Isokinetic contractile properties of the quadriceps with relation to fiber type

Fifteen subjects were assigned to three groups on the basis of the proportion of fast-twitch fibers (% FT) in their vastus lateralis muscles. Torque production per unit of fat free thigh volume was then determined during knee extension on an isokinetic dynamometer at 60, 120, 180, 240, and 300 degrees . s-1. Maximal isometric force was also obtained at 65 degrees from horizontal. Subjects with predominantly fast twitch muscle fibers demonstrated significantly greater peak power, rate of power production and work than subjects with predominantly slow twitch fibers at all but the lowest velocity of 60 degrees . s-1. Mean peak power for all subjects occurred at approximately 30% of maximal knee extension velocity (210 degrees x s-1) and 34% of maximal voluntary isometric contraction. When work, peak power and rate of power production were correlated with % FT fibers, then the highest correlations of 0.69, 0.57, and 0.73 respectively, all occurred at 180 degrees . s-1. These observations suggest that: 1) the torque-velocity, power-velocity relationship of the quadriceps is similar to the force-velocity, power-velocity relation found for excised muscle, 2) when using the knee extension exercise for the prediction of muscle fiber composition of the quadriceps the most appropriate speed for testing appears to be 80 degrees . s-1.