A portable exercising device for developing, strengthening and/or rehabilitating the muscles or joints of the body by working against a counterforce, at least comprising: - transmission means, being configured for being rotated by an electric motor and for transmitting this rotation to a rotating body, so as to link the electric motor to the rotating body, - a rotating body, being connected to the transmission means and wherein the rotating body is configured for being turned to retract a rope, cord, webbing or cable, - an electric motor, being connected to the transmission means and wherein the electric motor is configured for driving and/or retarding the rotation of the transmission means and the rotating body according to a specific parameter or parameters sent by a control unit, - a control unit, being connected to the electric motor, wherein the control unit is configured for at least commanding the electric motor rotation during the exercise of a specific motion on the basis of at least an input value selected by a user through a user interface, - a rope, cord, webbing or cable, being directly or indirectly attached at one end to the rotating body, and wound around an exterior surface of the rotating body and the opposed end of the rope, cord, webbing or cable is configured for being pulled by the user to exert a desired force, and - a user interface, for selecting at least a specific input value of the exercise and wherein the user interface is connected to the control unit and - means for supplying electrical current to the electric motor, - an outer enclosure (2, 12, 13) containing at least the transmissions means and the electric motor, - a support fixed on a fixed part of the exercising device and wherein the support is configured for fixing the exercising device on different training equipment or supports; the exercising device being compact and lightweight so it can be moved without much effort according to the user's preferences to many types of exercise equipment or other supports. - transmission means, being configured for being rotated by an electric motor and for transmitting this rotation to a rotating body, so as to link the electric motor to the rotating body, - a rotating body, being connected to the transmission means and wherein the rotating body is configured for being turned to retract a rope, cord, webbing or cable, - an electric motor, being connected to the transmission means and wherein the electric motor is configured for driving and/or retarding the rotation of the transmission means and the rotating body according to a specific parameter or parameters sent by a control unit, - a control unit, being connected to the electric motor, wherein the control unit is configured for at least commanding the electric motor rotation during the exercise of a specific motion on the basis of at least an input value selected by a user through a user interface, - a rope, cord, webbing or cable, being directly or indirectly attached at one end to the rotating body, and wound around an exterior surface of the rotating body and the opposed end of the rope, cord, webbing or cable is configured for being pulled by the user to exert a desired force, and - a user interface, for selecting at least a specific input value of the exercise and wherein the user interface is connected to the control unit and - means for supplying electrical current to the electric motor, - an outer enclosure (2, 12, 13) containing at least the transmissions means and the electric motor, - a support fixed on a fixed part of the exercising device and wherein the support is configured for fixing the exercising device on different training equipment or supports;