example of the electromagnetic type which are spaced along the axis 4 and are applied to a housing 40 which is fixed to the supporting structure 5 and surrounds the upper portion of the first tubular body 8 the operation of the fabric tension application device according to the invention is as follows when the knitting of the fabric begins the plug like element 11 is spaced downward from the first tubular body 8 which is connected to an aspiration device so as to draw inside it the initial portion of the formed knitted fabric the first piston 12 is engaged by the hook like elements 33 and the second piston 23 is in the maximum rise condition by virtue of the action of the springs 31 since the second chamber 24 is connected to the atmosphere by the electric valve 30 fig1 after the initial portion of fabric has been drawn into the first tubular body 8 the plug like element 11 is raised so as to enter the lower end of the first tubular body 8 locking the fabric at this stage the position of the electric valve 30 is switched so as to cause the progressive descent of the second piston 23.