Control of movement, postural stability, and muscle angular stiffness

Kinematic and static constraints of a 2 revolute joint articulate arm which are needed for the postural stability of the hand are presented. The author´s results enhance the previously proposed impedance control theory, and present new aspects for controlling hand stiffness. A new term, the angular stiffness of individual muscles, is introduced. using muscles with varying moment arms, it is shown that while individual muscles may have unstable angular stiffness, only the summation of the angular stiffness of the individual muscles determines the joint (and hand) postural stability. A computer model of a monkey´s arm, based on biomechanical measurements, with 17 muscles, is shown as an example