The ballistocardiograph worked by detecting the body's recoil when the heart:

1. The device was a diagnostic tool used in the second half of the twentieth century to study the strength of the heart. 2. About once a second, your heart beats, forcing blood into the aorta. 3. A force in the opposite direction is exerted on the rest of your body, by Newton's third law. 4. That tiny whole-body recoil is what the instrument measured. 5. The electrocardiogram and the echocardiogram are more widely used in cardiology today. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 8 'Linear Momentum and Collisions', sections 8.1-8.7_