How Exercise Works

Cross Training

To become a world-class athlete or to get the most out of your exercise, you want your muscles to get the oxygen they need most efficiently. To do that you need to increase:

  • cardiac output
  • respiration
  • the amount of oxygen carried by the blood

You can do this by resistance training, possibly in combination with cross-training, training for more than one sport at a time or for multiple fitness components (strength, endurance and flexibility) at the same time.


The main effects of training on the cardiac output appear to be an increase in stroke volume (that is, a larger heart) and a decrease in the resting heart rate. The increased stroke volume allows the heart to pump more blood with each beat. Because there is a limit to the maximum heart rate (180-190 beats/min), then a slower resting heart rate (50-60 beats/min in the trained athlete vs. the normal 70-80 beats/min) allows the heart to have a greater increase in heart rate during exercise. The greater increase in heart rate during exercise along with the larger volume increases cardiac output and blood flow to working muscle.

Training can help the respiratory system by decreasing the resting rate of breathing, increasing the respiration rate during exercise and increasing the volume of air exchanged with each breath (tidal volume). These changes allow the lungs to take in more air during exercise. Training can also boost the amount of oxygen that the working muscles take from the blood, which probably reflects the increases in metabolic enzymes.

You have probably heard about runners or cyclists who train in the mountains. This kind of training can actually increase the amount of oxygen carried by the blood forcing the body to develop more hemoglobin in the blood. Because there is less oxygen in high altitudes, the body responds by producing a hormone called erythropoietin (EPO), which causes the bone marrow to produce more red blood cells and more hemoglobin. Some athletes try to take a shortcut by injecting EPO directly into the bloodstream, but this is a dangerous practice. The International Olympic Committee has banned the use of EPO because it increases the thickness of the blood, which can lead to circulatory problems such as a heart attack or stroke.

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