Aerobic exercise is performed at an intensity that increases the heart rate but still relies on aerobic metabolism. An activity is generally considered aerobic if a person can talk, but not sing, while exercising.
Aerobic training causes physiological changes in the cardiovascular system and the muscles. These changes increase the aerobic capacity (VO2max), which is the amount of oxygen that can be delivered to and used by the muscle cells. This increases overall endurance.
Regular aerobic exercise strengthens the heart muscle and increases stroke volume. When stroke volume is increased, the heart can deliver more blood with each beat.
This means that the heart does not need to beat as many times to deliver enough blood to body cells at rest, and during exercise it increases the amount of oxygen that can be delivered to the muscle.
Therefore, more fit people have lower resting heart rates. They can perform higherintensity activity before reaching their maximum heart rate.
Aerobic training also causes other changes that increase aerobic capacity. It increases the number of capillaries in the muscles, so that blood is delivered to muscles more efficiently.
It increases the total volume of blood and the number of red blood cells. This increases the amount of hemoglobin in the blood, which allows more oxygen to be transported at any given time.
Aerobic training also causes changes at the cellular level that make it easier for muscle cells to use oxygen for energy. The number and size of muscle-cell mitochondria increase. There also is a boost in the activity of mitochondrial enzymes that are needed for aerobic metabolism and fatty acid breakdown.
More fatty acids are brought to the mitochondria, and more triglycerides are stored and broken down in the muscle. These changes boost the cell's capacity to burn fatty acids to produce ATP. The use of fatty acids preserves glucose stores (glycogen), which delays the onset of fatigue.
Training also makes it easier to store glucose in the muscle as glycogen. Because trained athletes store more glycogen and use it more slowly, they can sustain aerobic exercise for longer periods at higher intensities than untrained people can.
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