Contents lists available at ScienceDirect Human Movement Science journal homepage: www.elsevier.com/locate/humov Full Length Article The effects of dual tasking on gait synchronization during over- ground side-by-side walking Ari Z. Zivotofsky a,1, ⁎ , Hagar Bernad-Elazari b,1 , Pnina Grossman a,f , Jeffrey M. Hausdorff b,c,d,e a Gonda Brain Research Center, Bar Ilan University, Ramat Gan, Israel b Center for the Study of Movement, Cognition, and Mobility, Neurological Institute, Tel Aviv Sourasky Medical Center, Tel Aviv, Israel c Department of Physical Therapy, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel d Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel e Rush Alzheimer’s Disease Center and Department of Orthopaedic Surgery, Rush University Medical Center, Chicago, IL, United States f City College, New York, United States ARTICLE INFO Keywords: Stride length Stride time Synchronization Human Gait Dual tasking ABSTRACT Recent studies have shown that gait synchronization during natural walking is not merely an- ecdotal, but it is a repeatable phenomenon that is quantifiable and is apparently related to available sensory feedback modalities. However, the mechanisms underlying this phase-locking of gait have only recently begun to be investigated. For example, it is not known what role, if any, attention plays. We employed a dual tasking paradigm in order to investigate the role attention plays in gait synchronization. Sixteen pairs of subjects walked under six conditions that ma- nipulated the available sensory feedback and the degree of difficulty of the dual task, i.e., the attention. Movement was quantified using a trunk-mounted tri-axial accelerometer. A gait syn- chronization index (GSI) was calculated in order to quantify the degree of synchronization of the gait pattern. A simple dual task resulted in an increased level of synchronization, whereas a more complex dual task lead to a reduction in synchronization. Handholding increased synchroniza- tion, compared to the same attention condition without handholding. These results indicate that in order for two walkers to synchronize, some level of attention is apparently required, such that a relatively complex dual task utilizes enough attentional resources to reduce the occurrence of synchronization. 1. Introduction Two people who walk together from one point to another must match their gait speeds if they wish to stay together. To achieve this goal, each person can select from an array of cadence and stride length combinations. Surprisingly, casual observation suggests and recent studies confirm that a significant fraction of people who ambulate together do this in synchrony, with near identical cadence and stride lengths. Step length and step time can be varied in numerous ways to ensure that walking partners move forward at the same rate and arrive at the desired destination together. However, instead of achieving this goal through a random, time- varying, combination of these parameters, remarkably, couples often seem to March to a single drummer. The first paper to examine this phenomenon (Zivotofsky & Hausdorff, 2007) analyzed it in a qualitative manner. More recently, this finding was quantitatively https://doi.org/10.1016/j.humov.2018.03.009 Received 9 November 2017; Received in revised form 19 March 2018; Accepted 20 March 2018 ⁎ Corresponding author at: Gonda Brain Research Center, Bar Ilan University, Ramat Gan 52900, Israel. 1 These authors contributed equally. E-mail address: ari.zivotofsky@biu.ac.il (A.Z. Zivotofsky). Human Movement Science 59 (2018) 20–29 0167-9457/ © 2018 Elsevier B.V. All rights reserved. T