pivoting section 140 are separated then it is replaced with another hand back supporting member and engaged in addition a velcro tape is adhered to an inner surface of one of the hand back supporting members 150 151 so that a second cushion pad refer to fig2 152 is adhered by the velcro tape further a rubber washer 155 covering the bolt 163 is provided to between the hand back pivoting section 140 and one of the hand back supporting members 150 151 thereby preventing the hand back pivoting section 140 and one of the hand back supporting members 150 151 from being damaged due to a tightening force of the bolt 163 hereinafter the rail 115 and the slider 120 provided to the wrist section 110 will be more specifically described referring to fig4 again a slide recess 113 is formed in the middle of the width of the wrist section 110 in a direction from the leading end to the rear end and the rails 115 are formed on both opposed inner surfaces of the slide recess 113 in a longitudinal direction of the slide recess 113 a plurality of engaging recesses 117 are provided to the rails 115 along the length thereof meanwhile fig6 is a partial sectional view of the slider shown in fig4 and fig7 is a cross sectional view of the slider moving along the rail shown in fig4 referring to fig6 and 7 the slider 120 located in the slide.