plateau even though the heart rate increases this is caused by a decrease in stroke volume the heart no longer has time enough to be completely refilled after each heart beat when heart rate increases even further cardiac output will decline rapidly since the blood filling of the heart now becomes more severely impeded line 1b designates cardiac output at a first activity level as the diagram shows the plateau shifts toward a higher cardiac output and faster heart rate line 1c shows how an additional increase in the level of activity shifts the plateau additionally since cardiac output does not increase when rate increases at the plateau the ideal heart rate is the rate at the transition to the plateau at this rate the heart will deliver maximum cardiac output for the activity level involved while the heart rate is the lowest possible thereby reducing the load on the heart transition to the plateau for lines 1a 1b and 1c physiologically corresponds to the situation in which the heart is just able to pump the blood flowing back to the heart through the veins in other words the heart does not impede the return flow by beating too slow nor does it impede the blood filling of the heart by beating too fast when a person at rest becomes active blood flow to the heart increases an increased heart rate is then necessary to accommodate the increased flow if the heart rate then increases.