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Cardiorespiratory endurance is the ability to take in, deliver, and extract oxygen for physical work. It is the ability to persevere at a physical task at a given intensity level. Cardiorespiratory endurance improves with regular participation in aerobic activities, such as speed walking, jogging, cycling, swimming, and cross country skiing. The term aerobic literally means "with oxygen," but when applied to exercise, it refers to activities in which oxygen demand can be supplied continuously during performance. Aerobic performance depends on a continuous and sufficient supply of oxygen to burn the carbohydrates and fats needed to fuel such activities. In other words, the intensity or the energy requirement is within the capacity of the performer to sustain for longer than a couple of minutes. Cardiorespiratory endurance is also referred to as aerobic capacity, or maximum oxygen consumption. Cardiorespiratory endurance is the most important component of physical fitnessand is the foundation of total fitness. The physiological changes that result from cardiorespiratory training are referred to as the long term, or chronic, effects of exercise. The effects of training are measurable and predictable. Heart Rate A few months of aerobic training lowers the resting heart rate by 10 to 25 beats per minute (bpm). It also lowers the heart rate for a given workload. For example, a slow jog may produce a heart rate of 165 beats per minute before training and 140 beats per minute after a few months of training. The trained heart is a stronger, more efficient pump that is capable of delivering the required oxygen with fewer beats. This is accomplished because the trained heart is capable of ejecting more blood per beat. Stroke Volume Stroke volume is the amount of blood that the heart can eject in one beat. Aerobic training increases the stroke volume by Increasing the size of the cavity of the ventricles, which results in greater filling of the heart with blood, Increasing the contractile strength of the ventricular wall, so contraction is more forceful and a greater amount of blood is ejected from the ventricles. The increase in stroke volume, both at rest and during exercise, is one of the primary effects of endurance training and one of the major mechanisms responsible for improvement in aerobic fitness. Respiratory Responses The chest muscles that support breathing improve in both strength and endurance. Vital capacity, which is the amount of air that can be expired maximally following a maximal inspiration, increases slightly. A corresponding decrease occurs in "dead space" air or residual volume, which is the amount of air remaining in the lungs after a maximal expiration. Training substantially increases maximal pulmonary ventilation (the amount of air moved in and out of the lungs). Before training, the lungs can ventilate approximately 120 liters of air per minute. Pulmonary ventilation increases to about 150 liters of air following a few months of training. Highly trained athletes commonly ventilate 180 to 200 liters of air per minute. Blood flow to the lungs, particularly to the upper lobes, appears to increase after strength training. This results in a larger and more efficient surface for the exchange of oxygen and carbon dioxide.
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