272 and the spacer 274 are made of rubber when assembled the pulley systems 23 are assembled as detailed above the first ends 2324 of the pivot shafts 232 are fastened in the connecting holes 216 of the upper support 21 the pinion gears 266 are fastened to the movable rack 25 as detailed above the resisting device 27 is mounted to the movable rack 25 as detailed above the cable 242 is assembled to the movable rack 25 the pulley systems 23 and the coil spring 292 of the fixing module 29 as detailed above the rack gears 267 are fastened along the two sides of the main support 22 respectively then the upper support 21 and the movable rack 25 are coupled to the main support 22 with the side sheets 214 of the upper support 21 and the resisting device 27 mounted to the movable rack 25 facing the main support 22 the upper support 21 is fixed to the main support 22 the second ends 2326 of the pivot shafts 232 engages in the connecting holes 229 of the main support 22 the rack gears 267 engage with the pinion gears 266 correspondingly the shaft 297 is extended through the main support 22 the bearing 296 and the washers 295 thereby fixing the inner end of the coil spring 292 to the main support 22 thus the adjustable stand 20 is assembled the support bracket 30 is fixed to the movable rack 25 and the base 40 is fixed to a.