post 26 having at its upper end a cross sleeve 27 which receives a horizontal tube 28 the horizontal tube is provided at one end with a cross sleeve 29 as shown in fig5 journalled in the cross sleeve 29 is a clutch shaft 30 one end of which protrudes from the cross sleeve and receives a pair of clutch discs 31 the clutch discs 31 are provided with clutch slots 32 and clutch rollers 33 the clutch slots 32 are so arranged that when the clutch shaft 30 is turned in one direction one clutch disc 31 turns therewith and when the clutch shaft is turned in the opposite direction the other clutch disc turns therewith the clutch shaft 30 is joined to a short cross sleeve 34 disposed coaxially with the cross sleeve 29 the cross sleeve 34 forms the base end of a patient actuating unit the cross sleeve 34 is joined to a radially extending sleeve 35 which receives an extension member 36 the extremity of the extension member 36 is provided with a lateral arm 37 for engagement by the patient the clutch discs 31 includes peripheral portions which are received in force adjusting units including a common housing 38 secured to the cross sleeve 29 by a bracket 38a and has a pair of slots 39 which receive peripheral portions of the clutch discs 31 referring to fig7 the housing 38 forms a pair of oppositely directly cylinders 40 which receive a pair of pistons 41 one end 42 of each piston engages a corresponding clutch disc 31 the piston ends 42 are disposed in alignment with backup discs 43 at the opposite side of the corresponding clutch.