Chronic heart failure and sensor built force-frequency relation during exercise. Blue curve: exercise. Red curve: recovery. All patients had severely reduced LVEF% and dilated left ventricle. Left and middle panel: the force-frequency relation is biphasic, with an initial up sloping trend followed by a down sloping at critical heart rate (left panel, critical heart rate = 90 bpm; middle panel, critical heart rate = 130 bpm). Right panel: the upper rate of the paced heart rate is 110 bpm, and the negative down sloping of the biphasic FFR is avoided.