Which fibers produce lactic acid

They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers. 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 primarily use anaerobic glycolysis as their ATP source.

They 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 they 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. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods.

Most muscles possess a mixture of each fiber type. The predominant fiber type in a muscle is determined by the primary function of the muscle. ATP provides the energy for muscle contraction. The three mechanisms for ATP regeneration are creatine phosphate, anaerobic glycolysis, and aerobic metabolism.

Creatine phosphate provides about the first 15 seconds of ATP at the beginning of muscle contraction. Anaerobic glycolysis produces small amounts of ATP in the absence of oxygen for a short period. Aerobic metabolism utilizes oxygen to produce much more ATP, allowing a muscle to work for longer periods.

Muscle fatigue, which has many contributing factors, occurs when muscle can no longer contract. An oxygen debt is created as a result of muscle use. SO fibers use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. Long-term muscle use requires the delivery of oxygen and glucose to the muscle fiber to allow aerobic respiration to occur, producing the ATP required for muscle contraction. If the respiratory or circulatory system cannot keep up with demand, then energy will be generated by the much less efficient anaerobic respiration.

In aerobic respiration, pyruvate produced by glycolysis is converted into additional ATP molecules in the mitochondria via the Krebs Cycle. With insufficient oxygen, pyruvate cannot enter the Krebs cycle and instead accumulates in the muscle fiber.

With pyruvate accumulation, lactic acid production is also increased. This further inhibits anaerobic respiration, inducing fatigue. Lactic acid can be converted back to pyruvate in well-oxygenated muscle cells; however, during exercise the focus in on maintaining muscle activity. Lactic acid is transported to the liver where it can be stored prior to conversion to glucose in the presence of oxygen via the Cori Cycle. The amount of oxygen required to restore the lactic acid balance is often referred to as the oxygen debt.

With extensive exercise, the osmotically active molecules outside of the muscle are lost through sweating. In extreme cases, this can lead to painful, extended maintenance of muscle contraction or cramp. Nerves are responsible for controlling the contraction of muscles, determining the number, sequence, and force of muscular contractions. Most movements require a force far below what a muscle could potentially generate, and barring disease nervous fatigue is seldom an issue. However, loss of desire to exercise in the face of increasing muscle soreness, respiration, and heart rate can have a powerful negative impact on muscle activity.

Depletion of required substrates such as ATP or glycogen within a muscle result in fatigue as the muscle is not able to generate energy to power contractions. With sufficient training, the metabolic capacity of a muscle can change, delaying the onset of muscle fatigue. Muscle specified for high-intensity anaerobic exercise will synthesise more glycolytic enzymes, whereas muscle for long endurance aerobic exercise will develop more capillaries and mitochondria.

Additionally, with exercise, improvements to the circulatory and respiratory systems can facilitate better delivery of oxygen and glucose to the muscle.

Muscle fibers shrink or are lost and surrounding connective tissue hardens, making muscle contraction slower and more difficult.

Exercise throughout life can help reduce the impact of aging by maintaining a healthy oxygen supply to the muscle. Privacy Policy. Skip to main content. Skeletal muscle contains different fibers which allow for both rapid short-term contractions and slower, repeatable long-term contractions. Skeletal muscle fibers can be further subdivided into slow and fast-twitch subtypes depending on their metabolism and corresponding action. Most muscles are made up of combinations of these fibers, although the relative number substantially varies.

Slow-twitch fibers are designed for endurance activities that require long-term, repeated contractions, like maintaining posture or running a long distance.

The ATP required for slow-twitch fiber contraction is generated through aerobic respiration glycolysis and Krebs cycle , whereby 30 molecules of ATP are produced from each glucose molecule in the presence of oxygen. The reaction is slower than anaerobic respiration and thus not suited to rapid movements, but much more efficient, which is why slow-twitch muscles do not tire quickly. However, this reaction requires the delivery of large amounts of oxygen to the muscle, which can rapidly become rate-limiting if the respiratory and circulatory systems cannot keep up.