Sitting balance I: trunk–arm coordination and the contribution of the lower limbs during self-paced reaching in sitting

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.

Introduction

Sitting balance involves the ability not only to maintain 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]. Reaching 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], [3], [4], [5], [6], trunk movements [7], [8], [9] or support 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 quality of life. Individuals with movement disorders, following stroke for example, frequently have difficulty coordinating movement of body segments and balancing during reaching tasks [12] and poor sitting balance has been associated with poor functional outcome [13], [14], [15]. Information about the biomechanical characteristics of reaching movements performed under varying conditions while seated is critical to planning rehabilitation 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 manipulating reach distance and task on the relative contributions 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 of the task and the properties of the object have been shown to influence coordination [17], [18], [19], [20], [21]. For example, differences in movement duration and timing of peak velocity of arm trajectory during pointing as compared with grasping tasks have been reported [18]. To date, however, it appears that there is only one study which has examined experimentally the contribution of trunk motion to the transport of the hand to a target during seated reaching actions. Kaminski et al. [22] investigated fast pointing movements to five target locations, two of which were within arm’s length while three were beyond arm’s length. They reported that the trunk was involved in hand transport to targets beyond arm’s length but not when the target was placed at or within arm’s length. Subjects were, however, constrained by a belt securing the shank segments and for the present study it was anticipated that trunk movement may differ if the thighs and shanks were free to move. In order to examine the effect of task on segmental coordination and muscle activity, two different tasks were examined: reaching to pick up a glass of water and reaching to press a button.

The second aim of this study was to examine whether or not the lower limbs are actively involved in self-paced reaching and, if so, to determine the effects of both reach distance and type of task. Few studies have examined the contribution of the lower limbs during reaching in sitting. Moore et al. [23] suggested that they are not actively involved and that their contribution is passive as part of a large base of support. Such a contribution has been demonstrated experimentally by Chari and Kirby [24] who reported that subjects could reach further when their feet were in contact with the ground than when they were not. More recently, however, several studies using rapid reaching to perturb balance have provided evidence that the lower limbs play an active role in fast reaching movements [25], [26], [27], [28]. Crosbie et al. [28] found that, during fast reaches to targets beyond arm’s length, vertical ground reaction forces (GRF) through one or both feet increased according to the direction of the reach, and that a number of leg muscles were activated during the reach. The role of the lower limbs may, however, be different in self-paced reaching and would be likely to differ according to the position of the target relative to the body.