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- Discussion:
    - muscle contraction is regulated by calcium ions, which will change thin filament into an activated state by binding to troponin;
         - release of calcium actives the myosin-ATP complex, which motors the sliding action between actin and myosin;
    - in absence of calcium, troponin & tropomyosin interfere w/ formation of active complexes;
    - in absence of ATP, myosin and actin form rigor complexes whether or not calcium is present;
         - after death, there is a gradual decrease in ATP concentration, so less ATP is available to disassociate the myosin head in a
              rigor complex from the actin molecule