3-D trajectory of body sway angles: A technique for quantifying postural stability

The article focuses on a non-invasive method of quantifying human postural stability. Recent alternatives to quantify human postural stability have several limitations – the major one being the evaluation of only two physical quantities of body movement in 3D space – however, a complex movement pattern can be described better using three physical quantities. A cheap 3DOF orientation tracker (Xsens MTx unit) placed on patient's trunk was used to measure roll, pitch and yaw. Using a novel method based on the total length of the 3-D trajectory of body sway angles, we are able to evaluate 3-D movements of the trunk. The trajectory length obtained by plotting roll, yaw and pitch vs. each other (i.e. curve in the 3-D plot) was used to identify a pathological balance control. In this study, ten patients with progressive cerebellar ataxia and eleven healthy subjects were measured and a statistical analysis was performed. The results yielded by new method show that the total trajectory lengths of patients with cerebellar disease are significantly larger than the total trajectory lengths of healthy subjects. It is evident from the median of the total trajectory lengths that the method based on the data obtained by an inexpensive orientation tracker may be used to quantify human postural stability and enables for studying body sway in 3-D space. For example, the 3-D deviations of the trunk angles in a time period that are caused by a tremor in 3-dimensional space can be studied accordingly by the method.