If you have a friend who runs marathons, you know that he/she has a "low resting heart rate". What this means is that, when your friend is sitting on the couch watching TV, his/her heart beats much more slowly than the heart of a "normal person". A normal resting pulse is 70 to 75 beats per minutes. An athlete in great shape can cut that number in half.
Why do athletes have low resting heart rates? This article has the answer:
Heart rate and cardiovascular efficiency
Cardiovascular efficiency depends on a number of factors. One measure is called stroke volume, which is the volume of blood pumped per heartbeat. A fit individual has a larger stroke volume, which means a greater volume of oxygen is delivered to the body per heartbeat. This is also the reason fit people have a lower resting heart rate. As mentioned, muscles reqire a given amount of fuel, even at rest, which doesn't change with exercise training. Because exercise training has increased the volume of oxygen that can be delivered to muscles per heartbeat, the heart needs to beat less to do the same job. Therefore, the heart rate is lowered.
For more info see How your heart works
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