Force exerted and power generated were measured during short-term exercise performed on a bicycle ergometer that had been modified by the addition of an electric motor driving the cranks at a chosen constant velocity. Five subjects made a series of 20-s maximum efforts at different crank velocities (range 23--171 rev/min). The forces exerted were continuously monitored with strain gauges bonded to the cranks. Peak force was exerted at approximately 90 degrees past top dead center in each revolution. During the 20-s effort peak force declined from the maximum level (PFmax) attained near the start of exercise, the rate of decline being velocity dependent. PFmax was found to be inversely and linearly related to crank velocity and when standardized for upper leg muscle (plus bone) volume (ULV) was given by PFmax (kgf/l ULV) = 27.51--0.125 crank velocity (rev/min). Integration of the force records with pedal velocity enabled power output to be calculated. Maximum power output was a parabolic function of crank velocity, the apex of the relationship indicating that the velocity for greatest power output was 110 rev/min. At this velocity our subjects achieved a maximum mean power output, averaged over a complete revolution, of 840 +/- 153 W (85 +/- 5 W/l ULV). This was compared with the calculated value for maximum mechanical power output from aerobic sources, which was 272 +/- 49 W (30 +/- 1 W/l ULV).