Balance In Elderly

Journal of Motor Behavior, 2002, Vol. 34, No. 1, 37–44

Postural Muscle Responses Following Changing Balance Threats in Young, Stable Older, and Unstable Older Adults
Sang-I Lin
Department of Physical Therapy National Cheng Kung University Tainan, Taiwan

Marjorie H. Woollacott
Department of Exercise and Movement Science University of Oregon, Eugene

ABSTRACT. The authors determined thepostural muscle response to support surface perturbations, in relation to aging and level of stability of 16 young adults and 32 older adults who were classified into stable (SOA) and unstable (UOA) groups on the basis of their functional balance abilities. Forward and backward support surface translations of various amplitudes and velocities were used so that postural responses of the standingadults could be elicited. The thigh and leg postural muscle responses were recorded with surface electromyography (EMG). The older groups had significantly longer onset latency in the anterior postural muscles, smaller integrated EMG in the posterior muscles, and greater extent of integrated EMG attenuation over time. The UOA showed longer onset latency in the gastrocnemius following slow backwardperturbation and used a greater percentage of the functional capacity of the gastrocnemius muscle than the SOA did. Those findings indicate that the SOA and UOA had limited ability to adapt to changing balance threats; the UOA were more limited than the SOA. When designing balance training programs, therefore, therapists should consider the adult’s level of functional stability. Key words: aging,electromyography, posture, stability

aintaining upright standing following threats to balance depends upon the ability of the central nervous system (CNS) to generate organized postural muscle responses. In previous research on balance control, investigators have used threats to balance similar to those that are found when an individual is standing on a bus or train that starts or stopssuddenly, resulting in forward or backward sway of the body. In response to forward body sway, the associated postural muscles on the posterior side of the body are activated sequentially in an ascending pattern, from the leg, to the thigh, to the trunk (Nashner, 1977; Nashner, Woollacott, & Tuma, 1979). The muscles on the anterior side of the body are activated in the same sequence in response tobackward sway. In previous research on balance control in healthy older
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adults, specific and identifiable changes in postural muscle response characteristics have been found to occur with aging. In response to external threats to balance, healthy older adults show the following changes: (a) increased onset times of muscle response synergies, (b) occasional disruption in response organizationso that proximal muscles are activated before distal muscles, and (c) a breakdown in the correlation of the amplitude of responses within a synergy (Manchester, Woollacott, Zederbauer-Hylton, & Marin, 1989; Stelmach, Teasdale, DiFabio, & Phillips, 1989; Woollacott, Shumway-Cook, & Nashner, 1986). In situations in which the sizes of the postural threats are changing, individuals need to adapt tothose changes by adjusting the timing, amplitude, or both, of their postural muscle responses in order to meet the specific postural demands of each impending postural threat (Dietz, Horstmann, & Berger, 1989; McIlroy, & Maki, 1993). Older adults, whose response capacity may be limited by slowness, weakness, or both, may need to modify their postural muscle responses differently from young adults.Because in previous studies examining postural muscle responses in older adults, fixed perturbation sizes have been used, we do not yet know the extent to which older adults are limited in their ability to adapt to changing balance threats. What has also been left unanswered is how older adults with balance problems respond to postural threats. It is known that there are different levels of...