Infant Behavior & Development 33 (2010) 30–38 Contents lists available at ScienceDirect Infant Behavior and Development Development of proximal arm muscle control during reaching in young infants: From variation to selection Hanneke Bakker a , Victorine B. de Graaf-Peters a , Leo A. van Eykern a , Bert Otten b , Mijna Hadders-Algra a,* a Department Paediatrics – Developmental Neurology, University Medical Center Groningen, University of Groningen, Hanzeplein 1, 9713 GZ Groningen, The Netherlands b Institute of Human Movement Sciences, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands article info Article history: Received 1 September 2008 Received in revised form 8 April 2009 Accepted 23 October 2009 Keywords: Motor development Infants Reaching EMG Variation Kinematics abstract Reaching movements are initiated by activity of the prime mover, i.e. the first activated arm muscle. We aimed to investigate the relationship between prime mover activity and kinematics of reaching in typically developing (TD) infants in supine and sitting position. Fourteen infants were assessed at 4 and 6 months during reaching in supine and sup- ported sitting. Kinematics and EMG-activity of deltoid, pectoralis major, biceps (BB) and triceps brachii were recorded. Kinematic analysis focused on number of movement units (MUs) and transport MU (MU with longest duration). Prime mover use was variable, but at 6 months a dominance of BB emerged in both testing conditions. Kinematics were also variable, but with increasing age the number of MU decreased and the relative proportion of the transport MU increased. BB as prime mover at 6 months was related to a larger transport MU. Conclusion: Between 4 and 6 months BB prime mover dominance emerges which is related to relatively efficient reaching characteristics. © 2009 Elsevier Inc. All rights reserved. 1. Introduction Reaching seems a simple daily activity. But, on close inspection the task of reaching towards an object is far from simple: the multi-jointed upper limb has to be directed towards the object while simultaneously the position of hand and fingers has to be prepared for the pick-up of the object (Jeannerod, 1990). Therefore it is not surprising that the development of reaching runs a protracted course which extends into adolescence (Kuhtz-Buschbeck, Stolze, Jöhnk, Boczek-Funcke, & Illert, 1998). Our knowledge on typical development of reaching in early infancy is still limited. Nevertheless such information is a prerequisite for the understanding of deviant development. What we do know is the following. Immediately after birth the infant mainly produces spontaneous non-goal-directed activity (Hadders-Algra, 2004). Nevertheless, the neonate has some capacity for goal-directed arm activity (Hopkins & Prechtl, 1984; Van der Meer, Van der Weel, and Lee, 1995). From 3 months onwards the infant starts to produce more types of goal-directed arm activity (Thelen et al., 1993; Van der Fits, Klip, Van Eykern, & Hadders-Algra, 1999), in particular when an interesting object is present. In response to the presence of an object the infant may put the hands into the mouth or it may produce oscillating movements of the extended arm in the direction * Corresponding author. Tel.: +31 50 3614247; fax: +31 50 3619158. E-mail address: m.hadders-algra@med.umcg.nl (M. Hadders-Algra). 0163-6383/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.infbeh.2009.10.006