Aging-induced shifts from a reliance on sensory input to muscle cocontraction during balanced standing.

BACKGROUND: Peripheral sensation is the most important sensory system in the maintenance of upright posture in all age groups. With aging, visual and somatosensory processing change their prospective contribution to the maintenance of quiet standing, at debated percentages. Aging is associated with a decrease in balance abilities that, in turn, increases the risk of falling. We used force plate data to show that, with aging, while vision plays a significant role in regulating postural stability (PS), the individual's perception of his/her stability becomes more significant than vision. Moreover, under experimental conditions, electromyography (EMG) of the ankle musculature of elderly people reveals the adoption of a different strategy, a cocontraction strategy, with or without visual input. The aim of this study was to look at two distinct age groups to determine whether or not a shift takes place in the sensory modality typically relied on while maintaining PS during a static, postural-related task.

METHOD: The participants comprised two groups: a "young" (Y) group of 20 people aged 20-35, and an "old" (O) group of 32 people aged 65-84. The role of vision was tested with regard to two differently sized bases of support. They were tested during quiet upright standing on a single force plate in wide base and then in narrow base conditions. Surface EMG was recorded from the tibialis anterior, soleus, rectus femoris, and semitendinous muscles.

RESULTS: The older group differed from the younger group when performing the task under the narrow base condition. When participants stood naturally, our EMG data indicated that, unlike the Y group, the O group used cocontraction around the ankle in order to deal with changing conditions and sensory inputs. Significant increases were found in the area, length, and mean velocity of body sway in the older group as compared with the younger group.

DISCUSSION: The visual contribution to postural stabilization is significantly greater in the younger population than in the elderly population. Across the older group, lack of vision seemed to interfere less with PS; however, the EMG data indicated that, unlike the Y group, the O participants used cocontraction around the ankle in order to deal with changing conditions and sensory inputs.

CONCLUSION: To cope with the deterioration in their sensory input and processing ability, elderly individuals seemed to have developed a strategy of stiffening and freezing their lower legs during upright standing.