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C h e c k o u t A B M P 's l a t e s t n e w s a n d b l o g p o s t s . Av a i l a b l e a t w w w. a b m p . c o m . 43 FUNCTIONAL ANATOMY education Skeletal Muscle Fiber By Christy Cael There are many causes of muscle pain and dysfunction, including both hypertonicity and chronic ischemia. One strategy for addressing these underlying issues is selecting treatment methods according to the dominant type of skeletal muscle fibers in the area or region of discomfort. Skeletal muscles are made up of two distinct types of muscle fibers that vary in both structure and function. The primary variable that differentiates these fiber types is how each one produces the energy needed to drive muscle contractions. Remember, energy is released in the body when a single phosphate bond is broken from the adenosine triphosphate (ATP) molecule. These molecules are formed within the muscle cells either aerobically (in the presence of oxygen) or anaerobically (without oxygen) through a process called glycolysis. ENERGY PRODUCTION METHODS Aerobic energy production is extremely efficient, producing a large number of ATP molecules from a given amount of glucose or fuel. Delivery of adequate amounts of oxygen and fuel to the mitochondria of muscle cells is essential for continual aerobic energy production, as is the removal of carbon dioxide, a metabolic by- product of aerobic glycolysis. This is the preferred method of energy production in the body, and the one utilized for most activities of daily living. The process continues indefinitely, as long as fuel and oxygen are delivered to the mitochondria and carbon dioxide is removed. Sometimes muscles need to perform activities that are fast and forceful for short periods of time. Skeletal muscles are capable of exceeding the force production offered through aerobic means, but with less efficiency and only for short periods of time. Anaerobic energy production occurs without oxygen or mitochondria. This process occurs in the cytoplasm of the cell and only yields a couple of ATP molecules per unit of glucose. Since it does not utilize oxygen, there is an increase in acidity in the surrounding intercellular environment, as the phosphate bonds are broken and energy is released. The subsequent change in pH as well as rapid depletion of stored fuel make this type of energy production short-lived, lasting between 12 seconds and 2 minutes. Characteristic Type I Type IIa Type IIb Energy System Aerobic Aerobic/Anaerobic Anaerobic Contraction Speed Slow Fast Fast Force Production Low Intermediate High Endurance High Intermediate/Low Low Capillary Density High Intermediate Low Fiber Diameter Small Intermediate Large Mitochondria Density High Intermediate Low Myoglobin/Color High/Red Low/White Low/White Lateral view Cross-sectional view A Slow-twitch fibers Note smaller diameter and darker color from myoglobin B Fast-twitch fibers Note larger diameter and paler color Image ©2010 Wolters Kluwer Health | Lippincott Williams & Wilkins FIBER TYPES There are two distinct types of fibers that make up skeletal muscles: slow-twitch, also called Type I fibers, and fast-twitch, or Type II fibers. Twitch refers to the amount of time it takes a motor unit or group of fibers to develop force and relax. Fast-twitch fibers are divided further into Type IIa and Type IIb. Each fiber type has unique characteristics: All skeletal muscles have a combination of fiber types, with the ratio and distribution of these fibers being genetically determined. This explains why some individuals have a

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