Types of Fibers

Types of Fibers

Most skeletal muscle fibers are called fast fibers because they can reach peak twitch tension in 0.01 seconds or less after stimulation. They are large in diameter, containing large reserves of glycogen, relatively few mitochondria, and densely packed myofibrils. Muscles with fast fibers produce powerful contractions because the produced tension is proportional to the number of myofibrils, yet they fatigue quickly because adenosine triphosphate is used in large amounts. Slow fibers are only about half the diameter of fast fibers, taking three times as long to reach peak tension. They can contract longer than fast fibers and are surrounded by a larger network of capillaries. They have a much higher oxygen supply to support their mitochondria. They also contain the red pigment myoglobin, which is similar to hemo-globin. Both pigments reversibly bind oxygen molecules. Myoglobin allows resting slow fibers to hold large oxygen reserves to be used during contractions. Slow fibers give skeletal muscles a dark red appearance because of the extensive capillaries and the large amounts of myoglobin.

Intermediate fibers look more like fast fibers, but their properties are mostly “in between” those of fast and slow fibers. They are paler because they contain low amounts of myoglobin. However, they are more resistant to fatigue than fast fibers. Muscles that have mostly fast fibers are often referred to as white muscles, whereas muscles that have mostly slow fibers are often referred to as red muscles.

The diaphragm is a dome-shaped muscle located below the lungs. (The diaphragm muscle is only dis-cussed briefly here.) As it moves downward, the tho-racic cavity enlarges and atmospheric pressure forces air into the airways. While it is contracting and mov-ing downward, the ribs are raised and the sternum elevates, allowing more air to enter the airways. As the diaphragm and external intercostal muscles relax after inspiration, the lungs and thoracic cage return to their original shapes. The diaphragm is pushed upward to force air inside the lungs out through the respiratory passages. The abdominal wall mus-cles increase pressure and force the diaphragm even higher against the lungs to squeeze additional air out of them.