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

Hejda J., Cakrt O., Socha V., Schlenker J., Kutilek P.

Biocybernetics and Biomedical Engineering

2015

Vol. 35, no. 3

185--191

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.

Differences in evaluation methods of trunk sway using different MoCap systems

Kutilek P., Socha V., Cakrt O., Svoboda Z.

Acta of Bioengineering and Biomechanics

2014

Vol. 16, no. 2

85--94

The position of the trunk can be negatively influenced by many diseases. Several methods can be used for identifying defects in balance and coordination as a result of pathology of the musculoskeletal or nervous system. The aim of this article is to examine the relationship between the three methods used for analysis of trunk sway and compare two fundamentally different MoCap systems. We used a camera system and a 3DOF orientation tracker placed on subject’s trunk, and measured inclination (roll) and flexion (pitch) during quiet stance. Ten healthy participants in the study were measured with eyes open and closed. The pitch versus roll plots of trunk were formed, and the area of the convex hull, area of confidence ellipse and total length of the trajectory of the pitch versus roll plot were calculated. The statistical analysis was performed and strong correlation between the area of the convex hull and area of the confidence ellipse was found. Also, the results show moderate correlation between the area of the confidence ellipse and total length of the trace, and moderate correlation between the area of the convex hull and total length of the trace. In general, the different MoCap systems show different areas and lengths but lead to the same conclusions. Statistical analysis of the participants with eyes open and eye closed did not show significant difference in the areas and total lengths of the pitch versus roll plots.