Stability criterion for controlling standing in able-bodied subjects.

A new stability criterion that can be used to assess the standing condition of a subject from center of pressure (CoP) measurements is presented. This criterion can be applied, for example, to control a standing prosthesis, which should allow a paraplegic subject to stand up, sit down and stand safely without using hands for support. Experiments conducted with able-bodied subjects enabled us to establish a relationship between its stability and the subject's CoP position. Four CoP stability zones were identified: high preference, low preference, undesirable and unstable zones. The high preference zone is defined as the area where the CoP is found 99% of the time during quiet standing. The area where the CoP is found during the remaining 1% of the time is called the low preference zone. The undesirable zone is defined as the CoP area where the subject is forced to change posture in order to maintain balance, and the unstable zone is defined as the CoP area in which the subject is forced to step forward, backward or sideways to maintain stability. A general model of the proposed four stability zones was derived, which can be used to compute stability zones a priori for any subject and thus allows one to assess the subject's stability condition from the CoP measurements.