Originally Posted by RISKDR1

my theory is that your working muscles generate the oxygen demand and your heart responds to that. Some days our muscles may be fatigued and do not generate the the oxygen demand and your heart will not respond by going as high. It probably indicates that you need more recovery in your working muscles. Also, the aerobic system is more than just the heart. It is the lungs, the blood, the arteries, veins, capillaries and the blood itself including your red blood cell count. People who are on beta blockers eventually develop those other elements to compensate for the lower heart rate. You can increase the stroke volume through targeted training. Increase RBC count, capillary beds, etc. If your perceived effort is high but your power output is low our muscles are not fresh.