Seatbelt for chairs

Office chairs have curvy backs and a thousand levers for control, but I've never sat in one that has been comfortable for an extended period. This is caused by the weight of the torso being supported at the top of a jointed structure, rather like the plate-on-stick balancing clowns. In this case muscles are required to resist the inevitable moment generated as the centre of mass lies at a point longitudinally separate from the base of the spine.

Loosely strap the upper body against the back of the chair to take the forward pitching moment of the torso and relegate poor posture and consequent back problems to history. For those who currently slight their posterior forwards in order that the moment is resisted by the back of the chair (adopting the deep sofa position), a lap belt can be provided as well.