One of the classic successes in Mathematical Physiology is A.F. Huxley's (1957) model of muscle contraction through cross bridge dynamics. This mathematical model is based on the microscopic structure of protein filaments within muscle cells, and it can accurately model macroscopic dynamic properties of muscle.  As molecular details have been revealed through modern experimental techniques, the mathematical work originated by Huxley has been expanded and extended.  In this talk I will describe the Huxley model and our recent extensions which accurately capture muscle dynamics when stretched.  The model is based on a conservation law and is essentially a simple evolution equation for the density of bound cross-bridges.