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.
Muscle also can atrophy .
If the nerve fibers are injured, the muscle cannot contract. The amount of actin and myosin in the fibers reduce, and the fibers themselves grow smaller. Even if there is no damage to the nerve fibers, the muscle can atrophy from disuse. By and large, the greatest cause of atrophy in muscle is aging. Muscular strength will peak at the age of thirty.