When you’re dead, you stiffen up. We all know this. But why? What is the process that goes on, inside a dead body, that causes muscle tissue to contract so tightly that the limbs can be impossible to move? We’ll take you into the wonderful world of rigor mortis.
…Rigor mortis is famous for setting in from the head down, but actually all muscles experience it equally. The stiffness is just more noticeable in the smaller muscles around the face and head than in the large muscles of the legs and abdomen. The stiffness generally goes away after about two days.
Source: What Causes Rigor Mortis?
FYI: The stiffness goes away as the body starts to decompose. So be careful how you stash a corpse, in case you have to move it within a day or two. 😉