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It is widely assumed that healthy aging includes a decline in the stability of standing body sway. Certainly, the spatial magnitude of postural sway increases with age. However, the interpretation of this effect as a decline in the ability to stabilize posture rests, in part, on assumptions about the nature and definition of stability in stance. In this article, I review data on the control of standing posture in healthy older adults. I focus on a growing list of studies that demonstrate the retention, among healthy older adults, of the ability functionally to modulate postural sway in support of “suprapostural” activities. I address laboratory research, but also field studies carried out in a setting that dramatically challenges the control of stance: life on ships at sea. I argue that it may be possible, and certainly will be useful, to address directly the functional control of stance in older adults.

Ordinary behavior, such as walking, reading, and throwing, depends on real-time perceptual guidance. In this article, I discuss the nature of perceptual information that, in principle, might be sufficient for the guidance of movement to achieve behavioral goals. I argue that we achieve behavioral goals by controlling movements relative to multiple physical referents. Movement relative to different physical referents causes changes in the structure of different forms of ambient energy (e.g., light, sound) and, therefore, to changes in sensory stimulation. I claim that movement always is controlled simultaneously relative to multiple referents, such that no single form of ambient energy can, in principle, contain information that is sufficient for successful control. The needed perceptual information exists, I claim, solely in the global array, that is, in emergent, higher-order patterns that extend across different forms of ambient energy. I review formal and empirical examples, and discuss implications for kinesiology.

We demonstrated that postural responses to imposed optic flow are to some extent voluntary. In a moving room, participants either stood normally or were instructed to resist any influence of visible motion on their stance. When participants attempted to resist, coupling of body sway with motion of the room was significantly greater than when the eyes were closed, but was significantly reduced relative to coupling in the normal stance condition. The results indicate that the use of imposed optic flow for postural control is not entirely automatic or involuntary. This conclusion motivates a search for non-perceptual factors that may influence the degree to which body sway is coupled to imposed optic flow.

We examined the potentially exploratory and performatory nature of postural sway. Subjects stood upright or leaned forward, with eyes open or closed. Postural data were analyzed using a statistical mechanics analysis of center of pressure (COP) trajectories, which examines the fractional Brownian nature of postural sway. Positive correlations (persistence) over short time scales are hypothesized to reflect exploratory behavior, and negative correlations (antipersistence) over long time scales are hypothesized to reflect performatory behavior. When leaning, subjects exhibited decreased levels of persistence (decreased correlation) and increased levels of antipersistence (increased correlation) than when upright. With eyes open, subjects showed decreased levels of persistence and decreased levels of antipersistence than with eyes closed. Effects of vision were more pronounced when leaning. Evidence for direction-specific exploration (based upon root mean square variability analysis) was considered. Task-specificity and trade-offs between biomechanical and task constraints in models of postural control were discussed.

This study was undertaken to determine whether young children, after only a few weeks standing experience, could respond adaptively to the dynamical constraints imposed by different support surfaces. The spontaneous postural motions of young children (13-14 months old) were observed as they stood on surfaces that differed in length, friction, and rigidity. There were no externally imposed perturbations to stance. Children's postural control was remarkably adaptive: There were few falls on any of the surfaces. Moreover, the children showed surface-specific utilization of manual postural control (holding onto wooden poles), suggesting that manual control is an adaptive strategy for postural control. Finally, kinematic analysis suggested that, in some instances, children were able to employ independent control of the hips, contrary to previous models which had suggested that hip motions could not be controlled before the age of 3 years. Small, slow hip movements useful in controlling spontaneous sway (unperturbed stance) may serve as a basis for the development of larger, faster hip movements that are associated with imposed perturbations.

Surfaces shorter in extent than the feet elicit multi-joint coordination that differs from what is elicited by stance on extensive surfaces. This well-known effect arises from the mechanics of the actor-environment interaction. Multi-joint control of stance is also known to be influenced by non-mechanical aspects of a situation, including participants' task or intention. Intentional constraints do not originate in mechanics, and for this reason one might suppose that constraints imposed by mechanics would dominate constraints imposed by intentions, when the two were in conflict. We evaluated this hypothesis by varying participants' supra-postural task during stance on a short surface. While standing on a 10-cm wide beam, participants were exposed to optic flow generated by fore-aft oscillations of a moving room. Participants faced a target attached to the front wall of the moving room and were asked either to look at the target (with no instruction to move) or intentionally to track it with their head (i.e., to keep the target-head distance constant). Within trials, we varied the frequency of room (and target) motion, from 0.15 to 0.75 Hz, in steps of 0.05 Hz. In both conditions, ankle and hip rotations exhibited antiphase coordination, but behavior was not identical across conditions. Coupling between motion of the room and the head was stronger for the tracking task than for the looking task, and the stability of ankle-hip coordination was greater during tracking than during looking. These results indicate that the influence of support surface mechanics did not eliminate the influence of the supra-postural task. Environment-based and task-based constraints interacted in determining the coordination of hips and ankles during stance.

When two standing people converse with each other there is an increase in their shared postural activity, relative to conversation with different partners. We asked pairs of participants to converse with each other or with experimental confederates while standing on rigid and nonrigid surfaces. On the rigid surface, shared postural activity was greater when participants conversed with each other than when they conversed with confederates. In addition, the strength of interpersonal coupling increased across trials, but only when members of a dyad conversed with each other. On the nonrigid surface, postural sway variability increased, but we found no evidence that shared postural activity was different when participants conversed with each other, as opposed to conversing with confederates. We consider several possible interpretations of these results.