BioMechanics
August 2006

Postural stability measurement balances technology and treatment
A dynamic assessment of sway may help practitioners design treatment for a range of patients.

By: Michelle Butera

http://www.biomech.com/showArticle.j...leID=192700650

Measuring and evaluating postural stability is an effective way for practitioners to diagnose balance problems and to decide on treatment for their patients at risk for falling. New tools for measuring available data give practitioners a more complete picture of how their patients move and can help researchers develop new methods for treating stability issues. This in turn can advance evaluation, rehabilitation, and performance for people with postural stability problems.

Continually changing technology is challenging some of the processes used to provide data for assessing movement. New ways to analyze the comprehensive data available from testing devices such as the Biodex Stability System and the NeuroCom International Balance Manager System are being developed.

The NeuroCom EquiTest system comprises a movable visual surround and a force plate that can both move backward and forward and rotate. The system protocol includes the Sensory Organization Test (SOT) and the Equilibrium Score (ES), a composite of the results of the SOT. The SOT looks at how proprioception affects postural sway. The ES averages the weighted scores of a patient's performance under the six sensory conditions of the SOT, which all take place in the sagittal plane. It identifies changes of coordination and interaction of the visual, somatosensory, and vestibular systems that are the focus of the SOT.

But there are some important balance and stability factors the ES doesn't consider. The ES assumes a maximum anteroposterior sway angle of 12.5 degrees before a normal person will fall. But that assumption doesn't take into account individual differences due to age, sex, height, or weight. By averaging the single maximum points of anterior and posterior sway, the ES also can't distinguish between small amounts of sway in each direction and a large sway angle in one direction and none in the other. A subject who, say, can control posterior movement (no sway) but not anterior (large sway angle) is more likely to fall than someone who moves a little in both directions-but they may both have the same ES.

A newer measure proposed for use in conjunction with the SOT is the Postural Stability Index (PSI).1,2 A composite of movements in several directions, the PSI uses data obtained through the SOT to augment what can be learned from the ES. The PSI also calculates an average sway angle. But by incorporating sway across the time of the trial in the final number, the PSI can give a fuller picture of a subject's functional balance.

Outcomes and findings

A study to test the efficacy of the PSI was conducted at the facilities of the VA Medical Center in East Orange, NJ.1 The balance of 20 subjects, 10 with chronic fatigue syndrome (CFS) and 10 armed services veterans with no known illness, plus 10 healthy controls was assessed on the NeuroCom EquiTest system. Prior to testing, none of the subjects had been diagnosed with any balance problems, though the CFS patients and vets were chosen because of links between CFS and balance problems and the fact that the veterans had some of these symptoms. None took any medications that are known to cause problems pertaining to balance. Each subject participated in three identical 20-second trials for each of the six conditions. Researchers calculated ES and PSI outcomes at five-, 10-, and 20-second intervals to assess how time in the trial would affect final scores. For both measures, a score of 100 represents perfect stability: the lower a subject's score (on either measure), the more impaired his or her balance.

ES and PSI both have a place in the measurement of postural stability and AP sway but the PSI was created because of a need for deeper analysis of postural stability. It can offer more practical, reliable, and valid information for a few reasons.

Clinicians generally accept that ankle stiffness is directly related to postural stability. The more rigid a patient's ankles, the less stable the patient. The ES considers the foot and ankle as one unit with the body swaying above the ankle. The PSI recognizes movement of the foot independent of the ankle for a more accurate representation of how ankle stiffness (less independent foot movement) affects balance. The researchers found that PSI decreased as ankle stiffness increased, suggesting a negative correlation between ankle stiffness and postural stability. The ES does not have this capability because it does not take ankle stiffness (defined as "the rate of change of torque at the ankle with respect to the displacement (in radians) of the COM [center of mass]") into consideration.2

The study found that a dependable estimate of stability could be obtained with the PSI even if a subject fell during testing. Because only the extremes of movement are included, the ES for a subject who falls is zero for that trial. This is not true with the PSI because movement through the whole of a trial is considered. In addition, of 2000 data points collected from the SOT, only two values of data for each trial (maximum anterior sway and maximum posterior sway) are used to form the ES. The PSI, on the other hand, uses all the data points plotted by the SOT to measure stability patterns.

The Postural Stability Index is one of many tools that analyze information gathered from force plate technology and can provide unique information to the practitioner.