Athletes must work harder than non-athletes to achieve maximum heart rates because athletes have an increased tolerance for intense exercise due to high levels of physical conditioning, according to AZ Central. The increase in exercise tolerance for athletes is due to physiological adaptations that result from years of training.
The physiological adaptations that make it more challenging for athletes to reach their maximum heart rates include increases in stroke volume, arteriovenous oxygen difference, lactate threshold and maximal oxygen uptake, according to AZ Central. Stroke volume is the amount of blood pumped per heart beat. Athletic training increases the stroke volume; this results in the heart not needing to beat as much to pump the same amount of blood. Because of this, the athlete requires a longer period of exertion to reach his maximum heart rate.
The arteriovenous oxygen difference, or AVO2 difference, is the change in the amount of oxygen in the blood before and after the blood enters a muscle. This difference is higher in well-trained athletes. Because the muscles takes more oxygen out of the blood, the heart needs to pump less to meet oxygen demand, which makes it harder for the athlete to reach his maximum heart rate.
The athlete also has an increased lactate threshold and maximal oxygen uptake. The lactate threshold is the point where the body can no longer metabolize lactate acid. At this point, breathing becomes labored and the heart starts to work harder. Non-athletes reach this point well before highly-conditioned athletes do. The maximal oxygen uptake refers to oxygen capacity. Higher lactate threshold and oxygen capacity make it more difficult for an athlete to achieve a maximum heart rate.