A computation tool to simulate trunk motion and predict muscle activation by assigning different weights to physical and physiological criteria

A central problem in motor control is to understand how the many biomechanical degrees of freedom are coordinated to achieve a goal. A common assumption is that Central Nervous System (CNS) will plan tasks based on open-loop optimal control theory which simultaneously predicts state variables and motor commands based on a compound objective function. A 3D computational method incorporated with 18 anatomically oriented muscles is used to simulate human trunk system. Direct collocation method allows us to convert a constrained optimal control problem to a common nonlinear programming problem to assume the spinal stability condition. Trunk movement from the upright standing to 60 degrees of flexion is simulated based on this method. Incorporation of the stability condition with the open-loop optimal controller resulted in an increase of antagonistic activities which would increase the joint stiffness around the Lumbosacral joint in response to gravity perturbation. Results showed that different patterns of trunk movement and back muscles activity can be explained based on change in the coefficient of two performance indices.