an instrument that records the slight recoil of the body, while on a special bed, caused by the contractions of the heart: used to measure cardiac pumping power and the elasticity of the aorta
See ballistocardiograph in American Heritage Dictionary 4
(bə-lĭsˌtō-kärˈdē-ə-grăfˌ)
noun
A device used to determine the volume of blood passing through the heart in a specific period of time and the force of cardiac contraction by measuring the body's recoil as blood is ejected from the ventricles with each heartbeat.