SUTER, E., W. HERZOG, J. SOKOLOSKY, J. P. WILEY, and B. R. MACINTOSH. Muscle fiber type distribution as estimated by cybex testing and by muscle biopsy. Med. Sci. Sports Exerc., Vol. 25, No. 3, pp. 363–370, 1993. The purpose of the present study was to derive a regression equation relating variables obtained from a series of noninvasive functional tests in a normal subject population to the fiber type distribution of vastus lateralis muscle (VL) determined using muscle biopsy. All functional tests were designed to distinguish between basic properties of Type II fibers (fast twitch fibers) and Type I fibers (slow twitch fibers) and included assessment of peak torque, power output at nine different angular velocities (60°.s-1 to 300°.s-1), as well as a fatigue test consisting of 60 consecutive contractions at 90°.s-1 to establish fatigue resistance of the knee extensor muscles. Using a stepwise multiple regression procedure, relative torque after 53–55 contractions (T55) in the fatigue test and power output at an angular velocity of 280°.s-1 normalized for fat free mass of the thigh (FFMT) were the best predictors for fiber type distribution, explaining 51.8% of the variance in the proportion of Type II fibers in VL. No other measured variable met entering criteria. Subgroup analyses revealed a higher peak torque/FFMT, higher power/FFMT values at angular velocities of 200°.s-1 and higher, and lower relative torque beyond 30 contractions in the fatigue test for the fast twitch group, FTG (subjects with >60% Type II fibers, N = 8) as compared with the STG (subjects with <45% Type II fibers, N = 9). Results from the present study suggest that two simple functional tests on a Cybex dynamometer yield reasonable estimates of the fiber type distribution in VL.