Physical basis of ballistocardiography. II. The quantities that can be measured with different types of ballistocardiographs and their mutual relations.

It is investigated which quantity concerning the common center of gravity of a subject is recorded by a low-frequency, a middle-frequency, and a high-frequency ballistocardiograph. It appeared that the displacement of these types of ballistocardiographs represents the displacement, the velocity, and the acceleration of the center of gravity, respectively, if the external circumstances meet the given requirements. Corresponding amplitude and phase characteristics are calculated. A relation between the quantities of displacement, velocity, and acceleration of the center of gravity is deduced, from which follows the method by which these three quantities can be found from each of the three types of ballistocardiographs. In this paper the binding between body and ballistocardiograph is assumed to be infinitely strong.