The invention relates to a backrest assembly of a seat of a chair, in particular a wheelchair, having a seating frame (4) comprising tubes (4', 4"), with the backrest assembly comprising: a backrest for supporting the back of a person； two hollow backrest tubes (6, 7； 60, 70) connecting the backrest to the seat on each side of the seat: at least two joints (14； 90, 91) for allowing a movement of the backrest tubes (6, 7； 60, 70) between at least a first end position and a second end position； two locking means (130) for locking and/or unlocking the joints (14； 90, 91)； and an hollow handle tube (8； 80) having only one handle (10； 100) for effecting the locking means (130) to lock and/or unlock the joints (14； 90), wherein the handle (10； 100) is arranged in the middle between the backrest tubes (6, 7； 60, 70) and is mechanically connected to the locking means (130) via cables (120) running substantially inside the hollow tubes (6, 7, 8； 60, 70, 80) to effect at least the unlocking of the locking means (130) and the movement of the backrest tubes (6, 7； 60, 70) via the joints (14； 90, 91) when gripped by one hand (20) of an user, without releasing the respective grip of the handle (10； 100), which is engineered to be effected by at least partly sliding, rotating, squeezing, pushing and/or pulling, and wherein the locking means (130) automatically re-lock after effecting the unlocking of the joints (14； 90, 91) and reaching the first or second end positions.
