On perturbation and pattern coexistence in postural coordination dynamics

In studies of postural control, investigators have used either experimentally induced perturbations to stance or unperturbed stance. The distinction between perturbed and unperturbed stance has gained renewed importance in the context of in-phase and antiphase coordination of the hips and ankles. Several contributions have replicated the findings published over the past decade, suggesting the possibility of a unified view of postural control. However, any proposed unified view depends on how so called perturbed and unperturbed are defined. The authors argue that, to date, there is no explicit and general definition of those terms. The main reason is that all perturbations are relative and depend on appropriate frames of reference for perception and action. Arguments about empirical or theoretical unification of perturbed and unperturbed stance are premature.