10.5-10.6

• Two criteria to consider when classifying the types of muscle fibers are how fast some fibers contract relative to others, and how fibers produce ATP. Using
• Three Types of Muscle Fibers:

  1. Slow Oxidative: fibers contract relatively slow to others and use aerobic respiration to produce ATP
  2. Fast Oxidative: fibers contract fast and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), they can fatigue more quickly than SO fibers
  3. Fast Glycolytic: have fast contractions and primarily use anaerobic glycolysis.
• The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyzes ATP to produce cross-bridge action.
• The primary metabolic pathway used by a muscle fiber determines whether the fiber is classified as oxidative or glycolytic. If a fiber primarily produces ATP through aerobic pathways it is oxidative. More ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue. Glycolytic fibers primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. As a result, glycolytic fibers fatigue at a quicker rate.
• The oxidative fibers contain many more mitochondria than the glycolytic fibers, because aerobic metabolism, which uses oxygen (O2) in the metabolic pathway, occurs in the mitochondria. The
• The myoglobin stores some of the needed O2 within the fibers themselves (and gives SO fibers their red color).
• SO fibers produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time.
• SO fibers are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.
• FO fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers.
• FO fibers are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However,
• However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers. FO
• FO fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. FO fibers are useful for this type of movement because they produce more tension than SO fibers but they are more fatigue-resistant than FG fibers.
• FG fibers have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension.
• Because FG fibers do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color.
• Although muscle cells can change in size, new cells are not formed when muscles grow.
• Instead, structural proteins are added to muscle fibers in a process called hypertrophy, so cell diameter increases. The reverse, when structural proteins are lost and muscle mass decreases, is called atrophy.
• Sarcopenia: age-related muscle atrophy
• Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions.
• production. Endurance exercise can also increase the amount of myoglobin in a cell, as increased aerobic respiration increases the need for oxygen.
• Myoglobin is found in the sarcoplasm and acts as an oxygen storage supply for the mitochondria.
• The training can trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis, to supply oxygen and remove metabolic waste. To
• To allow these capillary networks to supply the deep portions of the muscle, muscle mass does not greatly increase in order to maintain a smaller area for the diffusion of nutrients and gases.
• Long-distance runners have a large number of SO fibers and relatively few FO and FG fibers.
• The proportion of SO muscle fibers in muscle determines the suitability of that muscle for endurance, and may benefit those participating in endurance activities.
• Postural muscles have a large number of SO fibers and relatively few FO and FG fibers, to keep the back straight (
• Resistance exercises require large amounts of FG fibers to produce short, powerful movements that are not repeated over long periods.
• Resistance exercise affects muscles by increasing the formation of myofibrils, thereby increasing the thickness of muscle fibers.
• Resistance training does not usually significantly increase mitochondria and capillary density, but it does increase the development of connective tissue, which adds to the overall mass of the muscle and helps to contain muscles as they produce increasingly powerful contractions.
• Resistance training also strengthens tendon to prevent tendon damage
• muscles gain mass as this damage is repaired, and additional structural proteins are added to replace the damaged ones.
• Anabolic steroids are one of the more widely known agents used to boost muscle mass and increase power output.
• Endurance athletes may also try to boost the availability of oxygen to muscles to increase aerobic respiration by using substances such as erythropoietin (EPO), a hormone normally produced in the kidneys, which triggers the production of red blood cells.
• Human growth hormone (hGH) is another supplement, and although it can facilitate building muscle mass, its main role is to promote the healing of muscle and other tissues after strenuous exercise.
• Increased hGH may allow for faster recovery after muscle damage, reducing the rest required after exercise, and allowing for more sustained high-level performance.
• Similarly, some athletes have used creatine to increase power output. Creatine phosphate provides quick bursts of ATP to muscles in the initial stages of contraction. Increasing the amount of creatine available to cells is thought to produce more ATP and therefore increase explosive power output, although its effectiveness as a supplement has been questioned.
• the effects of age are less noticeable in endurance athletes such as marathon runners or long-distance cyclists.
• As muscles age, muscle fibers die, and they are replaced by connective tissue and adipose tissue
• for aging, There may also be a reduction in the size of motor units, resulting in fewer fibers being stimulated and less muscle tension being produced.