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
ageing 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 ageing depresses the spontaneous electrical activity of the heart's
natural pacemaker, the sinoatrial node.
Aerobic capacity maintenance
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 utilised 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
cell called sinoatrial myocytes, or 'SAMs' from old mice just couldn't beat as
fast as SAMs from young mice."
The researchers recorded the tiny electrical signals from
the isolated cells and found that SAMs from old mice beat more slowly, even
when they were fully stimulated by the fight-or-flight response which can be
observed in these individual cells. The slower beating rate was due to a
limited set of changes in the action potential waveform, the electrical signal
that is generated by the cells.
The changes were caused by altered behaviour of some ion
channels in the membranes of the older cells. (Ion channels are proteins that
conduct electricity across the cell membrane. Imagine a balloon with little tiny
pinholes that open and close to let the air in and out; ion channels are like
Like most initial discoveries in basic science, this study
opens many more questions and avenues for further research. But the
significance of the study is that it raises the possibility that sinoatrial ion
channels and the signalling molecules that regulate them could be novel targets
for drugs to slow the loss of aerobic capacity with age.
In the meanwhile,
Proenza notes that "although maximum heart rate goes down for everybody
equally, regardless of physical conditioning, people can improve and maintain
their aerobic capacity at all ages by exercising."
This study will be published in the Oct. 14 Proceedings of
the National Academy of Sciences of the United States of America.