Anisotropic stiffness reduction during constrained multijoint arm movement

Human arm stiffness during a constrained multijoint movement was investigated. The subject was asked to move his hand from the start to end positions while the movement trajectory was constrained in a straight transversal path on a horizontal plane by a high gain servo of the manipulandum. Small perturbations were applied during movements, then stiffness was estimated by using reaction forces and positional shift caused by the perturbations. In the middle of the movement, it was found that the stiffness in the constrained direction was remarkably decreased compared with that during the unconstrained movement. As a result, the stiffness ellipses were inclined. This anisotropic stiffness change is effective to reduce resisting force from the constraints, suggesting that the strategy of multiple muscle control by the brain can be changed according to the external environment