This article reviews the skinned muscle fiber techniques and approaches that have been used in our laboratory in the study of both tension and Ca regulation over the past 15 years. The impetus for using these fibers was the finding that most membrane Cl channels in crayfish muscle are located within the transverse tubules at the junction (diad) with the sarcoplasmic reticulum (SR). This observation led to the postulate that a change in Cl concentration around the SR would cause it to release stored CA. This could be tested by using skinned fibers. Tensions induced by Cl release of SR- Ca or by direct application of Ca are highly dependent on the concentration of substrate (MgATP). A substrate-inhibition formulation accurately summarizes this dependence, but it is not integrated with current biochemical schemes of ATP hydrolysis. To facilitate development of a broad model for substrate regulation of tension, which includes concepts of a cross-bridge cycle, we developed a mammalian skinned fiber preparation and techniques for monitoring regulation in single fibers. Both optical and mechanical techniques are used to investigate contractile protein interactions and Ca binding.