Sense of Balance

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.