Glycogen, the stored form of the carbohydrate-derivative glucose, is an essential aspect of the ability of the body to generate fuel for both athletic and sedentary activities. The main storage centers for glycogen are within the liver and the skeletal muscles; approximately twice as much glycogen is stored in the muscles as is retained in the liver. For an athlete who is placing significant demands upon those energy reserves, the consumption of between 8 and 10 grams of carbohydrate per 2.2 lb (1 kg) of body weight per day will be required to keep the glycogen levels optimum.
Muscles that are supplied with appropriate amounts of glycogen for breakdown into glucose, and the further processes necessary to produce the fuel source, adenosine triphosphate (ATP), will lead inevitably to an increased endurance capacity on the part of the athlete. When glycogen levels in muscles are depleted, there is a resulting muscle fatigue, with a corresponding loss of protection to the joint. Sports such as Alpine skiing illustrate this progression. There is a greater incidence of downhill skiing accidents at the end of the ski workout, versus at the beginning. The repetitions on the hill cause a depletion in muscle glycogen, which presents a greater risk of injury to the athlete.
When the body is at rest, its energy requirements are 80% from fat stores, 15% from carbohydrates, and 5% from proteins. During exercise, the proportionate contribution from the energy sources within the body changes dramatically. Muscle glycogen and blood glucose (the product of the glycogen stored in the liver) become the predominate sources, with plasma fatty acids (the product of converted fat stored in adipose cells when released into the bloodstream) and intro-muscular fat stores playing a secondary role.
The factors that will influence the relative importance of muscle glycogen, blood glucose, and fats during exercise include: the intensity of the activity; the duration of the activity; the level of training previously attained by the athlete; the initial level of muscle glycogen available to the athlete; the amount of carbohydrate supplement consumed, if any, during exercise (providing an additional potential energy source).
The effectiveness of carbohydrate-loaded sports drinks during competition is restricted to a rather narrow window. These fluids contain carbohydrates intended to provide an additional mid-competition energy boost, typically a combination of simple sugars (monosaccharides), such as fructose and glucose, mixed with water and flavoring. When the amount of carbohydrates is greater than 6-8% by volume, the body will have difficulty digesting and processing the carbohydrates during the event.
The type of physical activity dictates which of the body's energy systems will be engaged, and for how long. These engagements will determine what impact is directed to muscle glycogen levels through and at the conclusion of the activity. For those sports that require short, explosive bursts of activity, the anaerobic alactic system is employed. These sports are typically ten seconds or less in the duration of any given interval, and the muscular activity will be fueled by ATP that is present in the muscle. These ATP stores recharge without significant strain on the bodily glycogen reserves.
Activities that are of approximately 90 seconds maximum duration require energy generation from the anaerobic lactic system. Those of greater duration will engage the aerobic system. Both of these energy systems will require supplies of glycogen for both the immediate conversion of glucose into ATP; the demands of the aerobic system are significantly greater due to the duration of the demand. Many athletes, particularly in events such as the marathon, speak of "hitting the wall," a euphemism for the sensation of being without energy and in an extreme state of fatigue. When athletes experience this physical state, they have a significantly reduced supply of glycogen in their muscles, particularly with respect to the muscles actively working and involved in the event.
As a contrast, when the athlete experiences a glycogen depletion in the liver, the athlete will experience the condition known as hypoglycemia, a dangerous reduction in blood sugar, which often causes dizziness and disorientation.
Diet is the most effective tool to maximize muscle glycogen, both pre- and post-competition and training sessions. An athlete in training will require appropriate carbohydrates on a daily basis to ensure glycogen stores for competition; the ingestion of carbohydrate drinks and similar supplements after exercise will tend to speed the return of the glycogen stores to their usual levels. The most difficult of endurance events will not prevent the athlete from returning to an accustomed muscle glycogen level within 24 hours.