The invention relates to a backrest assembly for a wheelchair with a backrest frame pivotally connected to a seat or seat frame, a carrying means provided by or attached to the backrest frame, a rotating means rotatable secured to the carrier means, a first sliding means slidable connected to the backrest frame on the rear side of the backrest assembly and connected to the rotating means via a first mechanical link, a second sliding means slidable secured to the backrest frame on the front of the backrest assembly and connected to the rotating means via a second mechanical link, and an actuator for adjusting the backrest assembly between an upright and reclined position with the actuator being connected to the first sliding means for driving the second sliding means via the rotating means to perform a sinusoidal movement.