Patterns of inter-joint coordination during a single-limb standing Wen Liu a, *, Marcio J. Santos b , Kevin McIntire c , Janice Loudon a , Heather Goist-Foley d , Gregory Horton d a Department of Physical Therapy and Rehabilitation Science, University of Kansas Medical Center, Kansas City, KS, USA b Physical Therapy Department, Santa Catarina State University, Florianopolis, Brazil c Department of Nephrology, Stanford University, School of Medicine, Palo Alto, CA, USA d Department of Orthopedic Surgery, University of Kansas Medical Center, Kansas City, KS, USA 1. Introduction The ability to maintain a controlled upright posture is essential for daily activities and is made possible through limited number of control strategies that have evolved over time. Although redun- dancies exist among various musculature and kinematical combinations, the central nervous system may simplify the control task with a limited number of muscular strategies/movement patterns to be chosen from [1]. For instance, ankle and hip strategies have been observed in a double-limb standing perturbed along the anterior–posterior axis [2–5]. When an ankle synergy is evoked the torso and legs move in the same direction about the ankle joint. When a hip strategy is evoked, the torso and legs move in the opposite direction about the hip joint. Muscles involved are not activated in response to a stretch of each individual joint, rather according to a muscular synergy leading to a synergistic movement pattern [2,6]. Control of a double-limb standing under a perturbation along the medio-lateral axis showed the coordination primarily in motions at the hip and trunk [7,8]. Studies of muscular synergies/movement patterns can help us to better understand human postural control [9]. Movement patterns during a single-limb standing has not been studied extensively. Biomechanically when compared to double- limb standing, the base of support against lateral bending and/or axial rotation in single-limb standing significantly decreases from the distance between two feet to the width of the supporting foot. The control strategies in maintaining upright single-limb standing, presented as movement patterns, are therefore expected to be different from that in double-limb standing. Single-limb stance always presents with large multi-joint movements in the lower extremity that limits the use of posturographic analysis of center of pressure trajectory because it provides no information on movement pattern or joint-coordination. Some investigators examined movement patterns during a single-limb standing in studies of functional ankle instability [10,11], low back pain, or knee osteoarthritis [12,13]. Tropp and Odenrick [14] observed a central role of the ankle joint in postural corrections during a single-limb standing. Hoogvliet et al. [15] suggested that lateral foot tilting strategy (ankle supination/pronation) might be an important corrective movement for postural control during a single-limb standing. Riemann et al., [16] compared corrective actions of the trunk, ankle, knee and hip joints during a single-limb Gait & Posture 36 (2012) 614–618 A R T I C L E I N F O Article history: Received 24 July 2011 Received in revised form 17 May 2012 Accepted 12 June 2012 Keywords: Posture Balance Single-limb Standing Coordination Ankle Hip A B S T R A C T Past studies have documented contributions of multiple joints in maintaining a single-limb standing, but no reports on patterns of inter-joint coordination. It is also unknown whether such inter-joint coordination, if exists, depends on visual feedback. Eight health young volunteers took part in this study. The inter-joint coordination during a single-limb standing were examined using 3D joint kinematics. There were five testing trials with eyes open (EO) and five trials with eyes close (EC) conditions. During each trial the subject stood on the right leg on an even platform for 20 s while 3D kinematic data was recorded. Recorded data was processed for an ‘‘adjusted coefficient of multiple determinations (ACMD)’’ to evaluate the inter-joint similarities in joint motions. Under both EO and EC conditions moderate to good similarities were found in axial rotation between the ankle and hip joints, and between ankle inversion/eversion and hip axial rotation. This pattern of the inter-joint coordination might be a unique feature of biomechanical configuration of the lower extremity. The significant increases in joint rotations but maintained inter-joint coordination from EO to EC condition may indicate a minimal influence of vision on the inter-joint coordination. Future studies to test patients with pathological conditions in single-limb stance need to examine any alternation/impairment of the inter-joint coordination pattern. ß 2012 Elsevier B.V. All rights reserved. * Corresponding author at: Department of Physical Therapy and Rehabilitation Science, University of Kansas Medical Center, 3901 Rainbow Blvd., Kansas City, KS 66160-7601, USA. Tel.: +1 913 588 6799; fax: +1 913 588 4568. E-mail address: wliu@kumc.edu (W. Liu). Contents lists available at SciVerse ScienceDirect Gait & Posture jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/g aitp os t 0966-6362/$ – see front matter ß 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gaitpost.2012.06.005