Abstract: A device for measuring at least one current mechanical state parameter of a mobile counterweight on an exercise machine with counterweights, in which the counterweight comprises a plurality of weights placed one on top of another, whose number can be selected using a selection pin, which can be attached as required to one of the weights, so as to define a counterweight with a predetermined weight. The device comprises means for generating a signal that indicates the instantaneous position of the pin relative to a predetermined reference part and means for detecting the signal. The generation means and the detector means are directly opposite one another and separated by the distance which the signal covers only once so that the length of the signal path is minimized.