This paper reviews recent work aimed at deriving tractable constitutive relations for skeletal muscle from biophysical cross-bridge theories. Discussion is focused on a model proposed previously by the first author (the Distribution-Moment Model), which emphasizes the important role of the moments of the actin-myosin bond distribution function. The theory leads to a relatively simple third order state variable model for contraction dynamics in which the state variables are the three lowest order moments of the bond-distribution function; further, these three moments have simple macroscopic interpretations as muscle stiffness, force, and elastic energy. New results are presented on the formulation of a compatible model for excitation-contraction coupling, and this model requires the introduction of only one more state variable—the free calcium concentration.

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