Dg. Kamper et Wz. Rymer, Effects of geometric joint constraints on the selection of final arm posture during reaching: a simulation study, EXP BRAIN R, 126(1), 1999, pp. 134-138
Significant debate exists regarding the neural strategies underlying the po
sitioning and orienting of the hand during voluntary reaching movements of
the human upper extremity. Some authors have suggested that positioning and
orienting are controlled independently, while others have argued that a st
rong 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 proposed independence of hand position
and orientation. Specifically, we analyzed the effects of geometric constr
aints on final arm postures using a 7 degree-of-freedom model of the human
arm. For 20 different hand configurations within the attainable workspace,
we computed sets of achievable joint angles by applying inverse kinematics.
From each set, we then calculated the locus of possible elbow positions fo
r the particular final hand posture. When the joints were allowed 360 degre
es of rotation, the loci formed complete circles; however, when joint range
s were limited to physiological values, the extent of the loci decreased to
an average are angle of 54.6 degrees (+/-27.9 degrees). Imposition of join
t limits also led to practically linear relationships between joint angles
within a solution 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 attain
able joint trajectories. Furthermore, it is conceivable that some of the co
rrelations observed between joint angles in the course of natural reaching
movements result from geometric constraints.