A portable device for measuring electromyographic signals of a user, said portable device comprising: - a first support layer comprising detection means including several electrodes arranged, in use, in contact with the skin of a user on at least one muscle, or part thereof, and configured to acquire a plurality of electromyographic signals, said first support layer being mechanically and electrically attached in a detachable manner by means of electro-conductive couplings to a second support layer, and said first support layer being incorporated in or attached to a wearable item of clothing including an armband, a strap for an upper or lower limb of the body of the user, an abdominal band, a trousers or a shirt; - said second support layer including electronic means configured to perform the conditioning by amplification and filtering of said plurality of acquired electromyographic signals, conversion of the electromyographic signals to a digital format and transmission of the conditioned and digitized electromyographic signals through a communication channel of the second support layer to a master electronic unit of the portable device; and - said master electronic unit is configured for controlling said electronic means and to further transmit the received conditioned and digitized electromyographic signals to a control unit for monitoring thereof, wherein the portable device comprises a plurality of second support layers, characterized in that the plurality of second support layers comprises electrical connectors adapted to be mechanically and electronically attached to each other to increase the detection area and to exchange both analog and digitized information by the electrical connectors and the communication channel, and wherein the plurality of second support layers are adapted to share said master electronic unit, wherein the electronic means of a second support layer being mechanically and electrically attached to another second support layer are configured to control some of adjacent electrodes of said first support layer attached to this another second support layer during the performing of said conditioning, wherein the plurality of second support layers are mechanically and electrically attached in a detachable manner to the first support layer, in different parts of said wearable item of clothing, the different parts selected depending on the muscle or muscles from which the electromyographic signals have to be measured according to the interest of the exercise carried out by the user, and wherein the master electronic unit is configured to control each one of said electronic means of the plurality of second support layers. - a first support layer comprising detection means including several electrodes arranged, in use, in contact with the skin of a user on at least one muscle, or part thereof, and configured to acquire a plurality of electromyographic signals, said first support layer being mechanically and electrically attached in a detachable manner by means of electro-conductive couplings to a second support layer, and said first support layer being incorporated in or attached to a wearable item of clothing including an armband, a strap for an upper or lower limb of the body of the user, an abdominal band, a trousers or a shirt; - said second support layer including electronic means configured to perform the conditioning by amplification and filtering of said plurality of acquired electromyographic signals, conversion of the electromyographic signals to a digital format and transmission of the conditioned and digitized electromyographic signals through a communication channel of the second support layer to a master electronic unit of the portable device; and - said master electronic unit is configured for controlling said electronic means and to further transmit the received conditioned and digitized electromyographic signals to a control unit for monitoring thereof, wherein the portable device comprises a plurality of second support layers, characterized in that the plurality of second support layers comprises electrical connectors adapted to be mechanically and electronically attached to each other to increase the detection area and to exchange both analog and digitized information by the electrical connectors and the communication channel, and wherein the plurality of second support layers are adapted to share said master electronic unit,