Muscle glycogen recovery is the process through which the muscles of the body are replenished with carbohydrate sources that have been depleted through the energy expended in exercise. The muscles that are the target of this recovery to an optimal glycogen level are the skeletal muscles, those structures that are actually engaged in the movement of the body; the other muscle groups, the smooth muscles of the internal organs and the cardiac muscles do not possess a capacity for glycogen storage or conversion to fuel in the muscle.
Glycogen is the form in which the carbohydrate glucose is stored within the body; the skeletal muscles typically contain two-thirds of the body's supply of glycogen at any time, or a quantity measuring approximately 2% of skeletal muscle weight. Most of the remaining glycogen in stored in the liver, where it is released into the bloodstream for transport to cells for energy production.
Exercise by its nature places demands on the body to provide energy to power the muscles in movement. When the energy sources, particularly glycogen, are constant at the commencement of the activity, these supplies will steadily diminish, and if not replaced to any degree during the activity, the glycogen must be recovered and the supplies restored at the conclusion of the activity. Most glycogen is entirely consumed from muscle stores within 15 minutes to 30 minutes from the commencement of the exercise; in disciplines such as sprinting, where the fast-twitch muscle fibers are the components of each muscle that are of primary importance, the athlete may exhaust all of the stored glycogen reserves with 10 minutes of muscle effort. Weight-lifters and other power sport competitors place similar demands on their muscle glycogen supply.
Of the glucose that is initially stored as glycogen and then converted for use in the energy-producing processes, 75% is directed to the brain and the functions of the related central nervous system; it is from the approximate 25% remainder that red blood cell production, skeletal muscle, and heart muscle functions are ultimately fueled.
Skeletal muscle glycogen depletion is a natural result of physical movement, which depletions are especially pronounced in athletic activity. Such depletion, if not corrected, carries with it risks to the structure and the function of the body. The primary danger associated with this depletion is damage to cells and muscle structures. When the body is unable to access muscle glycogen, it may trigger the breakdown of cell structures to create an alternative energy supply. Muscle glycogen depletion also places significant stress on the overall function of the immune system.
Muscle glycogen depletion is most effectively counteracted through diet; athletes who understand the demands placed on their muscle glycogen stores will plan how they shall achieve glycogen recovery through the foods ingested before, during, and after their workouts and their competitive events. There are a number of techniques employed by athletes to bolster their muscle glycogen levels, with each approach specific to the nature of the glycogen-depleting physical activity.
In sports where explosive, fast-twitch muscle movement is essential, the muscle glycogen is best restored through a post-activity meal that is rich in carbohydrates. This meal can be consumed within 30 minutes of the conclusion of the activity, as the body is most receptive to the recovery process at that time; the body's ability to ingest and synthesize carbohydrates into useful glycogen declines in the hours following the physical activity that depleted those reserves. Ideal muscle glycogen recovery will be performed through a series of small meals, as opposed to a single large meal that the body will not able to digest and process as readily. When the meals include a measure of protein replacement (the explosive muscle power activities that tax the glycogen stores often will require restoration of protein levels as well), a dual benefit is achieved.
A well-established method of ensuring that an endurance athlete has sufficient muscle glycogen is the process known as "carbo loading," when the athlete deliberately reduces carbohydrate consumption in the diet while continuing the training and energy expenditures. The athlete then loads carbohydrate-rich foods into the system, which tends to flood the muscles with glycogen. This technique, or a variation, is used after an endurance event such as a marathon race to restore muscle glycogen levels.
The development of specific sports fluid replacement products that contain carbohydrates (as well as useful electrolytes and water) has permitted athletes to more readily ingest carbohydrates during a workout or a competition. As a very general guideline, the body will process such carbohydrate sources most efficiently when the amount of carbohydrate in the fluid is between 6% and 12% by volume. This technique is effective in events that approach two hours or more in length, depending on the intensity of the athletic effort in question. For most athletes competing in events such as the Hawaii Ironman, such mid-race carbohydrate ingestion is essential.