Exercise makes muscles stronger. There are two ways that muscles become stronger.

• Many repetitions with a small resistance^* make muscle fibers get larger.
• Few repetitions with very large resistance tear muscle fibers. In a couple of days, they repair. After the repair, there are more fibers in the muscle.

A good exercise program includes a mixture of both types of exercise. A well-planned exercise program can:

• strengthen the heart.
• clear the lungs.
• make the body systems work more efficiently^*.
• remove harmful chemicals from the body.
• improve resistance to disease.
• make you feel better.

Exercise is usually classified as either aerobic or anaerobic.

In space, astronauts often use isometric exercise.