Contents lists available at ScienceDirect Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost Influence of age and falls incidence on tau guidance of centre of pressure movement during gait initiation Steven van Andel ⁎ , Michael H. Cole, Gert-Jan Pepping School of Behavioural and Health Sciences, Australian Catholic University, 100 Nudgee Road, 4014, Banyo, QLD, Australia ARTICLE INFO Keywords: Tau guidance General tau theory Gait initiation Postural control Aging Falls ABSTRACT Background: Prospective balance control can be assessed in terms of the characteristics of a tau-guidance function that summarizes the velocity profile of Centre of Pressure (CoP) movement during gait initiation. This allows the nature of CoP movement to be assessed on a continuum between controlled ‘soft’- and unstable ‘hard’ CoP-motion gap-closure. Previous research has shown less stable movement patterns with harder closures with increasing age, which makes movements more prone to overshooting and could possibly explain the increasing falls risk with age. Research questions: The primary research question was ‘what is the relationship between falls incidence and tau- guidance in the mediolateral centre of pressure movements during gait initiation?’ The secondary research question was ‘what are the influences of age and task demands on the variability of tau-guidance character- istics?’. Methods: Sixteen young adults and 76 older adults performed 33 gait initiations from a force platform, stepping onto stepping-targets imposing differing task demands. Older participants completed a one-year follow-up screening for falls. An analysis was performed investigating linear relationships between a tau-guidance function and the time-to-closure (tau) of the mediolateral centre of pressure motion-gap with coupling constant K (de- pendent variable). Results: Gait-related falls during the 12-month follow-up period were associated with higher tau-K values. Furthermore, longer movement preparation time was associated with lower K values, particularly in fallers. Previously-reported age-related increases of the tau-coupling constant values which were found in studies of unconstrained gait initiation were not present in our results. Significance: The presence of the targeting task provided a more prescriptive environment compared to un- constrained gait initiation and could explain the absence of age-related changes to the produced K values. Falls incidence was found to be associated with higher values of K, indicating less stable movement. Future studies should investigate the practical implications of these findings for falls prevention. 1. Introduction In light of the increasing number of older adults globally, falling has become one of the major problems faced by society. For older adults, falling leads to injury, decreases in quality of life and potentially even death [1]. Furthermore, falling is a major burden on society in terms of medical costs [2,3]. Given this relevance, it is important to further in- vestigate the mechanisms behind falling and the deficits in motor control that are associated with these incidents. Falls have been asso- ciated with age-related declines of the postural system [4,5] and the vast majority are reported to occur during gait [6]. It is known that the way in which gait is initiated changes with age [7]. The current study therefore, focusses on dynamic balance during gait initiation to better understand the mechanisms behind falling. The characteristics of closing motion gaps (i.e. the gap between a current state of the system and a goal state) can be described in terms of the relationship between the time-to-closure (tau) of the movement gap and a tau-guidance function (τ G )[8–10]. Such an approach has been shown to accurately describe the closing of motion gaps in numerous natural movements, such as club movements in golf putting strokes [11], frequency glides in singing [9] and infant suckling behaviour [12,13]. Similarly, in gait initiation, the mediolateral shift of the Centre of Pressure (CoP x ) when transferring weight onto one support leg can be conceived as a motion gap, and the control of its closure during gait initiation can be analyzed using the principles of tau-guidance [14–16]. Using an analysis of the change in tau during a movement (τ CoPx ), the https://doi.org/10.1016/j.gaitpost.2019.02.030 Received 5 October 2018; Received in revised form 26 February 2019; Accepted 26 February 2019 ⁎ Corresponding author. E-mail address: Steven.vanAndel@acu.edu.au (S. van Andel). Gait & Posture 70 (2019) 104–108 0966-6362/ © 2019 Elsevier B.V. All rights reserved. T