What is responsible for bradycardia affected in athletes?

Cardiac output = stroke volume x heart rate. Athletes hearts are stronger and able to push out more blood per beat (higher stroke volume) than non-athletes. This means that they can have a slower heart rate but same overall cardiac output as a non-athlete. When an athlete begins to exercise and the heart rate increases, they are able to deliver more oxygen more efficiently to peripheral tissues due to the increased stroke volume of their heart.