Glycogen

Glycogen is the stored form of glucose in skeletal muscle and the liver.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 200.} Glucose can be stored this way when it isn’t needed for energy right away.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 200.} About three-quarters of the body’s glycogen sits in skeletal muscle and the rest in the liver: roughly 300 to 400 g in muscle and 70 to 100 g in the liver.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 200.},^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 72.} The NSCA’s personal training text puts total storage at about 500 g, or about 2,000 kcal, and ties depletion to fatigue.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 52.}

Muscle glycogen is the more important fuel in moderate- and high-intensity exercise, becoming increasingly important above about 60% of maximal oxygen uptake.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 72.} Liver glycogen matters more at low intensities, and its share grows the longer exercise lasts.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 72.}

In practice

• Resistance training can cut muscle glycogen by 24 to 40%, with higher volumes at moderate loads having the most effect, especially in type II fibers.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 52.} In one study, depleted glycogen significantly reduced the reps performed in 3 sets of squats at 80% of 1RM.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 227.}
• Glycogen resynthesis starts as soon as exercise ends and is fastest over the first five to six hours.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 52.} Repletion appears best with about 1.2 g/kg of carbohydrate roughly every two hours after exercise.^{3Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 52.}
• With enough carbohydrate, muscle glycogen can be fully restored within 24 hours.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 72.} Sessions with a heavy eccentric component, which cause more muscle damage, may need longer.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 72.}
• Endurance athletes training 90 minutes or more a day need about 7 to 10 g/kg of carbohydrate daily to keep stores topped up.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 126.}