Force-Frequency relation during exercise in a normal subject wherein the memorized parameters are the heart rate, the ventricular contractile force, and the curve of force variation as a function of heart rate. Left panels: the force data is derived from the precordial cardiac sensor (maximum vibrations amplitude in the first 150 ms following the R wave). Right panels: the force data is derived from the ventricular force computed as SP/ESVi, which indicates the left ventricular end-systolic pressure volume ratio (end-systolic ventricular pressure divided by the end-systolic volume). Simultaneous recording is feasible, totally non-invasive. Echocardiography uses artificially generated cardiac reflections; the isovolumic systolic operator-independent force sensor simply records naturally generated heart vibrations.