What is the effect of contraction of muscle on size?

Effect of Contraction on Size of Muscle
 Exercise greatly benefits the health of all people. Forceful muscular activity over a prolonged period causes muscles to in­crease in size. This increase, called hypertrophy, occurs only if the muscle contracts to at least 75% of its maxi­mum tension. However, only a few minutes of forceful exercise several times a week are required for hypertro­phy to occur. The muscle fibers show a gain in metabolic potential, as well as an increase in their number of myofibrils. Contrary to former belief, the number of muscle fibers may also increase, resulting in muscles that can work longer before they fatigue. Some athletes take steroids, either testosterone or related chemicals, to pro­mote muscle growth. This practice has many side effects.
Muscles that are not used or that are used for only very weak contractions decrease in size, or atrophy. Atrophy can occur when a limb is placed in a cast or when the nerve serving a muscle is damaged. If nerve stimulation is not restored, muscle fibers are gradually replaced by fat and fibrous tissue. Unfortunately, atrophy causes muscle fibers to shorten progressively, leaving body parts con­tracted in contorted positions.
A surprising finding, however, is that health benefits also accompany less strenuous programs. A study of 12,000 men by Dr. Arthur Leon at the University of Minnesota showed that even moderate exercise lowered the risk of a heart attack by one-third. People with arthritis reported much less pain, swelling, fatigue, and de­pression after only four months of attending a twice-weekly, low-impact aerobics class. Increasing daily activity by walking to the comer store instead of driving and by taking the stai instead of the elevator can improve a person's health.
The benefits of exercise are most apparent with regard to cardiovascular health. Brisk walking for 2.5-4 hours a week can raise the blood level of high-density lipoprotein (HDL), a chemical that promotes healthy blood vessels. Muscles contract isotonically and isometrically. They have tone. Forceful contraction can cause them to increase in size, and lack of contraction vveakens them.