RESEARCH ARTICLE Asymmetry of foot position and weight distribution channels the inter-leg coordination dynamics of standing Zheng Wang • Karl M. Newell Received: 10 May 2012 / Accepted: 27 July 2012 Ó Springer-Verlag 2012 Abstract The study of quiet standing has mainly been conducted in the foot side-by-side position with the assumption that the contribution of the lower limbs is structurally and functionally equivalent. The purpose of this study was to examine how the two mechanical factors of foot position and weight distribution interact to influence postural control and inter-leg coordination dynamics. Participants were required, while standing in either a side-by-side, stag- gered, or tandem right foot forward position, to intentionally produce three different levels of weight distribution (50/50, 30/70, and 70/30) over the two feet. Our results showed that the interaction effects of the two mechanical constraints were represented in both linear and nonlinear analyses. The center of pressure (COP) mean velocity was predominantly influ- enced by body weight distribution in the side-by-side stance, whereas foot position was more influential in the tandem stance. The nonlinear analysis showed that the least experi- enced postural condition (i.e., tandem stance with a 70/30 loading level) had the lowest number and total duration of COP L –COP R phase synchronization epochs in the AP direction that were compensated by ‘‘stable’’ coordination dynamics in the ML direction. The findings revealed that the staggered stance represents a ‘‘hybrid’’ blend of the proper- ties of the side-by-side and tandem foot positions. Collec- tively, foot position and weight distribution interact to determine the stability and flexibility of inter-leg coordina- tion dynamics in postural control. Keywords Foot position  Weight distribution  Inter-leg coordination dynamics  Postural control Introduction To sustain standing posture, the neuromuscular control system must instantaneously support the passively unsta- ble, multi-leveled body components (e.g., head, torso, limb segments, muscles, etc.) against gravity to ensure the ver- tical projection of the center of mass (COM) is balanced within the base of support (Massion 1994; Riccio 1993; Nashner and McCollum 1985). Although stable control of undisturbed standing is well practiced and subconsciously processed for healthy subjects, it may be quite an achievement for individuals with central nervous system or peripheral musculoskeletal pathologies. Patients adapt different foot positions and/or asymmetric body weight- bearing strategies when required to stand upright. But, the general role of foot position and asymmetric body weight distribution in human postural balance is not well established. Upright standing in most previous studies has primarily been examined in the side-by-side position in which the lower limbs are aligned in parallel about hip or shoulder width apart assuming they are anatomically and function- ally identical. Independent control of limb position and contact forces has been found in a series of animal postural studies, but previous experiments with humans have not yet fully investigated the control mechanisms of foot position and weight distribution on upright stance (Lacquaniti et al. 1990; Lacquaniti and Maioli 1994a, 1994b). Winter et al. (1993, 1996) required participants to stand with different foot positions by loading their body weight evenly on the lower limbs and found that in side-by-side stance, COP NET motion in the anterior–posterior (AP) direction is pre- dominantly driven by ankle plantar- and dorsi-flexors and that motion in the medio-lateral (ML) direction is mainly determined by a hip loading/unloading mechanism. On the Z. Wang (&)  K. M. Newell Department of Kinesiology, The Pennsylvania State University, 23 Rec Building, University Park, PA 16802, USA e-mail: xiaoyaoxianshi2000@gmail.com 123 Exp Brain Res DOI 10.1007/s00221-012-3212-7