Muscle biopsies from m. vastus lateralis of two world class shot putters (shot putter 1 and 2) and the untrained brother of shot putter 1 were analyzed for fiber type distribution with ATPase staining and in situ hybridization for the expression of alkali myosin light chain (MLC) isoforms. Shot putter 2 had a predominance of type II fibers (67 X) and distinct hypertrophy of type I as well as type II fibers (fiber areas of 5939 and 8531 microm2). In shot putter 1, type II fibers amounted to only 40%, due to their selective hypertrophy, however type II fibers (10265 microm2) accounted for 67 2% of the total cross-sectional area. The type I fibers in shot putter 1 were similar in size to his untrained brother (3430 vs 3790 microm2). After 3 years of active detraining, type II fibers of shot putter 1 had reduced in size to values closer to those of his brother (7746 and 6340 microm2). The large difference between type I and type II fiber size, even in the untrained state, in both shot putter 1 and his brother is not usually seen in humans and maybe a genetic characteristic. We suggest that the ability to selectively increase the relative area of his type II fibers in the 15 years of strength training was a key element in his success as a shot putter. The observed increase in the expression of fast myosin light chain mRNAs in both fiber types is indicative of further adjustment of the myofibrillar apparatus towards the generation of very high peak power.