Cardiac output, the volume of blood the heart pumps per minute, is the product of heart rate and stroke volume, the amount of blood ejected with each beat. At maximal exercise, cardiac output determines aerobic capacity and VO2 max. The decline in maximal cardiac output with age is one of the primary physiological mechanisms behind the age-related decline in physical capacity, and understanding its drivers points directly to the interventions most likely to slow it.
The Maximum Heart Rate Decline
Maximum heart rate declines at approximately 1 beat per minute per year of age. A 40-year-old with a maximum heart rate of 180 bpm will typically have a maximum heart rate of approximately 170 bpm at 50. This is primarily caused by reduced responsiveness to catecholamines at the sinoatrial node and changes in cardiac autonomic control. Aerobic training does not prevent this decline in maximum heart rate, but it does improve stroke volume, partially offsetting the impact on maximum cardiac output.
The Stroke Volume Adaptation to Training
Unlike maximum heart rate, stroke volume responds substantially to aerobic training. Consistent endurance training increases left ventricular end-diastolic volume, the amount of blood the ventricle fills to before each contraction, and improves contractility. Masters athletes in their sixties who have trained consistently for decades have stroke volumes 40 to 50 percent higher than sedentary age-matched adults. This largely accounts for why their VO2 max and functional exercise capacity remain comparable to people decades younger.
Sources
- Ogawa, T., et al. (1992). Effects of aging, sex, and physical training on cardiovascular responses to exercise. Circulation, 86(2), 494-503.
- Fleg, J.L., et al. (1995). Impact of age on the cardiovascular response to dynamic upright exercise in healthy men and women. Journal of Applied Physiology, 78(3), 890-900.
- Bhella, P.S., et al. (2014). Abnormal haemodynamic response to exercise in heart failure with preserved ejection fraction. European Journal of Heart Failure, 13(12), 1296-1304.