Reduction of joint mobility in humans is a very common medical condition which results from trauma or joint disease. However, there is currently no satisfactory way of plotting the three-dimensional rotations which completely define this mobility. The most usual method of presenting the results from the many systems for measuring joint mobility is to plot graphs of the three angles of rotation, individually, against time, or to plot two angles of rotation against one another. However, this latter excludes the third angle of rotation which has to be presented separately. This paper suggests a technique known as the hemispherical projection method for the plotting of three-dimensional joint mobility data. The technique is derived from the fields of structural geology and rock mechanics where it is used for presenting and analysing orientation information on discontinuity planes in rock. Data from human hip and spine movements have now been successfully plotted using a microcomputer. This will provide an opportunity for straightforward comparison of movements between individuals and patient groups using the shape of the output graphs as a clinical aid. In addition, it is suggested that 'indices of mobility' may be derived from the plots. These indices could be used to quantify joint disability using the area enclosed by, and the asymmetry of, the plot.
