When an |exercise| lasts longer than a minute or two, the muscles get most of their energy from processes that require an increased supply of oxygen delivered to the muscles and tissues. These activities are called aerobic, meaning "with ~air~". Aerobic activities include running, brisk walking, |swimming|, |cycling|, rowing, cross-country skiing, rope skipping, and aerobic dance.
The most important element of physical fitness is cardiovascular endurance, which is the sustained ability of the |heart|, ~blood~ vessels, and ~blood~ to carry oxygen to the cells, the ability of the cells to process oxygen, and the ability of the ~blood~ to carry away waste products. Cardiovascular endurance is built up through |exercises| that enhance the body's ability to deliver ever larger amounts of oxygen to working muscles. The |exercise| must utilize the large muscle groups, such as those in the legs, and, most importantly, it must be sustained in order to achieve cardiovascular endurance.
When regularly performed, aerobic |exercise| helps keep elevated ~blood~ pressure at normal levels, reduces the risk of |heart| disease, and can help control weight gain. There is also indications that it can raise the level of HDL ("good") ~cholesterol~ in the ~blood~. An aerobic |exercise| regimen can also have positive psychological benefits, such as increased self-esteem, lessened anxiety, and even some relief of depression.
