Muscle fiber types and energy storage pathways for exercise

There are several types of muscle fibers. Type I, or slow-twitch muscle fibers, have a relatively slow contraction rate. They use predominantly aerobic metabolic pathways and contain many mitochondria with high levels of enzymes needed for aerobic energy production pathways (i.e., enzymes needed in the Krebs cycle and electron transport chain), and they have a higher capillary density to supply them with oxygen and energy substrates, and to remove waste products such as lactic acid.

Athletes with more type I muscle fibers have a higher blood lactate threshold because they can release pyruvate into the Krebs cycle faster and have less pyruvate converted to lactic acid, so they can perform longer and have a longer time to fatigue.

Type II, or fast-twitch, muscle fibers have a relatively fast contraction velocity and the ability to produce energy quickly anaerobic. They are divided into categories, two of which are well defined. Type II muscle fibers have a high contraction velocity and fairly well-developed aerobic and anaerobic energy production systems. Type II muscle fibers are the fastest and the most glycolytic. Most activities require a combination of fast-twitch and slow-twitch muscle fibers, capable of relatively slow muscle contractions with occasional short bursts of rapid muscle contraction.

Loads that require the involvement of a larger number of type II fibers, such as sprinting, intense walking, are quite heavily dependent on accumulated carbohydrate reserves. These loads are associated with a more rapid depletion of glycogen stores. The ratio of slow- and fast-twitch muscle fibers depends mainly on genetic predisposition. In humans, on average, 45-55% of muscle fibers are slow-twitch. However, training sessions can affect the distribution of muscle fiber types. In athletes involved in sports that require mainly aerobic energy supply (long-distance running), slow-twitch fibers make up 90-95% of the working muscles.

The energy of the chemical bonds of food is stored in the form of fats and carbohydrates and, to a lesser extent, in the form of proteins. This energy is transferred to ATP, which transfers it directly to the cellular structure or compound that needs it.

Three different systems can be used in the transfer of ATP energy: phosphagen, anaerobic-glycolytic, and aerobic. The phosphagen system transfers energy more rapidly, but its capacity is very limited. The anaerobic-glycolytic system can also transfer energy relatively quickly, but the products of this pathway reduce the pH of the cell and limit its growth. The aerobic system transfers energy more slowly, but has the greatest productivity, since it can use carbohydrates or fats as energy substrates. All of these systems can be used simultaneously in different cells of the body, and the cellular environment and energy needs determine the preferred energy transfer system.

• Availability of oxygen and energy substrates
• two important factors in the cellular environment.

Muscle fiber type and its inherent characteristics are key factors in determining the energy transfer system for muscle cells. Dietary manipulation and exercise training can alter the cellular environment and have a profound effect on energy transfer system performance as well as energy substrate reserves.