Described is a device for a gym machine comprising a frame (2), at least one resistant load (3) mounted on the frame having at least one coupling surface (4), at least one flexible cable (5) associated with the resistant load (3) for performing a physical exercise, the device being characterized in that it comprises at least one support (6) designed to be rigidly associated with the resistant load (3) at the coupling surface (4), at least one shaft (7) rotatably coupled to the support (6) on a first axis (8) at a first portion (9) and having a second portion (10) with a second, eccentric axis (11), power drive means (12) for rotationally driving the shaft (7) about the first axis (8), the shaft (7) being associated with the flexible cable (5) at the second portion (10) in such a way as to generate vibrations along the flexible cable (5).