The device (1) takes up the basic structure and operation of the McKibben type artificial muscles and similar, in which is provided a hollow cylindrical chamber (2), of resilient material, which is inflated in the active phase (K). The chamber (2) interacts with a braided sleeve (3), consisting of threads (31, 32), almost inextensible, arranged in crossed helical paths having a characteristic angle of inclination β with respect to the longitudinal axis (X) of the device (1). The sleeve (3) is connected at the ends with two rigid head members (4A, 4B) provided to be mechanically connected to external bodies. The invention provides adjusting means (5), associated to said rigid head members (4A, 4B) adapted to vary their distance (D) at rest, so as to require a proportional and consistent change in the angle of inclination β: if the latter is exactly 54.7°, the device (1), in the active phase (K) stiffens without dimensional changes, if the angle β is greater an axial extension is obtained, if the angle β is smaller an axial contraction is obtained.
