528 The authors are with the Kinesiology and Applied Physiology Department, University of Delaware, Newark, DE. Motor Control, 2010, 14, 528-544 © 2010 Human Kinetics, Inc. Effects of Task Complexity on Coordination of Inter-Limb and Within-Limb Forces in Static Bimanual Manipulation Vennila Krishnan and Slobodan Jaric Coordination of the hand grip (G; acting normally to the grasping surface) and load forces (L; acting in parallel) in bimanual static tasks was studied. L symmetry (either the magnitude or direction) and frequency were manipulated in healthy participants (N = 14). More complex tasks (i.e., the higher frequency and/or asymmetric ones) revealed expected deterioration in both the task performance (accuracy of the prescribed L force proiles) and force coordination (G/L ratio and G-L correlation) suggesting importance of L frequency and symmetry in prehen- sion activities. However, the same tasks revealed a more prominent deterioration of interlimb than the within-limb force coordination. This could be interpreted by two partly different and noncompeting neural control mechanisms where the coordination of interlimb forces may be based on ad-hoc and task-speciic muscle coordination (often referred to as muscle synergies) while the within-limb coor- dination of G and L could be based on more stable and partly relex mechanisms. Keywords: grip, load, coupling, performance, control, hand Grasping and manipulating objects are the most frequent motor activity of daily life. According to a simple kinetic model, it requires the exertion of a suficient grip force (G; acting normally to the object’s contact surface) to stabilize the object against the load force (L; acting in parallel) that performs the manipulative action. In general, G is adjusted to the physical properties of the object (such as the mass or the surface friction) to prevent slippage that could be caused by the action of L (Flanagan & Wing, 1995; Johansson & Westling, 1988). Although a brief period of G adaptation is needed at the initial phase of interaction with a novel object (Johans- son & Westling, 1984), the CNS maintains a close coordination of G and L over a variety of ongoing manipulative actions (Cole & Abbs, 1988; Flanagan & Wing, 1995; Johansson & Westling, 1984) virtually without any time delay between them (Flanagan & Wing, 1995; Jaric, Collins, Marwaha, & Russell, 2006; Zatsiorsky, Gao, & Latash, 2005). As a consequence, G and L control pattern maintains both a relatively low and stable G/L ratio (Johansson & Westling, 1984).