5 is provided with a first limit surface 101 and a second limit surface 102 the spindle drive rod 12 is provided with a third limit surface 103 the back protection plate 10 is provided with a fourth limit surface 104 a tenth limit surface 110 a twelfth limit surface 112 and a thirteenth limit surface 113 the second support rod 18 is provided with a fifth limit surface 105 the eighth link 19 is provided with a sixth limit surface 106 the fourth leg rod 7 is provided with a seventh limit surface 107 the second leg rod 5 is provided with an eighth limit surface 108 the locating plate 25 is provided with a ninth limit surface 109 the third link 9 is provided with an eleventh limit surface 111 a distance between mounting holes at two ends of the second support rod 18 is about 186 4 mm the foot rest board 1 is connected to an end of the first leg rod 3 and an end of the second leg rod 5 in a rotating manner through a spindle the foot board support 4 is connected to the first leg rod 3 and the second leg rod.