The human body has three types of muscles. The type used to walk up a fl ight of stairs, run around a track, or swim laps in a pool is called skeletal muscle.
Skeletal muscles are attached to the bones of the skeleton and are under voluntary control: They move when the person wants them to move. These muscles are responsible for all voluntary movement.
There are more than 600 skeletal muscles in the body. During exercise, the activity of the skeletal muscles increases.
The heart is made of cardiac muscle. Cardiac muscle is not under voluntary control. The heart beats, whether people want it to or not. Internal signals adjust the heart rate to meet the oxygen demands of the body tissues and to maintain homeostasis.
During exercise, the cardiac muscles work harder to deliver more oxygen-rich blood to the skeletal muscles. The third type of muscle is called smooth muscle. It lines the vessels that deliver blood to body cells, the air passageways in and out of the lungs, and the walls of digestive tract, glands, and other organs.
This type of muscle also is involuntary muscle; its contractions and relaxations are not under conscious control. Smooth muscle plays an important role during exercise. For example, it is the smooth muscles in the blood vessels that contract or relax to distribute blood to the tissues in need.
The mechanism that causes contraction is the same in skeletal, cardiac, and smooth muscle. Muscles are made up of bundles of muscle cells, which are called muscle fibers. Inside each muscle fiber are hundreds to thousands of rod-like structures called myofibrils, which are responsible for contraction. Each myofibril is made up of even smaller structures called filaments.
There are two types of filaments - thick and thin. Thick filaments are made of the protein myosin and thin filaments are made of long chains of the protein actin. In order for a muscle to contract, myosin must attach to actin and rotate, causing the thin and thick filaments to slide past each other.
This sliding increases the amount of overlap of the muscle filaments and shortens the muscle. This process requires energy from ATP.
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