A method and device for measuring force, torque and output on an ergometer or bicycle. When the ergometer or bicycle is used as intended, a torque is transmitted via a shaft onto a flexible drive and further onto the flywheel of an ergometer or the rear wheel of the bicycle by two pedal cranks. The flexible drive has a pulley or a sprocket wheel which is non-rotatably connected to the shaft in at least one direction of rotation. Elastic torsions of the shaft that result independently from the torque transmission are detected at both sides of the pulley or of the sprocket wheel as a measure of the forces applied to the shaft to determine the torque and output both of the left and right leg of the user of the ergometer or bicycle. Corresponding electrical sensor signals are generated and supplied to a memory and/or evaluation unit for further processing.