The function and organization of postural adjustments during voluntary pulls made by standing humans

Summary form only given. Postural adjustments preceding voluntary arm movements generally are assumed to stabilize standing balance relative to the base of support, body joint configuration, or orientation of one or more segments to the inertial reference frame. While this assumption is reasonable and consistent with much biomechanical data, postural adjustments also can contribute directly to the primary voluntary act. This role of postural adjustments has been demonstrated analytically for abrupt pulls made in the horizontal direction of the sagittal plane by freely standing humans. Under the constraints of the experiment, the biomechanical system has two kinematic degrees of freedom, which can be represented by the vertical and anterior-posterior coordinates of the body´s center of mass (CMV and CMAP). A model which treats the CMAP as a point mass at the end of an inverted pendulum that rotates about a fixed axis in a gravitational field is presented. The model has three parameters: stiffness of an elastic cord between the CMAP and the external world; the slack that has to be taken out before the cord exerts force on the CMAP; and the diameter of a weighted pulley centered at the axis of rotation, which generates torque that initiates pendulum motion.<>