The loss of tension is muscle fatigue . Eventually, even a strong stimulus will not cause a muscle response. The number of fibers in a muscle is fixed at birth. However, they do have larger muscle fibers and muscles with more connective tissue. Exercise stimulates the production of greater amounts of the contractile proteins actin and myosin.
Exercise makes more cross bridges available for more work.
With the increase in proteins, the myofibrils thicken and the fibers expand. Muscle enlargement is called hypertrophy . The heart also can hypertrophy. The cavity of the left ventricle enlarges in endurance athletes so that the heart can supply more blood to power prolonged activity. In athletes trained for spurts of strength, the left ventricle wall thickens enabling the athlete to cope with the sudden rise in blood pressure these sports produce.