About correlation between centre of pressure, trunk and head sways during quiet upright stance.

In order to better understand the mechanisms of orthostatic balance, centre of pressure (CoP), trunk, and head sways were simultaneously measured during quiet upright stance, and compared to each other, looking for possible correlation. Methods - A total of 16 healthy young adults served as subjects. They were asked to keep orthostatic position for 120 s, once with open and once with closed eyes. COP sway was measured by force platform, trunk oscillation by inclinometers placed at the sternum level, trunk and head angular velocities by miniature gyroscopes, placed at the sternum and the skull vertex, respectively. Cross-correlation functions were used to compare sways measured at different levels to each other. In particular, antero-posterior (AP) and medio-lateral (ML) components of CoP sway were correlated to the homologous components of trunk oscillation, and AP and ML trunk oscillation velocities to the homologous head oscillation velocities. Results – Clear and consistent positive correlation was found between trunk and head sways. In 83% of cases, cross-correlation functions presented one sharp peak near the origin, and much lower values elsewhere. Mean correlation delays in the different conditions (open or closed eyes) and directions (AP and ML) weren't significantly different from 0 at p=0.05 (t-test). Cross-correlation functions computed inside a 20 s sliding window showed that correlation was kept all along the test duration. CoP-trunk correlation was much less clear. Cross-correlation peaks as sharp as those observed between trunk and head velocities could never be observed. However, in many instances the two sways did show almost parallel time courses. This could either extend to almost the whole test duration or be limited to shorter periods, and was much more frequent in ML than in AP plane, where, in general, the two sways appeared to be uncorrelated. in a few instances, in the AP plane they semed to be phase opposed. Discussion – Sharp cross-correlation peaks with almost no delay show that trunk and head moved almost synchronously, like one rigid body. This was consistent with the fact that in either plane (AP or ML) and vision condition (eyes open or closed) the sway velocity distributions of trunk and head were similar to each other, and the corresponding average velocity ranges were not significantly different from one another at p=0.05 (t-test). Poorer correlation between CoP and trunk sways is a likely consequence of the presence of the hip joint, and the inertia of the trunk-arms-head system, linked to this joint. This makes the trunk over the hip intrinsically less steady than the head over the neck. Interestingly enough, CoP-trunk correlation was better in ML plane than in AP one, and this may easily be explained by the different stiffness of the hip joint in the two planes, due to the different geometry. Different degrees and modes of correlation between CoP and trunk sway suggest the use of different balance strategies. It is also possible that the two sways convey different, maybe complementary, information about balance control, and it could possibly make sense to consider also trunk movements in the diagnostic approach to balance control. This conclusion is consistent with previous suggestions by different authors.