Most models of muscle have assumed that the series elastic element has a constant stiffness, although experimentally this is not true, either at the level of individual cross-bridges or that of the whole muscle. In this paper elastic and viscous elements are introduced whose properties vary systematically with activity levels as has been found experimentally. The effects of these nonlinear variations on the methods for optimally controlling a pair of antagonistic muscles are calculated. Although the nonlinearities add some complexity to the derivation and are important to the mechanism of force generation in the muscle, they do not qualitatively change the overall control of movement.