To investigate the influence of skeletal muscle fiber composition on the mechanical characteristics of human skeletal muscle under isometric and dynamic conditions, ten well-trained track athletes with different muscle compositions (m. vastus lateralis) were used. The subjects were instructed to perform maximum isometric and dynamic knee extensions at maximal speed against increasing load. To determine the mechanical properties of the knee extensors a special dynamometer was used. The load was increased by adding weight discs to the electromagnetic part of the dynamometer. The load increased from 0.33-2.33 kg2 moment of inertia in six stages. The data were handled on the basis of Hill's characteristic equation. The subjects were divided into two equal groups, one with more and one with less than 50% of fast twitch fibers. The force-velocity curve was found to be different in the two groups, the subjects with a predominance of fast twitch fibers being able to develop higher power output at a given load. Significant correlations between percentage of fast twitch fiber and power, as well as velocity, were found at the four greater loads only. However, when the percentage of fast twitch fiber and fast twitch fiber cross-sectional areas were combined, significant correlations were also found between this combined parameter and power output at the two lowest loads.