Why must an athlete exercise harder and longer to achieve a max HR than a person who is not as physically fit?
because they are fit, their body is used to constantly being pushed. Their heart is used to pumping blood at higher rates, and they are used to the increase in blood level. Eventually, it isn't using force to supply the demand for oxygen and blood. It is doing it because it is used to it. Also, they have a higher stroke volume, which'll enable them to be able to work out longer without feeling tired or worn out.