“Hey there cadaver, whatcha thinking about?”
"Oh, just dead stuff."
Rigor mortis is the result of chemical changes in the muscles. When you die you stop oxygenating blood. When your blood isn’t oxygenated your cells can’t make that sweet, sweet ATP to provide energy for cellular processes. In the muscle, the lack of ATP affects SERCA pumps in the sarcoplasmic reticulum and calcium isn’t available to release the binding between the actin and myosin filaments. As a result, the muscles are super stiff. Over time enzymes degrade the myosin heads and the muscles relax and you get a floppy corpse.
The question burning in my mind is how this guy managed to hold this pose during death, like it was a casual breeze while he read a book, laying down in the park on a sunny day.