This question may be solved by CU researchers
Researchers at the University of Colorado have new insight into the age-old question of why maximum heart rate (maxHR) decreases with age. This decrease in maxHR not only limits the performance of aging athletes but it is also a leading cause for nursing home admittance for otherwise-healthy elderly individuals who no longer have the physical capacity required for independent living. We say we're just getting old and slowing down, but exactly what is it that is slowing down?
Everybody knows that aerobic capacity decreases with age. You know that chart in your gym that shows your target heart rate decreasing as you get older? Well, that's not a senior discount to let the elderly get off easy on their treadmill workouts. It's because older hearts simply can't beat as fast as younger hearts. So the older person who's doing 120 beats per minute is probably working harder - at a higher percentage of maximum heart rate - than the younger person who is at 150 beats per minute.
A new study by a group led by Catherine Proenza, PhD and Roger Bannister, PhD from the University of Colorado School of Medicine reports that one of the reasons for the age-dependent reduction in maximum heart rate is that aging depresses the spontaneous electrical activity of the heart's natural pacemaker, the sinoatrial node.
A dissertation from Eric D. Larson, a graduate from Proenza's lab in the Department of Physiology and Biophysics, is described in the article. Larson said, "I utilized a method to record ECGs from conscious mice and found that maximum heart rate was slower in older mice, just as it is in older people. This result wasn't unexpected. But what was completely new was that the slower maxHR was because the individual pacemaker cells - called sinoatrial myocytes, or 'SAMs' - from old mice just couldn't beat as fast as SAMs from young mice."