a-d: Length-adjustment mechanism. 2a shows the length-adjustment mechanism in detailed side view (see also Fig. 1c), 2b shows a 90 degree rotated side view section, longitudinally cut through the axle, 2c and 2d show transversal sections as viewed from above in 2b. The stopping mechanism for shortening consists of a stopping bolt (2 a) which can be rotated via an adjustment wheel (2) by an adjustment motor (2b). The position of the bolt is measured by a potentiometer (2c). The pre-stretching mechanism (3, 3a, 3b, 3c) can be independently adjusted. The adjustment wheels rest via bearings on two parts of a guiding tube (4) so that there is an air gap (4a) that prevents any direct contact between the adjustment wheels and the axle (1). A third bolt (1a), which moves parallel to the thumb lever (see 3 in Fig. 1a), is mounted on the axle.