Body size, age, and fitness level are factors in heart rate. why is this?

Well heart rate is like a variable, if someone is bigger blood has to flow farther, and takes more pressure to deliver the blood. As you age, cholesterol accumulates in arteries and makes each pump of the heart less efficient, and someone with good fitness needs less beats per minute to deliver all the blood/oxygen they need. Michael Phelps, for example, probably has a heart rate of less than 40 BPM; Whereas as person of the same age that is less involved in an exercise regiment may have a heart rate around 80 BPM.

By placing 2 fingers at the sternum (the spot where the lower ribs meet) the put the heel of your other hand next to your fingers. hope this