Because the resistance to muscle movement is much lower than resistance exercise, muscle enlargement (hypertrophy) is much less pronounced, if at all. However, muscle will adapt in another amazing way. Here the adaptation allows the trained muscle to have greater endurance by increasing its aerobic ATP generative capacity. In doing so there is an increase in the number of mitochondria in the trained muscle cells. Furthermore, the trained muscle develops more capillaries to deliver blood. The increase in the number of capillaries provides more oxygen and energy nutrients during exercise. The heart grows a little as well to provide a more powerful stroke and greater cardiac output (blood delivery) to working muscles. A greater heart stroke is often reflected by a slower heart rate when not exercising. Some top endurance athletes have resting heart rates as low as 40 to 45 beats per minute whereas inactive people tend to have heart rates between 60 and 75 beats per minute.
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