Exercise dumbbells have pairs of weights disposed at opposite ends of a handle. The chairs are provided with coupling devices with which they can be coupled to the load-bearing and traction cable. The transmission of torque to the machinery through the clutch is briefly interrupted or reduced when the engine risks stalling. This enables the diameter of the sprockets to be made smaller. The lower disk wheel moves forward and backward to conform to the contours of the body. The set of permutable functions includes chaotic maps generated by a composite function of first and second functions, and an inverse of the first function.