title: Optimal control of non-ballistic muscular movements: a constraint-based performance criterion for rising from a chair.
abstract: To understand how humans perform non-ballistic movements, we have developed an optimal control model to simulate rising from a chair. The human body was modeled as a three-segment, articulated, planar linkage, with adjacent links joined together by frictionless revolutes. The skeleton was actuated by eight musculotendinous units with each muscle modeled as a three-element entity in series with tendon. Because rising from a chair presents a relatively ambiguous performance criterion, we chose to evaluate a number of different performance criteria, each based upon a fundamental dynamical property of movement; muscle force. Through a quantitative comparison of model and experiment, we found that neither a minimum-impulse nor a minimum-energy criterion is able to reproduce the major features of standing up. Instead, we introduce a performance criterion based upon an important and previously overlooked dynamical property of muscle: the time derivative of force. Our motivation for incorporating such a quantity into a mathematical description of the goal of a motor task is founded upon the belief that non-ballistic movements are controlled by gradual increases in muscle force rather than by rapid changes in force over time. By computing the optimal control solution for rising from a static squatting position, we show that minimizing the integral of a quantity which depends upon the time derivative of muscle force meets an important physiological requirement: it minimizes the peak forces developed by muscles throughout the movement. Furthermore, by computing the optimal control solution for rising from a chair, we demonstrate that multi-joint coordination is dictated not only by the choice of a performance criterion but by the presence of a motion constraint as well.
authors.affiliation: Department of Kinesiology and Health Education, University of Texas at Austin, TX 78712, USA.
authors.forename: M G
authors.initials: MG
authors.lastname: Pandy
authors.forename: B A
authors.initials: BA
authors.lastname: Garner
authors.forename: F C
authors.initials: FC
authors.lastname: Anderson
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D004576:Electromyography; D006801:Humans; D008297:Male; D008433:Mathematics; D008954:Models, Biological; D009068:Movement / Q000502:physiology*; D009142:Musculoskeletal Physiological Phenomena; D011187:Posture / Q000502:physiology*; D012016:Reference Values; D012680:Sensitivity and Specificity; D013997:Time Factors