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The force that an isometric skeletal muscle will produce in response to time-varying stimulation ("dynamic isometric" force) is important both for understanding muscle function and for designing neuroprostheses. The paper reports a model for predicting the force produced by an isometric skeletal muscle at rest length in response to a wide range of stimulation patterns. The model consists of two linear, first-order systems separated by a static nonlinearity. The rate constant of the second first-order system varies with force level. The model was validated using three cat soleus and three cat plantaris muscles. The following whole-nerve stimulation trains were used: single pulses (twitches), 2-4 pulses, constant rates, triangularly modulated interpulse intervals, and randomly modulated interpulse intervals. The model reproduced most responses accurately. The model shows that a force-dependent rate constant is essential for model validity, and could be used in the control of neuroprostheses.