Abstract: An adjustable seat post assembly for a bicycle includes a movable seat for a rider of the bicycle and an adjustable mounting device for mounting the seat on the bicycle so as to enable the seat to be moved between a first, normal riding position and a second position upwardly and rearwardly of the first position wherein the lumbar spine of a rider sitting on the seat passively assumes a protective curvature. A braking assembly enables the seat to be stopped at any selected position along a linear path of movement between the first and second positions. In one embodiment, a pair of parallel elongate arms are secured to the seat and are movable axially to provide movement of the seat, and a mount through which the arms slide is secured to the bicycle. In another embodiment, the seat is affixed to a sliding block which is received in an elongate slot formed in a body member secured to the bicycle so that the seat can be moved to different positions along the body member under the control of the rider.