title: Evaluating the effect of co-contraction in optimization models.
abstract: The effect of co-contraction of antagonist muscles on spinal compression force is estimated using Karush-Kuhn-Tucker (K-K-T) multipliers. Co-contraction is modelled as an incremental increase in the lower bounds on the allowable muscle forces in an optimization model formation. The K-K-T multipliers associated with each lower bound provide an estimate of the partial derivate of the optimal objective function value with respect to a change in the lower bound. A model whose objective function is spinal compression force is analyzed to estimate the effect of co-contraction on spinal compression force. While the effect depends on the specific muscle and task under consideration, the marginal effect of co-contraction on spinal compression force can be as high as 5.52 N additional spinal compression force for every additional N of muscle force. Paradoxically, the co-contraction may slightly decrease predicted spinal compression in special circumstances.
authors.affiliation: Department of Industrial and Operations Engineering, University of Michigan, Ann Arbor 48109-2117, USA.
authors.forename: R E
authors.initials: RE
authors.lastname: Hughes
authors.forename: J C
authors.initials: JC
authors.lastname: Bean
authors.forename: D B
authors.initials: DB
authors.lastname: Chaffin
mesh_terms: D000465:Algorithms; D001696:Biomechanical Phenomena; D004576:Electromyography; D005544:Forecasting; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology*; D008954:Models, Biological; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000502:physiology*; D017568:Rectus Abdominis / Q000502:physiology; D013314:Stress, Mechanical