Studies of motor learning and robotic neurorehabilitation often involve walking with the patient suspended by means of a Body Weight Support (BWS) and guided by a robotic orthosis. This paper describes an innovative research carried on to establish the laws of motion of an active exoskeleton, called P.I.G.R.O. (Pneumatic Interactive Gait Rehabilitation Orthosis), designed by the authors and often used in suspended walking treatments as this avoid the iper-activation of the patient's antigravity musculature in the preliminary steps of the rehabilitation treatment. These experimental tests were carried on with ten healthy subjects wearing the robotic orthosis used as a means of acquiring hip-knee and ankle behavior in this special walking condition with the volunteers' joint angles recorded by the potentiometers of the orthosis. So the ESUWS (Experimental Spontaneous Unloaded Walking Set) curves were plotted. The latter were compared with the theoretical curves of the over-ground walking (Physiological Walking Set - PWS) in order to construct proper reference curves of the healthy spontaneous suspended walking (Authors' Unloaded Walking Set - AUWS), also compared with physiological curves in over-ground walking. The results obtained are highly satisfactory and useful for future neuro-rehabilitation training for brain strokes and ictus.