The accuracy and precision of a swimmer's estimate of heart rate was determined. The energy metabolism needed for any workload, and not the workload itself, is the deciding factor for heart rate adjustment as well as for the threshold for prolonged work. The smaller the muscle mass used to accomplish a given workload, the steeper the rise in heart rate and the sooner the work will be interrupted by fatigue. If the time required for oxygen intake, ventilation, and heart rate to return to resting values is measured after work of a steady-state nature, all variables will reach resting values at the same time. After exhausting work, the reaction is different.
Heart rate remains elevated anywhere from five to ten times longer than the other factors.
This is caused by the need for glycogen reserves to be restored. To accomplish that muscle metabolism must remain elevated because ATP is required to synthesize glycogen from glucose. This activity is aerobic in nature. Elevated heart rates after exercise support the mechanisms to replenish supplies of glycogen. Women must transport the same amount of oxygen to the working muscles with an average of 1.5 gm less hemoglobin per 100 ml of blood.