EVALUATING THE EFFECT OF COCONTRACTION IN OPTIMIZATION MODELS

The effect of co-contraction of antagonist muscles on spinal compressi on force is estimated using Karush-Kuhn-Tucker (K-K-T) multipliers. Go -contraction is modelled as an incremental increase in the lower bound s on the allowable muscle forces in an optimization model formation. T he K-K-T multipliers associated with each lower bound provide an estim ate of the partial derivate of the optimal objective function value wi th respect to a change in the lower bound. A model whose objective fun ction is spinal compression Force is analyzed to estimate the effect o f co-contraction on spinal compression force. While the effect depends on the specific muscle and task under consideration, the marginal eff ect of co-contraction on spinal compression force can be as high as 5. 52 N additional spinal compression force for every additional N of mus cle force. Paradoxically, the co-contraction may slightly decrease pre dicted spinal compression in special circumstances.