The isolated rat gastrocnemius was submitted to tetanic stimulation, with and without a spring in series with the muscle. The resulting isometric myograms allowed description of the properties of simple muscle models which contained linear and non-linear components. Mathematical comparisons demonstrated a difference of some 25% between the linear and non-linear representations of contractile and elastic tissue. In addition, the severe non-linearties of contractile and elastic components were judged to preclude the usefulness of a linear systems model when the state of activation of the muscle is changing rapidly. Although this method of deriving muscle properties is attractive because of its simplicity, some of the assumptions governing its use are considered of questionable validity.