If you were to engage in only one type of training, then exercise that develops cardiovascular or circulorespiratory endurance would be the best choice. Activities such as walking, swimming or distance running, that involve the large muscle groups of the body in continuous, submaximal contraction, are the most commonly chosen to develop aerobic fitness. Exercise HR is the most practical means of assessing and understanding the intensity of training.
HR generally represents college age individuals' threshold intensity for training improvement.
According to The American College of Sports Medicine, cardiovascular endurance may improve by the use of an exercise program that includes at least three 20 to 30 min. This may be achieved, for example, by an individual who weighs 170 lbs and swims fast crawl for 30 min. or by someone who weighs 130 lbs and swims breast stroke for 37.5 min. As the level of aerobic capacity increases so does the level of the threshold intensity for training improvement.