Reaction null-space filter: extracting reactionless synergies for optimal postural balance from motion capture data

Abstract

This paper introduces the notion of a reactionless synergy: a postural variation for a specific motion pattern/strategy, whereby the movements of the segments do not alter the force/moment balance at the feet. Given an optimal initial posture in terms of stability, a reactionless synergy can ensure optimality throughout the entire movement. Reactionless synergies are derived via a dynamical model wherein the feet are regarded to be unfixed. Though in contrast with the conventional fixed-feet models, this approach has the advantage of exhibiting the reactions at the feet explicitly. The dynamical model also facilitates a joint-space decomposition scheme yielding two motion components: the reactionless synergy and an orthogonal complement responsible for the dynamical coupling between the feet and the support. Since the reactionless synergy provides the basis (a feedforward control component) for optimal balance control, it may play an important role when evaluating balance abnormalities or when assessing optimality in balance control. We show how to apply the proposed method for analysis of motion capture data obtained from three voluntary movement patterns in the sagittal plane: squat, sway, and forward bend.

Keywords:

• Postural balance analysis
• optimal balance control
• unfixed-base dynamics
• reactionless synergy
• reaction null-space

Notes

No potential conflict of interest was reported by the authors.

1 For a given nonsquare matrix, there is an infinite number of generalized inverses (Ben-Israel & Greville 2003).

2 A nonzero implies larger variation of the CoP, depending upon the specific motion pattern.

Additional information

Funding

This work has been done without external financial support.