A seat back assembly (16) for a seat (10), comprising an upper seat back portion (24) for supporting an upper portion of a user's back and a lower seat back portion (22) for supporting a user's pelvis and pelvic region. The lower seat back portion (22) and the upper seat back portion (24) are independently adjustable. The upper seat back portion (24) can be reclined to one of a number of fixed angular positions and it is arranged and configured for dynamic angular movement under a load over a range of positions. The dynamic movement of the upper seat back portion (24) is controlled by a shock absorber (44) having adjustable damping means for providing a required level of tension and rebound in relation to the dynamic back action of the upper seat back portion (24).