Funeral pyres are a shared ritual of many religions and ethnic groups. And cremation continues to play a role in modern society, but the practice is typically carried out behind the scenes in a funeral home.
Reports of the deceased sitting up and performing a number of movements, including sitting straight up, during cremation are common. Is there a physiological reason for such motion, or can we dismiss this as the product of overactive imaginations?
In modern cremations, a blast of flame rains down on the corpse's chest in a specially designed chamber. The chamber allows for an extremely high internal temperature of 870–980° Celsius (1,598–1,796° Fahrenheit) to be reached and maintained.
At this temperature, skin ripples and burns away, fat sizzles, and organs shrink. Skin and muscle turn to carbon quickly, carbon that rapidly combines with oxygen in the air to create carbon dioxide. Only pieces of bone, surgical enhancements (like hip and joint replacements), and debris is left after an hour or so of exposure.
The pyres that we often see in historic accounts and fictional settings do not work as efficiently. Pyres use a system of scaffolding that increases the heat placed on the body as a function of height, reaching maximum temperatures of 600 to 800 °C (1110 to 1470 °F). These temperatures are attained by decreasing the size of each additional level of the pyre, which allows heat to funnel through a chimney-like system.
At temperatures of 600 to 800 °C, several hours are needed to cremate a body. Due to the decrease in temperature and lack of insulation in a funeral pyre, the muscles of a corpse will not immediately begin to shrink and essentially evaporate over a short period of time as they do in modern crematorium.
The temperatures attained in pyres are within a territory that causes the muscles to be "cooked." Akin to how a piece of bacon or roast will shrink and change in shape during cooking, the body's muscles change shape at temperatures attainable in a funeral pyre.
The most common "movement" observed in the heating of a corpse is a contraction of the arm muscles at around 670 °C. This contraction creates a pose similar to a boxer protecting their chest and occurs at the lower cremation temperatures observed in a pyre and sometimes as a body cools.
This "pugilistic pose" also features the corpse's fingers balled into a fist and a slightly tilted head. The movements could be viewed by pyre watchers as the act of a corpse sitting up, especially when combined with other body parts separating and falling away as the flames consume the body.
In order to see the pugilistic pose, cremation of the body has to take place prior to the degrading of muscle tissue. To optimize the possibility of seeing the pose, one needs to cremate the body prior to rigor mortis setting in. During rigor mortis, the muscles contract for a prolonged amount of time until actual muscle fibers are broken down by the remaining enzymes in the body.
Funeral pyres would be able to meet these requirements. The structures could be set up quickly, either before or during the first stages of rigor mortis, while providing a necessary (albeit inefficient) heat source. Bodies set aside for modern cremation are often in the middle of rigor mortis or in the aftermath, negating the possibility of movement even if the creation is carried out at a much lower temperature.
The nature of funeral pyres — smoldering towers of fire — draws individuals in, with people watching intently and gathering around the structure for hours. Any slight movement would be viewed by a number of people and probbaly hyper-analyzed. The pugilistic pose, combined with body parts separating during the course of cremation, easily provides the foundation for stories of bodies "sitting up" in a funeral pyre or performing a variety of movements.
So, it does appear that bodies do move in cremation, but only under a strict set of circumstances. If a body is burned at a low enough temperature and quickly after death, movements are possible. While the bodies do not sit up, these natural postmortem motions could be interpreted as such by those watching a funeral pyre from a distance.
Similarly, one would certainly expect some movement after death via self immolation, as the muscles will be "fresh" and the temperature could fluctuate. The same is true with individuals that die in a house fire, as they are often found by firefighters in a pugilistic pose.