What Causes Rigor Mortis?

The Scene of the Crime:

“What was the time of death detective?”

“When we arrived, rigor had begun to set in. I place time of death at 4 hours ago.”

This scene is typical of a CSI murder drama. The Medical Examiner steps on the scene and examines the body for signs of rigor mortis. Like clockwork, the muscles of a dead body begin to stiffen starting 3 hours after death. Full rigor comes on around 12 hours (the medical term for the body at this time is a “stiff”).

What’s the deal?

You’d like to think that when you die, you would finally get to relax. Why do your muscles want to postpone this well-earned relaxation by going into rigor? The answer has do to with the molecular motors of your muscle cells.

Myosin: Your muscles’ motors

When you move any part of your body, muscles contract to pull your bones the way they need to go. Your muscle cells do this by using a molecular motor called myosin, which tugs along a “rope” made out of protein. This is what it actually looks like (the red guy is myosin):

It doesn’t just do this on its own, you need energy to contract those massive biceps and rippling pectorals. Energy is something your body doesn’t have when its dead.

If myosin doesn’t have energy, why don’t muscles relax instead of tense up?

The thing about myosin is that, it acts like it is spring-loaded. You put in energy to reload the spring, then you release the energy to contract the muscle.

The process is similar to cocking a revolver. It takes some energy to pull the hammer back, but it doesn’t take any energy to get the gun to fire once its cocked.

To make matters worse for the muscle, myosin can’t release itself  from actin until it gets more energy to “re-cock” itself.

Result: The famous “stiff” corpse

When your killer comes around and does you in, your muscles don’t know that their energy supply is going to be running out. The unsuspecting myosin keeps going like nothing is happening, until it runs out of energy. Soon enough most of your muscle myosin is locked in the contracted position by the time the medical examiner finds you.

( dead man photo credit: The Rhumb Line; revolver photo credit: trawin)