Ann Arbor, MI. Dr Schultz's research concerns mobility impairments in older adults and other aspects of geriatic biomechanics. He has received a number of research awards, including an NIH Javits Neuroscience Investigator Award; the HR Lissner Award from the the American Society of Mechanical Engineers, and the Giovanni Borelli Award from the American Society of Biomechanics. Dr Schultz is a member of the National Acadamy of Engineering.

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    James A. Aston-Miller directs the Biomechanics Research Laboratory in the Department of Mechanical Engineering and Applied Mechanics at the University of Michigan (U–M). He also holds appointments as Research Scientist in the Deaprtments of Mechanical Engineering and Applied Mathematics, and Biomedical Engineering, and is Senior Research Scientist at the Institute of Gerontology at U–M. He obtained his MS degree in mechanical engineering from MIT in 1974 and his PhD in biomechanics from the University of Oslo, Norway in 1982. After moving to the University of Michigan in 1983, he spent more than a decade researching the biomechanics of the human spine, low back pain and the pathogenisis of childrens spine deformities. More recently he has concentrated on the biomechanics of fall-related injuries, mobility impairments and urinary incontinence in the elderly. He has authored over 85 publications on these and related topics.

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    Neil B. Alexander MD is an Associate Professor, Division of Geriatric Medicine, Department of Internal Medicine and Senior Associate Research Scientist, Institute of Gerontology, University of Michigan. He is also Research Scientist at the Ann Arbor VA Medical Centre Geriatric Research, Education, and Clinical Centre (GRECC). Dr Alexander develops assessments and interventions regarding daily mobility activities, such as rising from a bed and a chair, and regarding maintenance of upright stance and avoiding falls. He received a National Institute of Aging (NIA) Career Development Award and continues active NIA and VA funded projects.

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    Muturi Muriuki received his BS degree (summa cum laude) in Engineering Physics in 1997 from Hope College. He is currently working on his MS degree in Mechanical Engineering at the University of Pittsburgh. His research focuses on smart materials and their application to vibration control. He is a member of Sigma Pi Sigma and Sigma Xi.

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    Jodi James graduated in 1997 from Hope College with a BA in Dance and a BS in Engineering Physics. While at Hope, Jodie received an award from Sigma Xi for noteworthy undergraduate research. Jodi is currently pursuing a doctorate in dance science at the University of Utah.

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