Balance, Sense of
BALANCE, SENSE OF
In the 1990s specific factors contributing to falls in older adults were identified, including reduced postural or balance control. Age-related reductions in balance control may be due to impairments in the musculoskeletal system and/or the different nervous subsystems, including the neuromuscular, sensory, and higher-level adaptive systems.
Research has indicated that lower extremity muscle strength is reduced by as much as 40 percent between the ages of thirty and eighty, and is even further reduced in nursing home residents with a history of falls. This is accompanied by an age-related loss of muscle fibers, with type II (fast twitch) fibers being lost at a faster rate than type I (slow twitch). As a result, maximum isometric force development decreases, the muscles fatigue more rapidly, and the rate of tension development in muscles is slower. There is also an age-related decrease in range of motion and spinal flexibility, leading to a characteristic flexed or stooped posture in many older adults.
The first studies examining age-related changes in balance control during quiet stance were performed in the 1960s and showed that sway begins to increase at about fifty years of age. A study in the 1990s found a significant increase in sway in healthy older adults, with the greatest amount of sway found in older adults with a history of falls.
Though clinical examination of quiet stance may tell us something about age-related changes in balance, it is in dynamic conditions, where balance is threatened, that most falls occur. Thus research has addressed this question by using a moving platform to provide an external threat to balance. Measures of balance control have included postural muscle response characteristics (electromyograms or EMG), kinematics (body motion), and kinetics (muscle forces such as center of pressure, or COP, used to recover balance).
Studies examining age-related changes in postural muscle response characteristics elicited when balance was threatened showed that the muscle response organization of older adults and younger adults was similar, with responses being activated first in the stretched ankle muscle and radiating upward to the muscles of the thigh. However, clear differences between the two groups are also seen. These include: (1) slower onsets and smaller amplitudes for ankle muscle responses (resulting in a longer time to stabilize sway after a balance threat); (2) occasional disruption in muscle response organization; and (3) co-activation of the antagonist muscles along with the agonist muscles at a given joint (a strategy that stiffens the joints, possibly to compensate for other limitations in balance control). Figure 1 shows the research paradigm used to study reactive balance control in older adults and the difference in the center of pressure path of a young, healthy, and balance-impaired older adult when recovering balance. Note that the path is much longer for the balance-impaired older adult than the healthy and young adults.
When responding to a balance threat one can use one of three types of response strategies: sway primarily about the ankle joints, hip movement, or a step. Several labs have found that older adults use a strategy involving hip movements or stepping rather than ankle movements significantly more often than young adults. Use of a hip strategy for balance control in older adults may be related to pathological conditions such as ankle muscle weakness or loss of peripheral sensory function. Clinical tests show that scores for muscle strength and proprioceptive sensation (i.e., joint and muscle sensation, which contributes to a sense of where our limbs are located with respect to the rest of the body) are lower for unstable older adults than for young and stable older adults for most muscles tested.
Many of the changes in balance function in older adults may be due to deterioration in the different sensory systems contributing to balance, including the somatosensory (cutaneous and proprioceptive), visual, and vestibular (inner ear) systems. Tactile sensitivity decreases with age. Research examining reactive balance control in patients with peripheral neuropathy has found delays in muscle onset latencies in response to platform perturbations.
Age-related changes in the influence of vision on balance control can be tested by creating the illusion of postural sway through visual flow generated by an experimental moving room. Normally, young adults show small amounts of sway in response to visual flow, since their proprioceptive systems indicate no sway. However, healthy older adults respond to visual flow with increased sway, and balance-impaired older adults show the most visually induced sway of the three groups. This may be due to decreased somatosensory information available for balance in older adults, due to peripheral neuropathy.
Research on age-related reduction in the function of the vestibular system indicates a loss of 40 percent of the vestibular hair and nerve cells by seventy years of age. The vestibular system contributes to the amplitude of postural responses to balance threats, and thus, older adults with vestibular loss would show inappropriately small responses to balance threats. The vestibular system serves as an absolute reference system to which the other systems (visual and somatosensory) may be compared and calibrated, and is thus critical for optimal balance function.
Higher-level adaptive and cognitive systems
Higher-level systems, such as the cerebellum, are responsible for integrating information from the three sensory systems, and then adapting postural responses to meet the demands of changing sensory conditions. In the 1980s and 1990s a number of laboratories examined the ability of healthy and balance-impaired older adults to adapt senses to changing conditions during quiet stance using posturography testing. Results showed that healthy active older adults were not significantly different from young adults in amount of body sway except in conditions where both ankle joint inputs and visual inputs were reduced or absent. In these conditions, half of the older adults lost balance on the first trial for these conditions, requiring the help of an assistant. However, most of the healthy older adults maintained balance on the second trial within these two conditions. This suggests that they are capable of adapting postural responses to meet changing sensory conditions, but only with practice in the condition. Balance-impaired older adults had a larger percentage of falls in any condition with misleading somatosensory cues. Thus, a problem contributing to balance-impairment in many older adults is the inability to adapt responses to changing sensory conditions.
Studies in the 1990s began to determine if attentional requirements of postural control increase in older adults, by using a dual task paradigm in which older adults are asked to balance while performing a second cognitive task (e.g., a math task). Results indicate that balance is more attentionally demanding in older than younger adults and is highest in balance-impaired older adults. Attentional demands increase as the complexity of the balance task increases (e.g., responding to a slip).
To determine if balance function can be improved with training, in the 1990s research labs began to design and test different balance training programs. High-resistance muscle strength training studies have found that age-related declines in muscle strength are partially reversible, especially in frail older adults, such as nursing home residents. Dynamic balance training involving Tai Chi (an ancient Chinese discipline of meditative movements) also has been shown to reduce the risk of falls in healthy older adults. Studies focusing on sensory retraining, in which older adults practiced standing under changing sensory conditions (e.g., standing on foam, eyes open versus closed, head tilted) showed significant reductions in sway over ten days of training. Multidimensional exercise programs, including combinations of lower extremity strength and flexibility exercises, static and dynamic balance exercises, and participation in an aerobic activity (usually walking) have also improved balance and mobility function and reduced the likelihood of falls among older adults with a history of falling.
Marjorie H. Woollacott
See also Balance and Mobility; Vision and Perception.
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Lin, S-I. "Adapting to Dynamically Changing Balance Threats: Differentiating Young, Healthy Older Adults and Unstable Older Adults." Ph.D. diss., University of Oregon, 1997.
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See Pressure ulcers
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