This study sets out 31P-nuclear magnetic resonance (NMR) determinations of the muscle energy metabolism during anaerobic isometric exercise, histochemical analysis of the muscle biopsies, and the muscle characteristics. Subjects were six healthy males (controls, CON), five sprint-trained athletes (SPR), and four endurance-trained athletes (END). The three groups had similar fiber distribution and the percentages of fiber area in tibialis anterior muscles which were predominantly slow-twitch fiber. The mean of capillary numbers in END was higher than those in CON and SPR. The peak torque per unit muscle volume of the dorsiflexors was higher in SPR compared to those in CON and END. The subjects performed dorsiflexion at work load of 50% of maximal exercise intensity for 50 seconds with cuff-arrested circulation for estimation of the anaerobic metabolism. The ATP turnover in CON and SPR increased rapidly at 20 sec (p < 0.05) compared to the initial value and increased progressively afterwards. However, that in END remained relatively constant with time course. Mechanical efficiency (ME) in each group decreased gradually during the time course. A significant decrease was found in CON at 40 sec compared to that at 10 sec. ME in SPR decreased abruptly at 20 sec (p < 0.05). However, ME in END remained relatively stable during exercise as well as ATP turnover. The highest rates of PCr were observed in CON and END at 10 sec and in SPR at 20 sec; afterwards these decreased progressively. A significant decrease was found only in CON at 50 sec compared to that at 10 sec (p < 0.05). The highest rates of glycolysis were observed in all groups at 50 sec compared to those at 10 sec (p < 0.05), which indicated that these increased gradually with time course. The percentages of PCr depletion in three groups were greater than those of glycolysis until 30 sec. These results suggest that the anaerobic energy metabolism and muscle characteristics of the dorsiflexors adapt to a specific type of training used in athletes excepting the fiber types.