An aerobic activity is defined as one that the heart rate and the body's need for oxygen.
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The term aerobic literally means "with oxygen", and refers to the use of oxygen by muscles for generating energy. An aerobic activity will increase the rate of heat so that it can pump more oxygenated blood because of increase in muscle activity, the need of oxygen of body also increases.  

Hence, an aerobic activity is defined as one that increases the heart rate and increases the body's need for oxygen.

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