Further Studies on a Theory of the Ballistocardiogram

IN THIS THEORY of the normal human longitudinal ballistocardiogram we shall attempt to relate the record, in quantitative terms, to the events in the circulation that we believe to be its genesis. The variables concerned here fall into three groups: those concerned with the performance of the heart as a pump; those concerned with the position and elastic properties of the vessels that contain the blood and guide its movement; those concerned with the properties of body tissues themselves that influence the transfer of forces arising within the body to its support, the ballistocardiograph. Mainly the first two groups are discussed in this paper. The fourth group, comprising the properties of the instrument, has been discussed extensively. As the simplest approach to the problem we have taken as our starting point the movement of the body in space, when the body is free to move, with the events of the cardiac cycle. It is of interest to note that the advantages of this approach were recognized by Trotter in 1872, when he commented on Gordon's paper, the first on this subject. Such an approach is based on a well-known principle. Figure 1 shows a striking example that is within the experience of everyone. If an object is free to move, when the internal position of its center of gravity is altered by forces arising within it, the object alters its position in space so that the position of its center of mass in space remains the same. This is why the fisherman has so much difficulty recovering his hat. The body, placed in position in which it is