Increased cardiovascular fitness causes real physical changes in the structure of the heart. The muscles in the heart wall thicken, and the heart pumps more blood with each beat. That increased efficiency means an athlete’s resting heart rate falls to a level that could indicate trouble in a nonathlete. Many athletes don’t realize that high levels of conditioning also cause heart symptoms that doctors might mistake for serious problems. During exercise, the untrained heart beats faster, but not as efficiently as in trained athletes. Output during maximum exertion increases to 14 to 20 liters per minute, far below the 40 liters per minute of an elite athlete. The resting heartbeat of an athlete can fall below 40 beats per minute. When you exercise, your heart works harder when other muscles in the body pump more blood back to the heart. The major muscles of the legs have large veins that fill during the relaxation stage of the movement, and the muscle contraction in the active movement pushes the blood back into the circulatory system. Increasing blood flow triggers a faster heartbeat. Because the heart muscle of conditioned athletes enlarges and strengthens, the hearts of athletes pump more blood per beat. An athlete’s maximum heart rate, as well as resting heart rate, falls below the pace of the average heartbeat. Conditioning causes physical and electrical changes in the heart, creating a beneficial condition called “athlete’s heart.” Aerobic training, such as running or swimming, causes your heart to develop larger heart chambers. Participants in anaerobic sports, such as weightlifting, increase the thickness and strength of their heart muscles. Mixing the two types of training creates both types of heart improvement. During intense exercise, the hearts of highly trained athletes pump as much as twice the volume of blood as the hearts of untrained people.