contracted muscles are the ones that cause stiffness of the body, the normal operation of a muscle is contraction and relaxation, if the muscles are unable to relax then titigas talaga katawan naten
we have a "god molecule" for life which is ATP (adenosine triphosphate), the one and only energy for our cells
muscle contraction and relaxation are just the release and collection of calcium ions respectively, the management of calcium ions requires the "god molecule" or energy, when a living thing dies there will be no oxygen needed to produce ATP (though cells can produce ATP without oxygen but only for few minutes) then there will be no energy and the calcium ions will left un-collected therefore the muscles stay contracted until other living things (bacteria or animals) breaks the contraction by destroying the muscles themselves in the process called "decomposition"