Biomechanical Analysis of Posture and Movement Coordination in Standing Humans during Bending of the Trunk in the Sagittal Plane

Achievement of the behavioral aim of most motor acts requires coordination of posture and movement. We report here our studies of this coordination using body bending in the sagittal plane in standing humans as an example. These movements are particularly complex to study, as both components of the motor act (the main component, with the aim of producing a behavioral result, i.e., bending per se, and the “accessory” postural component, i.e., with the aim of maintaining balance during the movement) include movements of large body parts, which hinders the separation of these two components. These were discriminated on the basis of current views of the features of postural components as compared with those of the main target component. The main and postural components were shown to correspond to movements along the eigenvectors of the dynamic equation. These movements were unique, as they simultaneously demonstrated both “kinematic” and “dynamic” synergies, so they were termed “natural synergies.” The coordination of natural synergies in conditions of standing on wide and narrow supports was studied. The experimentally observed coordination was found to be close to the optimum coordination providing minimal movements of the center of pressure. This guarantees maintenance of the position of the center of pressure within the support area, as required to maintain balance.