Menashe DORNAY
Static Analysis of Posture and
Movement, Using a 17-muscle Model
of the Monkey's Arm
Abstract:The musculo skeletal apparatus of the arm transforms motor commands via muscle
forces into joint torques. A stable equilibrium posture with zero net force at the hand
is specified when the joint stiffness is stable, and the net joint torque is zero. The
transformation from a stable spring-like muscle space into joint and hand space
can potentially induce hand postural instability.
In this study I show kinematic and static constraints of the neuro-musculo-skeletal
arm of the monkey which are needed for the postural stability of the hand. My results
enhance the previously proposed impedance control theory, and present new aspects
for controlling the hand stiffness. A new term, the angular stiffness of individual
muscles, is introduced. Using muscles with varying moment-arms, I show that while
individual muscles may have unstable angular stiffness, only the summation of the angular
stiffnesses of the individual muscles determines the joint (and hand) postural stability.
A computer model of the arm, based on biological measurements, with 17 muscles,
is shown. The model is used to determine the effect of a given muscle activation patterns
on the posture and stability of the hand.
Using the equilibrium point hypothesis, arm movement is modeled as a gradual shift
of a stable equilibrium posture of the hand along a desired trajectory. Given initial
and final equilibrium postures of the hand, and using a minimum potential energy
change constraint, the simulator derives the motor commands to the muscles for creating
the desired trajectory.