Relative contributions of postural balance mechanisms reveal studying the CoP displacement alone may be incomplete for analysis of challenging standing postures.

Background: The mechanical consequences of the motor actions used to maintain upright standing balance can be discriminated in two mechanisms: i) moving the center of pressure (CoP) within the base of support (M1); and ii) modifying the whole-body angular momentum (M2). Since the contribution of M2 to the whole-body CoM acceleration increases with postural constraints, a postural analysis focusing only on the CoP trajectory (i.e. M1) could ignore the majority of the control actions in challenging postural tasks. The objective of this study was to determine the contributions of the two postural balance mechanisms across postures with different areas of the base of support.

Methods: Forty-one healthy young adults (19 females, 22.9 ± 2.9 years old) stood quietly on a forceplate, maintaining four different postures: bipedal, tandem, unipedal and unipedal on a 4-cm wooden bar; each with eyes open and for 60 s. Relative contributions of the two balance postural mechanisms were computed for each posture, in both directions of the horizontal plane.

Results: The posture impacted the mechanisms contributions, where the contribution of M1 decreased between each posture in the mediolateral direction as the area of the base of support was reduced. The contribution of M2 in the mediolateral direction was not negligible (about 1/3) in tandem and unipedal postures and became dominant (nearly 90% on average) in the most challenging unipedal posture.

Significance: This suggests the contribution of M2 should not be neglected for the analysis of postural balance, and particularly in challenging standing postures.