Coordination of rapid stepping with arm pointing: Anticipatory changes and step adaptation Eric Yiou a,b , Cyril Schneider a, * , Didier Roussel b a CIRRIS – Rehabilitation Research Center, Universite ´ Laval, Que ´bec, QC, Canada G1M 2S8 b Motor Control and Perception, UPRES EA 4042, UFR STAPS, Universite ´ de Paris-Sud, 91405 Orsay, France Available online 16 May 2007 Abstract The present study explored whether rapid stepping is influenced by the coordination of an arm pointing task. Nine participants were instructed to (a) point the index finger of the dominant arm towards a target from the standing posture, (b) initiate a rapid forward step with the contralateral leg, and (c) synchronize stepping and pointing (combined task). Force plate and ankle muscle elec- tromyography (EMG) recordings were contrasted between (b) and (c). In the combined task, the arm acceleration trace most often peaked around foot-off, coinciding with a 15% increase in the forward acceleration of the center of gravity (CoG). Backward displacement of the center of foot pressure at foot-off, duration of anticipatory postural adjustments (APAs) and ankle muscle EMG activity remained unchanged. In contrast, durations of swing phase and whole step were reduced and step length was smaller in the combined task. A reduction in the swing phase was correlated with an increased CoG forward acceleration at foot-off. Changes in the biomechanics of step initiation during the combined task might be ascribed to the postural dynamics elicited by arm pointing, and not to a modulation of the step APAs programming. Ó 2007 Elsevier B.V. All rights reserved. PsycINFO classification: 2330 Keywords: Stepping; Pointing; Arm/leg coordination; Anticipatory postural adjustments; Motor control 0167-9457/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.humov.2007.03.003 * Corresponding author. Present address: Neuroscience and Paediatrics, CHUL Research Center (CHUQ), Universite ´ Laval – Dept Rehabilitation, Que ´bec, QC, Canada G1V 4G2. Tel.: +1 418 654 2152; fax: +1 418 654 2753. E-mail address: Cyril.Schneider@rea.ulaval.ca (C. Schneider). Human Movement Science 26 (2007) 357–375 www.elsevier.com/locate/humov