Neuroscience Letters 381 (2005) 51–56 Coordination of hand grip and load forces in uni- and bidirectional static force production tasks Slobodan Jaric * , Elizabeth M. Russell, Jeffrey J. Collins, Rahul Marwaha Human Performance Laboratory, Department of Health, Nutrition, and Exercise Sciences, University of Delaware, 547 S. College Avenue, Newark, DE 19716, USA Received 9 January 2005; received in revised form 31 January 2005; accepted 31 January 2005 Abstract The purpose of the study was to explore the differences in coordination of grip (G) and load forces (L) in a unidirectional and bidirectional bimanual static force production task. Subjects (N = 10) exerted oscillatory isometric L profiles against an externally fixed hand-held device, modulated either in pure tension (unidirectional) or in alternating tension and compression (bidirectional) at a rate of either 1.33 or 2.67 Hz. The unidirectional task revealed a high level of coordination of both the ipsilateral (i.e., G and L of each hand) and contralateral pairs of forces (two Gs and two Ls) as assessed by correlation and stability of force ratios. The bidirectional task demonstrated a low level of inconsistently modulated Gs with respect to the change of L, which resulted in a deteriorated coordination, particularly between the ipsilateral forces. The overall effect of task on the force coordination was higher than the effect of frequency suggesting that the higher frequency of G modulation required in the bidirectional task is not likely to be the main cause of the observed phenomenon. We interpret these differences by a relative simplicity of the control mechanisms of the unidirectional task based on a single synergy of G and L muscles that allows simultaneous coordination of both the ipsilateral and contralateral forces. Due to the switching between two distinctive synergies involving G muscles, the bidirectional task could possess a higher control complexity causing a decoupled coordination of the ipsilateral forces, while retaining the coordination of contralateral forces at a relatively high level. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Human; Movement control; Muscle synergy; Isometric; Oscillatory; Manipulation An important requirement for successful manual manipu- lation of external objects is coordination of the hand grip force (G) with an externally imposed load force (L). Exam- ples include lifting and transporting objects, manipulating tools, or using external postural support. Specifically, G has to be maintained within a certain range that not only prevents the object’s slipping, but also avoids excessive forces that could crush the object or cause unnecessary muscle fatigue. When G is limited to the normal force component acting upon the surface of a hand-held object, while L is the tangen- tial force tending to cause slippage, a continuous adjustment of G to L variations has been reported with virtually no time lag between them, providing a stable grip-to-load (G/L) ra- tio [2,4,5,7,17,24]. It has been suggested that the G/L ratio * Corresponding author. Tel.: +1 302 8316174; fax: +1 302 8313693. E-mail address: jaric@udel.edu (S. Jaric). could be a controlled variable that is adjusted to the friction of the contact surface [4] and preserves a relatively small safety margin [7,18,20,21]. However, both a low level of the G/L ratio and a high level of coordination of G and L is difficult to maintain under increased difficulty of the task performed, such as an increase in movement frequency or dissimilar ac- tions of two hands [2,4,12,19,21,24]. A number of daily activities require bimanual actions, including those involving manipulation of hand-held objects. These actions also demonstrate a high level of coordination of G and L [3,18], as well as the coordination of contralateral forces, such as two Gs or two Ls in both similar and dissimilar tasks [3,14,15,20]. A bilateral interaction of two Gs and Ls in an asymmetric manipulative task [18] was explained by contralateral neural interactions that particularly affect the performance of dissimilar tasks by the two hands [1,6,22,23], as well as by stronger bimanual 0304-3940/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2005.01.086