ABSTRACTBELL, D. G. and I. JACOBS. Muscle fiber-specific glycogen utilization in strength-trained and untrained males and females.Med. Sci. Sports Exerc., Vol. 21, No. 6, pp. 649–654,1989. This study evaluated the effect of strength training on glycogen utilization in slow twitch (ST) and fast twitch (FT) muscle fibers during repeated maximal unilateral isokinetic leg extensions at 180° ± s-1. Strength-trained (5 males, 4 females) and untrained (4 males, 6 females) subjects performed three sets of 50 maximum voluntary contractions (MVC) at this velocity with 10-min rest intervals between sets. Biopsies were taken from the vastus lateralis muscle before and after each exercise session. Glycogen content of the fibers was quantified as optical density (OD) using microspectrophotometric densitometry on serial cross-sections of muscle tissue stained with a periodic acid Schiff reagent stain after individual fibers were identified as ST or FT according to a stain for myofibrillar ATPase activity. Analysis of variance with repeated measures yielded the following results: OD, i.e., glycogen, was reduced similarly in both fiber types after exercise, but only in the males (P= 0.02); there was no significant main effect of training statusper se(i.e., strength-trained vs untrained). These results indicate that years of strength training do not change the pattern of muscle fiber-specific glycogen utilization during repeated dynamic MVCs.