Dynamic Comparison of Current Ballistocardiographic Methods: Part III. Derivation of Cardiovascular Force from Body Motions

In part I was discussed the nature and magnitude of force artefacts in the ballistocardiogram (BCG) arising from coupling the body to ground. The effect of this grounding on the reading of cardiovascular motion, momentum and force was explained, using the simple one-mass dynamics. Part II dealt with the effects on the ballistocardiogram of a second mass, a platform supporting the body. The body itself is a cutoff filter (of the resonant type) by its springy supporting tissues which couple the body and platform masses. In the displacement ballistocardiogram this attenuates the upper frequencies recorded from stiffly sprung platforms, or from the direct-body whatever its coupling to earth. In the acceleration record, the body mass and spring cut off the lower ballistocardiographic frequencies, while platform mass and body-spring cut off the higher frequencies. The size of these errors and their bearing on multilateral recording were discussed.)