Biomechanical Stability of Multi-Joint Posture

Past studies of human and animal arms (limbs) suggest that mechanical behavior of the neuro-musculoskeletal system play a significant role in the arm posture and movement control. Based on the observation that arm muscles of a single or multiple joint system interact to stabilize the limb at some joint configuration by producing restoring forces opposing perturbation displacement, the concept of equilibrium position control was developed which describes how the central nervous system may coordinate the activity levels of agonist and antagonist muscles to control posture and movement stability (Feldman, 1966; Bizzi, 1991; Kelso, 1977; Mussa-Ivaldi, 1985; Hogan, 1985; Shadmehr, 1993). Especially for multi-joint posture, the restoring forces were shown to be dominated by a convergent force field, meaning that the behavior of the arm can be represented by the arm stiffness (Mussa-Ivaldi et al., 1985; Hogan, 1985). While many of the geometric features of the force field and regular patterns of their variations associated with posture have been identified, factors that contribute to the regular variations are yet to be clearly understood (Hogan, 1985; Mussa-Ivaldi, 1985). Previous examination of geometric and inertia effects of musculoskeletal anatomy indicates that the mechanical constraints cannot fully account for the observed patterns of postural stiffness (Hogan, 1985; Mussa-Ivaldi, 1985). There is a clear evidence that the restoring forces are nonlinearly related to larger displacements from equilibrium (Shadmehr et al., 1993) and that contributions from neural input and/or muscle properties seem to act in a way as to counteract changes in hand stiffness produced by musculoskeletal geometry (Mussa-Ivaldi, 1985).