When the body takes in carbohydrates, they are broken down into glucose, which is used immediately for energy or stored for later use. The storage form of this glucose is glycogen, which acts as the body’s reserved fuel tank. Glycogen is primarily housed in the liver and the skeletal muscles, each serving a distinct purpose. Liver glycogen maintains stable blood sugar levels for the entire body, while muscle glycogen is reserved exclusively for the muscle cells that hold it. Understanding this storage capacity is important, as this limited fuel source directly influences physical performance and endurance.
Calculating Muscle Glycogen Capacity
The amount of glycogen a person’s muscles can hold is not a fixed value but a concentration measured in grams per kilogram (g/kg) of muscle tissue. In an average, well-fed, untrained person, the resting concentration is generally around 10 to 15 grams of glycogen per kilogram of muscle mass. For a 70-kilogram adult, who has about 40% skeletal muscle, this translates to roughly 300 to 400 grams of total muscle glycogen. Since each gram of carbohydrate yields about four calories, the skeletal muscles can store approximately 1,200 to 1,600 calories worth of glycogen.
This capacity is significantly influenced by training status and diet. Highly trained endurance athletes, who maximize carbohydrate intake, can often store 20 to 25 grams of glycogen per kilogram of muscle, sometimes reaching up to 750 grams of total muscle glycogen. Muscle glycogen is considered “selfish” because it cannot be released into the bloodstream to raise blood sugar. Instead, it serves as an immediate, localized fuel source for muscle contraction.
Fueling Exercise: How Muscle Glycogen is Used
Muscle glycogen is the preferred fuel source for high-intensity activity, where energy demand outpaces the body’s ability to use fat or oxygen. During short, intense bursts of effort, such as sprinting or heavy weightlifting, the muscle rapidly breaks down stored glycogen through anaerobic pathways. This process generates energy quickly but causes a fast depletion of local glycogen stores in the active muscle fibers.
Prolonged, moderate-intensity aerobic exercise, like long-distance running, relies on a mix of fat and carbohydrate for fuel. Muscle glycogen still contributes significantly to total energy expenditure, especially as intensity increases above 60% of maximum effort. Even at moderate intensities, the slow, sustained depletion means stores will eventually run dry after about 90 to 120 minutes of continuous activity.
Severe depletion of muscle glycogen marks a performance-limiting state often called “hitting the wall” or “bonking.” This sudden onset of extreme fatigue results from the muscle running out of its primary fuel. The body is forced to slow down drastically to rely almost entirely on less efficient fat metabolism. Performance is impaired because the metabolic pathways shift, reducing the muscle’s ability to maintain a high rate of work.
Strategies to Increase Muscle Glycogen Stores
Athletes seeking to maximize muscle energy reserves often employ specialized nutritional protocols known as carbohydrate loading or glycogen supercompensation. The classic approach involves intense exercise to deplete glycogen, followed by several days of rest and a high-carbohydrate diet (8 to 12 grams per kilogram of body weight). This manipulation allows muscle cells to overcompensate, storing a greater amount of glycogen than normal.
Consistent endurance training is a powerful stimulus that increases the muscle’s inherent capacity to store glycogen, regardless of acute dietary changes. Training enhances the activity of enzymes and transport proteins, such as GLUT-4, responsible for bringing glucose into the muscle cell and converting it into glycogen. This adaptation means a trained muscle can both store more fuel and replenish stores faster than an untrained muscle.
Post-exercise nutrition is a strategy for store replenishment, particularly in the first few hours following a strenuous workout. Consuming carbohydrates immediately after exercise, often with protein, takes advantage of the muscle’s heightened sensitivity to insulin and increased permeability to glucose. This rapid intake is essential for kickstarting glycogen synthesis, ensuring the muscle is adequately refueled and prepared for the next training session.