A chair seat backrest assembly, in particular for a wheelchair, comprising: a backrest for supporting the back of a person; two hollow backrest tubes (1.14) adapted for connecting the backrest to a seat on each side of the seat; at least two joints (4.1) for allowing a movement of the backrest tubes (1.14) between at least a first end position and a second end position relative to the seat; two locking means (2.3) for locking and/or unlocking the joints (4.1); and at least one hollow handle tube (1.13) carrying a handle (1.9, 1.10), which is arranged in the middle between the backrest tubes (1.14) and is mechanically connected to the locking means (2.3) via cables (1.3) running substantially inside the hollow tubes (1.14, 1.13), with the cables (1.3) being torsion cables guided from the two hollow backrest tubes (1.14) via two handle tubes (1.13) into the handle (1.9, 1.10) and being connected to each other via a connection rod (1.2), wherein the handle (1.9, 1.10) • is providing a twist lock (1.1), with the twist lock (1.1) having only one idle position in which the locking means (2.3) is locked and being adapted to unlock the locking means (2.3) upon rotation of the handle (1.9, 1.10), and • is suited to move the backrest tubes (1.14) via the joints (4.1) by pushing or pulling after unlocking, the handle (1.9, 1.10) is adapted to be rotated by a user only in one direction for unlocking the locking means (2.3) in any one of the end positions, and the handle (1.9, 1.10) is adapted to automatically re-lock after unlocking the locking means (2.3) and releasing the grip, such that rotating and pushing or pulling of the handle (1.9, 1.10) can be achieved by gripping the handle (1.9, 1.10) by one hand of an user, without releasing the respective grip of the handle (10; 100), characterized by a cable guide (1.6, 1.7) for each of the two cables (1.3), with the cable guides (1.6, 1.7) being arranged within the handle (1.9, 1.10) and attaching the connecting rod (1.2) between the two cable guides, and with each cable (1.3) being connected to one locking means (2.3) via an elastic element being suited to be compressed or expanded within a predetermined range, wherein each cable guide (1.6, 1.7) provides a cable deflection slope, and each cable (1.3) is redirected or deflected inside the handle (1.9, 1.10) via its respective cable guide (1.6, 1.7) upon rotation of the handle (1.9, 1.10) against the spring force of the respective elastic element for unlocking the locking means (2.3) and automatically returned into its idle state upon release of the handle (1.9, 1.10) due to the release of the spring force such that the handle (1.9, 1.10) automatically returns into its idle position. a backrest for supporting the back of a person; two hollow backrest tubes (1.14) adapted for connecting the backrest to a seat on each side of the seat; at least two joints (4.1) for allowing a movement of the backrest tubes (1.14) between at least a first end position and a second end position relative to the seat; two locking means (2.3) for locking and/or unlocking the joints (4.1); and at least one hollow handle tube (1.13) carrying a handle (1.9, 1.10), which is arranged in the middle between the backrest tubes (1.14) and is mechanically connected to the locking means (2.3) via cables (1.3) running substantially inside the hollow tubes (1.14, 1.13), with the cables (1.3) being torsion cables guided from the two hollow backrest tubes (1.14) via two handle tubes (1.13) into the handle (1.9, 1.10) and being connected to each other via a connection rod (1.2), wherein the handle (1.9, 1.10) • is providing a twist lock (1.1), with the twist lock (1.1) having only one idle position in which the locking means (2.3) is locked and being adapted to unlock the locking means (2.3) upon rotation of the handle (1.9, 1.10), and • is suited to move the backrest tubes (1.14) via the joints (4.1) by pushing or pulling after unlocking, • is providing a twist lock (1.1), with the twist lock (1.1) having only one idle position in which the locking means (2.3) is locked and being adapted to unlock the locking means (2.3) upon rotation of the handle (1.9, 1.10), and • is suited to move the backrest tubes (1.14) via the joints (4.1) by pushing or pulling after unlocking, the handle (1.9, 1.10) is adapted to be rotated by a user only in one direction for unlocking the locking means (2.3) in any one of the end positions, and the handle (1.9, 1.10) is adapted to automatically re-lock after unlocking the locking means (2.3) and releasing the grip, such that rotating and pushing or pulling of the handle (1.9, 1.10) can be achieved by gripping the handle (1.9, 1.10) by one hand of an user, without releasing the respective grip of the handle (10; 100), characterized by a cable guide (1.6, 1.7) for each of the two cables (1.3), with the cable guides (1.6, 1.7) being arranged within the handle (1.9, 1.10) and attaching the connecting rod (1.2) between the two cable guides, and with each cable (1.3) being connected to one locking means (2.3) via an elastic element being suited to be compressed or expanded within a predetermined range, wherein each cable guide (1.6, 1.7) provides a cable deflection slope, and each cable (1.3) is redirected or deflected inside the handle (1.9, 1.10) via its respective cable guide (1.6, 1.7) upon rotation of the handle (1.9, 1.10) against the spring force of the respective elastic element for unlocking the locking means (2.3) and automatically returned into its idle state upon release of the handle (1.9, 1.10) due to the release of the spring force such that the handle (1.9, 1.10) automatically returns into its idle position.