The effect of training with concentric and eccentric contractions on fiber hypertrophy and isometric torque production was investigated in 20 healthy subjects. One group (eight female and two male subjects) performed concentric contractions of their quadriceps femoris muscles at an intensity of 90% of their maximal concentric power. The other group (six female and four male subjects) performed eccentric contractions at the same relative power level. Both groups exercised three times per week for 4 wk at a constant speed of 60 degrees.s-1 on a Kin-Com dynamometer. Needle biopsies were obtained from the vastus lateralis before and after the exercise program. Fiber-type differentiation was performed using a myosin ATPase stain at a preincubation of 10.5. Maximal isometric knee extension torque was also measured before and after the exercise program. An analysis of covariance was used to determine whether there were significant differences between the exercise groups in: 1) the post-exercise fiber areas and 2) maximal isometric torque (MIso), while controlling for initial differences. Results showed a significant difference between the Type II fiber areas (P < 0.01) and the MIso (P = 0.01). These data indicate that, when exercising at the same relative power level, a subject performing concentric contractions will show greater muscle hypertrophy and improve in MIso production more than a subject training with eccentric contractions.