Identification of the relationship between center of pressure and ankle angle during standing of normal subjects

The relationship between movement of center of pressure (COP) and the ankle angle in one foot in the sagittal plane was examined during standing of a normal subject. The body was regarded as an inverted pendulum, where bracing prevented flexion of the hip and knee joints. The center of pressure was calculated from recorded force plate data, where the subject was standing with one foot on a force plate. The ankle angle was measured using 3D coordinates from optical markers. A 2nd order transfer function between COP and ankle angle was estimated using a system identification method. The results showed that COP can be related to the ankle angle during standing, when the body is regarded as an inverted pendulum. This indicates that COP provides information about the body movement. It is suggested that COP possibly can be used as feedback to stabilize the inverted pendulum