Cardiac Changes with Improved Fitness
So when we have improved cardiorespiratory fitness, what happens to your heart? What are the changes in your heart that really drive that cardiorespiratory fitness improvement? The first is increased contractile force, so you're able to contract your heart with greater force, and again, that's not a voluntary contraction, but your heart is stronger, it's able to pump blood out with more force. Your heart is also better able to expand to let more blood in, which means—because more blood can flow in and you're able to contract with more force—you have an increased stroke volume. Additionally, a very fit person will have a lower heart rate than an unfit person when doing the same amount of work, and you could really think about this as your body becoming more efficient, your heart becoming more efficient in particular; more blood can be pumped out with fewer beats.