In order to evaluate relationships between structure and concentric function of the quadriceps muscle, isokinetic maximum knee extensor output at different velocities of angular motion (30-180 degrees s-1), integrated electromyographic activity (iEMG), cross-sectional area (CSA) of m. quadriceps and fibre composition of m. vastus lateralis were analysed in five sprinters and five marathon runners (all males). Both output and input parameters, concerning peak torque (PT), instantaneous power (IP), contractional work (CW) and the summed iEMG were closely correlated to the m. quadriceps CSA, as well as to the calculated absolute type II (A + B), IIA and IIB fibre areas of m. vastus lateralis. When PT, IP, CW and the summed iEMG were corrected for CSA, such relationships were found only at 180 degrees s-1 and only for type IIA fibre area. The electromyographic findings indicate that m. vastus lateralis was representative for the total mechanical output. The calculated mean power output (CW s-1) and output/input balance (CW/iEMG) describe similar relationships with the velocity of angular motion and were not related to the CSA or to the absolute area of any fibre type. The calculated optimal CW per second and CW/iEMG occur for sprinters at about 450 degrees s-1 and for marathon runners at about 270 degrees s-1, that is, velocities well above the capacity of the isokinetic dynamometer. Our observations indicate that the peak torque or instantaneous power, during single maximum knee extensions, at least at 180 degrees s-1, express characteristics of the muscle structure.