Two strap members are respectively connectable with a seat member and a backrest member and are joined for relative pivoting movement. An inner and a cooperating outer gear annulus are provided on the respective strap members in engagement with one another. A brake sleeve is provided on one of the strap members and a helical spring is located within the brake sleeve and normally frictionally engages an inner surface thereof. The spring has two inwardly angled end portions. An output member is turnably mounted within the sleeve and has a first section extending between the end portions of the spring, and a turnable control member is adjacent the output member and has a second section also extending between the end portions. A torsion spring engages the input and control members and urges the sections thereof into engagement with the respective end portions of the helical spring.