Posture control depends on balancing act.

Even when standing still, the human body is in constant motion. That subtle swaying causes the center of pressure under an individual's feet to shift randomly within a radius of a centimeter or two.

Researchers have now shown that monitoring the path followed by the center of pressure of a quietly standing person furnishes useful information about the body's response to small impulses, or shoves. Such measurements may eventually help in diagnosing balance disorders in people, says mathematician Carson C. Chow of Boston University's Center for Biodynamics.

Chow, biomedical engineer James J. Collins, and their coworkers at the center describe their findings in the Jan. 12 Physical Review Letters.

Whether in studies of how infants learn to sit up and stand or in efforts to correct balance problems, human posture control has long been a subject of considerable interest. Although researchers sometimes measured gross characteristics of center-of-pressure fluctuations, until now they have paid little attention to the details of the overall pattern of movement.

Several years ago, Collins recognized that the location of the center of pressure, as measured on a sensitive apparatus known as a force platform, follows a specific type of random walk. Each successive step of the walk occurs in a randomly selected direction. The overall path is constrained, however, almost as if the wanderer were connected to the central point by an elastic band.

According to Chow, such fluctuations can be described by simple equations that also apply to a string or polymer strand glued flat to an elastic membrane. As random perturbations displace the strand, the membrane pulls it back to its straight equilibrium position.

In the mathematical model developed by Chow and his colleagues, the motion of the center of pressure corresponds to the movement of a single point of the strand. Experiments in which subjects stood still or were pulled backward by a mechanical device demonstrated that the observed movements fit the model quite well.

The model enables the researchers to link the body's intrinsic, random movements when standing still to its response when disturbed by a small impulse. It suggests that the same neuromuscular mechanisms control posture, whether fluctuations are random and intrinsic or caused by an external, perturbing force.

This finding may greatly simplify the tests that clinicians use to determine a person's susceptibility to falling. "All you nave to do is stand on a force platform and do nothing for 30 seconds," Chow says. "By measuring the fluctuations, we can determine how you respond to perturbations."

He adds that "we want to come up with a software package that a clinician can use with a patient standing on a platform to characterize his or her body's stiffness or reaction time."