Physical aspects of the "direct" recording of body displacement, velocity, and acceleration by shinbar ballistocardiographs.

THE APPLICATION of physical principles to problems inherent in taking and recording ballistocardiograms has resulted, not only in greater understanding of the genesis of the record, but also in several important advances in other directions. Thus, under the impact of this analysis a new type of instrument, the ultra-low frequency ballistocardiograph, has been reintroduced and the older high-frequency instrument has been much improved. A similar criticism, based on physical principles, has not as yet been applied extensively to some of the simpler and more popular methods of ballistocardiography; that is the aim of this paper. It can always be objected that the physical formulae used in such studies were derived either from theory, or from experiments with objects such as steel springs and oil damping, which bear little obvious resemblance to the complicated physical conditions that exist in the body where a large number of masses, some small, some large in size, are held together by bindings whose properties vary. The answer to this objection is two-fold. First the larger movements of the body studied, such as its movement as a whole on its supports, do conform closely to expectations from physical theory; and, second, the knowledge gained from calculations based on physical theory has resulted in improved methods and technics. So we