Thresholds for step initiation induced by support-surface translation: a dynamic center-of-mass model provides much better prediction than a static model

The need to initiate a step in order to recover balance could, in theory, b e predicted by a static model based solely on displacement of the center of mass (COM) with respect to the base of support (BOS), or by a dynamic mode l based on the interaction between COM displacement and velocity. The purpo se of this study was to determine whether the dynamic model provides better prediction than the static model regarding the need to step in response to moving-platform perturbation. The COM phase plane trajectories were determ ined for 10 healthy young adults for trials where the supporting platform w as translated at three different acceleration levels in anterior and poster ior directions. These trajectories were compared with the thresholds for st ep initiation predicted by the static and dynamic COM models. A single-link -plus-foot biomechanical model was employed to mathematically simulate term ination of the COM movement, without stepping, using the measured platform acceleration as the input. An optimization routine was used to determine th e stability boundaries in COM state space so as to establish the dynamic th resholds where a compensatory step must be initiated in order to recover ba lance. In the static model, the threshold for step initiation was reached i f the COM was displaced beyond the BOS limits. The dynamic model showed sub stantially better accuracy than the static model in predicting the need to step in order to recover balance: 71% of all stepping responses predicted c orrectly by the dynamic model versus only 11% by the static model. These re sults support the proposition that the central nervous system must react to and control dynamic effects, i.e. COM velocity, as well as COM displacemen t in order to maintain stability with respect to the existing BOS without s tepping, (C) 2000 Elsevier Science Ltd. All rights reserved.