Study methods

Dynamic systems research typically examines postural control by disturbing posture and then observing how the system returns to its original state. The current study examines how people con-trol their posture in the absence of such externally generated disturbances. Previous studies have examined postural coordination and have shown that postural sway is modulated in a task specific manner (Bardy, Marin, Stoffregen, and Bootsma, 1999; Marin, Bardy, and Bootsma, 1999; Bardy, Oullier, Bootsma, and Stoffregen (2002). However, this research has not examined postural coordi-nation modes amidst multiple simultaneous constraints (e.g., task, environment). Our previous re-search has demonstrated that different coordination strategies, as measured by hip-ankle ratios, emerge as a function of support surface during quiet stance. In this study we manipulated both the surface of support (hard surface vs. foam roller) and suprapostural task (tracking frequency) in a continuous manner by ramping frequency up or down within a trial. In this experiment, we sought to determine the dynamical nature of coordinative strategies based on changing suprapostural task to-gether with support surface