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Figure 6: Single stance test (ST): model of interpretation. The impairment of stability is faced first by widening the postural cone (PIxy) and then by activating the precautionary strategy (Ps, black area of the cone in %). Then, if necessary, Ps increases further while PIxy tends progressively to become smaller. This behavior is observed both in eyes open and eyes closed (EC) conditions, even if in the latter condition it is more evident. The stability index of EC trials is an indicator of the effectiveness of proprioceptive control, even if the vestibular component is not excluded. From top to bottom higher values of stability index indicate more refined proprioceptive control. Didactically four functional zones can be identified (their dimensions are approximate). At the lower level of stability, the subject is unable to maintain the single stance without continuous hand support and cannot walk without aid. ST levels correspond to different antigravity movement performances.