Validity of force platform measures for stance stability under varying sensory conditions.

Maintaining stance stability under varying sensory environment is an essential function in the elderly and among patients. Testing sensory organization ability of standing balance, the Sensory Organization Test (SOT), has become a standard procedure in many clinical and laboratory settings. The stance stability can be quantified by two forceplate measures in the SOT: the equilibrium score (ES) and the sway area (SA). This study compares the validity of the ES and the SA in detecting gender, trial, and sensory effects on stability in twenty (ten male, ten female) healthy young adults. Subjects were tested under six sensory conditions: eyes open (EO), eyes closed (EC), sway-referenced vision (Vs), sway-referenced support surface (Ss), eyes closed sway-referenced support surface (ECSs), and sway-referenced visual surround and support surface (VsSs). A visual surround and/or the support surface were tilted proportionately to the subject's spontaneous sway in the sway-referenced conditions. Three trials, 20-second for each trial, were repeated for each sensory condition. Above results demonstrated that the Pearson correlation coefficients between the ES and the SA were all highly significant (p < .0001) except for the first trial of the EO condition. The consistencies in which the two measures discriminated among sensory conditions were tested by the Kendall's coefficient of concordance. The Kendall's coefficient for the ES (W = .843) and the SA (W = .866) were high and similar. Separate ANOVA procedure for the ES and SA revealed that both measures satisfactorily detected a significant sensory condition and trial effects and insignificant gender effect. We can conclude that ES and SA are valid measures of stance stability during the SOT. Our results confirm that healthy young adults have a poorer postural stability when the visual and somatosensory inputs are simultaneously altered. Moreover, the learning effect is observed during repeated trials within test conditions.