Physiological adaptations that make athletes more efficient when exercising also make it harder for them to reach their near-maximum heart rate. But, short periods of work at near-maximal intensity -- such as running sprints -- enhances fitness markers, including oxygen-intake capacity, body composition and speed, shows research including that published in a 2011 issue of Medicine and Science in Sports and Exercise. Conditioned athletes must work extra hard to garner these benefits because their bodies are more conditioned to the stress of exercise and become more efficient at working at high intensities.
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Elasticity of the Heart
Regular exercise enlarges and strengthens the heart muscle so it can pump more blood each time it contracts. This means an athlete delivers nutrients and oxygen to working muscles more effectively.
In an untrained exerciser, stroke volume -- or the amount of blood pumped out of the left ventricle of the heart in one contraction -- rises to about 120 milliliters with exercise. Trained athletes can increase this amount to 200 milliliters or more. As a trained athlete increases his workload, his heart beats faster -- and with each beat, he's pumping all that blood through his system. Because he can continue to supply blood and all the nutrients at high intensity levels, his heart rate doesn't need to increase as quickly as someone who can't process as much blood per beat.
An athlete's body is also more efficient at removing waste products, including carbon dioxide and metabolites, that induce the "burn" you feel in your muscles when exercising intensely. An athlete has to work harder and sometimes longer to achieve the burning feeling that accompanies near-maximal heart-rate because his system effectively processes these waste products. At some point, they do affect his ability to keep going -- but it's much later than someone whose body isn't conditioned to process them as quickly.
Taking in breath and processing oxygen so it gets into the bloodstream more quickly also improves the more you exercise. An athlete pulls air into the respiratory system in larger quantities and at a quicker pace. This means he has more access to oxygen.