Abstract Significant debate exists regarding the neural strategies underlying the positioning and orienting of the hand during voluntary reaching movements of the hu- man upper extremity. Some authors have suggested that positioning and orienting are controlled independently, while others have argued that a strong interdependence exists. In an effort to address this uncertainty, our study employed computer simulations to examine the impact of physiological limitations of joint rotation on the pro- posed independence of hand position and orientation. Specifically, we analyzed the effects of geometric con- straints on final arm postures using a 7 degree-of-free- dom model of the human arm. For 20 different hand con- figurations within the attainable workspace, we comput- ed sets of achievable joint angles by applying inverse ki- nematics. From each set, we then calculated the locus of possible elbow positions for the particular final hand posture. When the joints were allowed 360° of rotation, the loci formed complete circles; however, when joint ranges were limited to physiological values, the extent of the loci decreased to an average arc angle of 54.6° (±27.9°). Imposition of joint limits also led to practically linear relationships between joint angles within a solu- tion set. These theoretical results suggest a requirement for coordinated interaction between control of the joints associated with hand position and those involved with hand orientation in order to ensure attainable joint trajec- tories. Furthermore, it is conceivable that some of the correlations observed between joint angles in the course of natural reaching movements result from geometric constraints. Key words Upper limb · Posture · Geometric constraints · Human Introduction Reaching movements of the human upper extremity re- present a significant control problem due to the number of degrees of freedom involved. A number of researchers have hypothesized that humans might simplify the control process by dividing the task into parallel, independent components, as is often done in robotics (Hollerbach 1988). Accordingly, shoulder motion, incorporating ab- duction/adduction, flexion/extension, and internal/external rotation, and elbow flexion/extension would position the hand in space. Forearm pronation/supination, wrist flex- ion/extension, and ulnar/radial deviation would govern orientation of the hand. Movement of the limb to the prop- er location, positioning of the hand in the proper orienta- tion, and grasping the object would be controlled indepen- dently (Arbib 1981; Jeannerod 1984, 1992). Additional experimental results seemed to support this hypothesis (Lacquaniti and Soechting 1982). In contrast, several oth- er studies have shown strong interdependence between grasp and transport (Chieffi and Gentilucci 1993; Gent- ilucci et al. 1996), as well as systematic links between the positioning and orienting of the hand (Soechting and Flan- ders 1993; Desmurget et al. 1996, 1998). The geometry of the limb itself may fundamentally influence its control. For each hand position and orienta- tion, a locus of possible elbow locations exists (Korein 1985). This locus lies along a circle centered on the line connecting the center of the shoulder and wrist coordi- nate axes (Fig. 1) (Engelbrecht 1997). In the absence of limitations on joint range of motion, any point on the circle can be attained, so the orienting and positioning of the hand can be separated. However, if the locus does not include the entire circle, then either one cannot prop- erly orient the hand for a given posture of the upper arm that places the hand in the correct position, or one cannot attain the necessary hand position for a distal solution that generates the desired hand orientation. Thus, as the percentage of the circle spanned by the elbow locus di- minishes, the need for greater interaction between con- trol of position and orientation grows. D.G. Kamper ( ✉ ) · W. Zev Rymer Sensory Motor Performance Program, Suite 1406, Rehabilitation Institute of Chicago, 345 E. Superior Street, Chicago, IL 60611, USA e-mail: d-kamper@nwu.edu Tel.: +1-312-908-4269, Fax: +1-312-908-2208 Exp Brain Res (1999) 126:134–138 © Springer-Verlag 1999 RESEARCH NOTE Derek G. Kamper · W. Zev Rymer Effects of geometric joint constraints on the selection of final arm posture during reaching: a simulation study Received: 7 December 1998 / Accepted: 14 January 1999