Why do you tense up when you die?

Answer 1

Your body doesn't tense up. If your dead, why would your muscles start contracting? Your muscle cells are dead, so they wouldn't function, unless it gets electrocuted.

The above answer is incorrect - your skeletal muscles do, in fact contract after death but the condition is temporary, beginning a few hours after death and continuing for about 3 days, at which time the muscles again relax. This condition is called rigor mortis and is one of three body conditions used in determining the time of death, the other two being algor mortis (the cooling of the body), and livor mortis (the settling of the blood to the lowest point of the body).

Rigor mortis occurs due to the very complex physiology of the muscle system and how the muscles are able to move in the first place. This cannot be explained effectively in a paragraph or two, but briefly has to do with the lack of blood flow to the muscles after death, and the absence of signals from the brain.

Answer 2

Tensing up when you die is a natural response as the muscles in the body contract due to lack of oxygen and energy. This occurs as the body shuts down and rigor mortis starts to set in after death.