Estimation of the centre of mass for the study of postural control in Idiopathic Scoliosis patients: a comparison of two techniques.

The objective of the present study is to quantify the position of the Centre of Mass (COM) during quiet standing using a force plate and compare this technique to the quantification of the COM with an anthropometric model. The postural control of 18 healthy adolescents and 22 IS patients was evaluated using an Optotrak 3D kinematic system, and two AMTI force plates during quiet standing. The position of anatomical landmarks tracked by the Optotrak system served to estimate the position of the COM of both groups using an anthropometric model (COM(anth)). The force plate served to estimate the position of the COM through double integration of the horizontal ground reaction forces (COM(gl)). The mean position and root mean square (RMS) amplitude of COM(gl, )in reference to the base of support (BOS) and the first sacral prominence (S1) were quantified in the Anterior-Posterior (A/P) and Medial-Lateral (M/L) directions. There was a significant difference between the control subjects and IS patients for the displacement of the COM(gl) in reference to the BOS in both the A/P and M/L directions. There was no difference between groups for the mean position of the COM(gl), however, 63% of the IS and 43% of the controls had a lateral position of the COM(gl )in reference to S1 of greater than 5 mm. There was a significant difference between groups in the A/P and M/L directions for the amplitude of error between the COM(gl) and COM(anth) techniques.