Most of us are familiar with the idea that during endurance exercise muscle fibers become depleted of carbohydrate or better known as muscle glycogen. This is why adequate carbohydrate needs to be ingested by endurance athletes, or they will become fatigued very quickly. Dr. Per Tesch recently showed that glycogen loss in both fast and slow twitch fibers also occurs with weight training. Muscle glycogen depletion of fibers from a thigh muscle was determined by a muscle biopsy before and after five sets of 10 reps of knee extension exercises. Sets were completed using 30, 45 and 60% of a one rep max. Significant glycogen loss in both fast and slow twitch fibers was shown after all three workouts, but was greatest after the sets at 60% of the one rep max.
Per Tesch, et. al. Skeletal muscle glycogen loss evoked by resistance exercise. Journal of Strength and Conditioning Research. 12: 67-73, 1998.