Head Movement Analysis Based on Areas of Convex Hull and Confidence Ellipse Obtained Using Two Motion Capture Systems
Abstract
This paper proposes two methods that are potentially suitable for the analysis of movements and quantitative assessment of the postural stability of the head. A camera system and a gyroscope system are used to measure the roll and pitch of the subject's head.
The pitch versus roll plots of the head are created, and the areas of the shape of a convex hull and those of a confidence ellipse are calculated. The pitch is the rotational motion of the head around the medial-lateral axis, and the roll is the rotational motion around the anterior-posterior axis.
In the case of the convex hull, its shape is defined by the pitch and roll excursions when these variables are plotted on an x-y plot. The confidence ellipse is a two-dimensional interval in which lie the points in the x-y plot.
In the case of the 95 % confidence ellipse, its shape bounds 95 % of the plotted points of the pitch and roll excursions. The expected values of these excursions are determined via a statistical analysis of data obtained from ten healthy subjects measured with eyes open and closed.
The results show a strong correlation between the areas of convex hulls and areas of confidence ellipses. Also, the results show a strong correlation between the areas measured by each system separately.
Statistical analysis of the participants with eyes open and closed does not show a significant difference in the areas. The proposed quantitative approaches, based on the data measured by the camera system or the gyroscope system, are suitable for the study of the postural stability.
Although the results differ when using different motion capture systems to measure the angular movements of the head, the motion capture systems identified the same subject behaviors.