a of the belt 2 rests on a plate or base 5 e of which the surface which comes into contact with the belt is treated in order to reduce friction the beam 5 may be of one piece construction or consist of two parallel members 5 b 5 c which form slit 5 a the members being joined two at a time by a lateral member 5 d beam 5 regardless of the embodiment may also be joined at the level of its lower surface and on the side of its external arm to another member 6 essentially having the function of further increasing the rigidity of beam 5 in addition member 6 projects beyond the transverse sides of beam 5 and may have mounting feet 7 in order to install the drive rollers 8 and 9 of the belt 2 in a conventional manner one of the rollers 8 is motor driven for example roller 8 may have an integral motor or be connected by any known appropriate means to an independent motor 10 so as to drive each of the belts 2 in a direction that is opposed to the direction of walking the chassis assembly as defined rests on floor support plate 4 with the force meters 3 in between them for instance the lower side of each of the beams 5 has a precision ground rail 11 to which one of the ends of the force meters 3 is attached the other end of the force meters is attached to parallel rails 4 a of the common floor support plate 4 both the chassis are absolutely identical and are arranged opposite each other in parallel and are joined to the common floor plate 4 via the force meters 3 leaving a very small gap e.