"rigor mortis is a muscle stiffening that sets in a few hours after death." it is due to muscles being in a contracted state. it dissipates after several days as muscle proteins degenerate and cells break down. rigor mortis is one sign that coroners take into account when estimating a time of death. at a cellular level, what is the mechanism behind rigor mortis?

Explanation:

For the normal contraction and relaxation of the muscles the energy molecules in the form of ATP is required. Thus this process is considered as ATP dependent. For maintaining the levels of ATP glycogen is required. After death the glycogen levels depletes. The glycogen available after death facilitates the synthesis of ATP which leads to the contraction within the muscles also accounts for the hardness and rigidity of the muscles. With the depletion of the glycogen the anaerobic respiration and lactate formation takes place in the muscles. Hence, no further ATP synthesis will occur and the muscles begins to loose the stiffness and becomes soft and flexible.  

the correct answer in apex is tissue development, i know because i just had this question