Gait and Posture 10 (1999) 135–146 Sitting balance I: trunk – arm coordination and the contribution of the lower limbs during self-paced reaching in sitting Catherine Dean *, Roberta Shepherd, Roger Adams School of Physiotherapy, Faculty of Health Sciences, The Uniersity of Sydney, PO Box 170, Lidcombe, NSW 2141, Australia Accepted 11 January 1999 Abstract The effects of reach distance and type of task on the functional relationship between the trunk, upper limb segments and the lower limbs during self-paced reaching in sitting were examined. Two-dimensional kinematic, kinetic and electromyography (EMG) data were collected as six healthy subjects reached forward under three distance (60, 100, 140% arm’s length) and two task (reaching to press a switch, reaching to grasp a glass) conditions. The results demonstrate that type of task affected primarily the temporal aspects of coordination, with the grasp task taking consistently longer than the press task. In contrast, reach distance affected both the spatio-temporal aspects of coordination between the trunk and arm segments and the active contribution of the lower limbs. As reach distance increased, the magnitude of trunk and upper-arm segmental motion increased, whereas forearm segmental motion decreased. However, at each reach distance the path of the hand was relatively straight and there was remarkable consistency in the relationship between trunk and arm segments both within and between subjects suggesting that despite the presence of redundant degrees of freedom, the individual uses a parsimonious coordinative pattern. The vertical ground reaction force (GRF) and EMG data demonstrated that the lower limbs actively contributed to support the body mass when the object was located at 140% arm’s length. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Trunk; Arm segments; Vertical ground reaction force www.elsevier.com/locate/gaitpost 1. Introduction Sitting balance involves the ability not only to main- tain the seated posture without falling over but also to balance the body mass over the base of support while performing a variety of self-initiated actions [1]. Reach- ing to targets at various distances from the body is a common action which perturbs balance since it involves complex interactions between the arm, the upper body and the base of support which is provided by the pelvis and thighs on the seat and the feet on the floor. To date, research pertaining to the relationship between posture and movement has primarily focussed on able- bodied individuals in the standing position, using rapid arm movements [2–6], trunk movements [7–9] or sup- port surface variations [10,11] to perturb balance. There has been little investigation of the organisation and control of self-paced reaching in sitting. The ability to perform reaching tasks while seated is fundamental to an individual’s independence and qual- ity of life. Individuals with movement disorders, follow- ing stroke for example, frequently have difficulty coordinating movement of body segments and balanc- ing during reaching tasks [12] and poor sitting balance has been associated with poor functional outcome [13 – 15]. Information about the biomechanical characteris- tics of reaching movements performed under varying conditions while seated is critical to planning rehabilita- tion which aims to improve reaching and balance in sitting [16]. The purpose of this study was, therefore, to investigate the effects of both reach distance and type of task on segmental coordination and muscle activity during self-paced reaching actions performed in the seated position. The first aim was to determine the effects of manipu- lating reach distance and task on the relative contribu- tions of trunk and upper limb segments to transporting the hand to the object. There have been several studies of arm movement during reaching and both the nature * Corresponding author. E-mail address: c.dean@cchs.usyd.edu.au (C. Dean) 0966-6362/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII:S0966-6362(99)00026-0