The human body has three types of muscle tissue: smooth, skeletal and cardiac. Cardiac muscle, as the name suggests, is found in the heart. Smooth muscles surround the body's other organs, and are involuntary—meaning they do what they do without any conscious decision on your part, such as the diaphragm expanding with breath.
Skeletal muscles, however, are voluntary. Classed as either slow-twitch or fast-twitch, they're the muscles you use to stand, sit, tango, climb trees and ice skate. When muscles are challenged on a regular basis, chemical changes occur on the cellular level. The mitochondria, or energy-producing units within the cell, increase. Blood capillaries around the slow-twitch organ muscles increase in number as well, enhancing blood flow—and therefore improving oxygen capacity.
"The heart is a muscle. As it contracts and relaxes, it becomes stronger," explains University of San Francisco exercise physiologist Christin Anderson. "Regular exercise strengthens the cardiovascular and nervous systems, lowers diastolic blood pressure and decreases the risk of disease, including breast cancer, diabetes and heart disease."
Bones benefit as well. David Lemberg, DC, a New York City chiropractor and author of Commitment to Fitness: Real Fitness for Real People (Universe, 2000), says that during exercise, the increase in stress to the long bones that bear the body's mechanical load makes them denser and stronger. "Skeletal maturity is reached in men around the age of 20, and in women around the age of 21," says Lemberg. "After this time, changes have to do with the quantity of bone mass."
While nutrition definitely plays a role in bone health, for women especially estrogen levels are also a determining factor. The lowered estrogen levels in postmenopausal women cue their bodies to absorb less calcium from the blood. Without exercise, Lemberg explains, bones have no reason to produce new bone, and the result can be the brittle bone condition known as osteoporosis.