title: The 'extrapolated center of mass' concept suggests a simple control of balance in walking.
abstract: Next to position x and velocity v of the whole body center of mass (CoM) the 'extrapolated center of mass' (XcoM) can be introduced: xi = chi + nu/omega 0, where omega 0 is a constant related to stature. Based on the inverted pendulum model of balance, the XcoM enables to formulate the requirements for stable walking in a relatively simple form. In a very simple walking model, with the effects of foot roll-over neglected, the trajectory of the XcoM is a succession of straight lines, directed in the line from center of pressure (CoP) to the XcoM at the time of foot contact. The CoM follows the XcoM in a more sinusoidal trajectory. A simple rule is sufficient for stable walking: at foot placement the CoP should be placed at a certain distance behind and outward of the XcoM at the time of foot contact. In practice this means that a disturbance which results in a CoM velocity change Deltav can be compensated by a change in foot position (CoP) equal to Deltav/omega 0 in the same direction. Similar simple rules could be formulated for starting and stopping and for making a turn.
authors.affiliation: Laboratory of Human Movement Analysis, Center for Rehabilitation, University Medical Center Groningen, The Netherlands. a.l.hof@med.umcg.nl
authors.forename: At L
authors.initials: AL
authors.lastname: Hof
mesh_terms: D001696:Biomechanical Phenomena; D049628:Body Size; D003198:Computer Simulation; D005528:Foot; D005684:Gait; D006112:Gravitation; D006801:Humans; D007699:Kinesthesis; D008962:Models, Theoretical; D004856:Postural Balance; D012371:Robotics; D013008:Somatotypes; D016138:Walking; D016474:Weight-Bearing