A young and healthy heart can double its stroke volume in a heartbeat. However, this capability diminishes with an ageing heart, especially one that has been subjected to high blood pressure, heart valve issues or metabolic conditions such as poor glucose control over many years. These factors cause the left ventricle to stiffen in many people as they age, limiting their left ventricle's  ability to change its stroke volume. Consequently, any change in cardiac output in older individuals, or those with a stiff heart, increasingly relies on increasing the heart rate.

If the heart rate fails to increase with physical activity, cardiac output becomes constrained. Somewhat scarily, doctors call this "Chronotropic Incompetence" or "CI" for short. 

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