Pathophysiology of Rigor Mortis: Causes and Mechanisms

Introduction: Description of Pathology

Rigor mortis, or postmortem stiffness, is one of the first signs that decomposition has begun. It occurs when the body's muscles stiffen due to a lack of adenosine triphosphate (ATP), the cell's primary energy molecule. ATP is necessary for muscle contraction, and without it, the muscles are unable to relax. Rigor mortis typically begins 2–6 hours after death and can last up to 24–48 hours, though it is not uncommon for muscles to begin relaxing after 8 hours. Depending on environmental conditions, the body begins to decompose and the muscles start to break down, leading to a decrease in stiffness following the initial onset of rigor mortis, making it a temporary condition only.

In some cases, rigor mortis can be used to estimate time of death. However, this is only possible if environmental conditions are consistent with the body temperature at the time of death. If rigor mortis has not yet set in, it may be indicative of temperature changes that occurred after death.

Normal Anatomy of Muscle

There are three types of muscles in the human body: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and support movement. Smooth muscles are found in the walls of internal organs and assist with functions such as digestion. Cardiac muscle is found in the heart and pumps blood throughout the body. All muscles are composed of cells that contract when they receive a signal from the nervous system, and it is this contraction that allows the body to move.

Muscles are composed of bundles of long, thin fibers. Each fiber is made up of even thinner filaments of protein. The filaments slide past each other when the muscle contracts, causing the muscle to shorten and produce force. Skeletal muscles attach to bones via tendons, while smooth and cardiac muscles attach via sites on connective tissue. When all the fibers in a muscle contract simultaneously, it is called an isometric contraction — this type does not produce movement because the muscle's length does not change. An isotonic contraction occurs when some fibers contract while others relax, shortening the muscle and producing movement. There are two types of isotonic contractions: concentric and eccentric. In a concentric contraction, the contracting fibers become shorter; in an eccentric contraction, the contracting fibers become longer (Vang & Niznik, 2020).

No matter the type of contraction, all muscle activity requires energy in the form of ATP. ATP fuels the chemical reactions that cause filaments to slide past each other and generate force. Muscles obtain ATP from either aerobic or anaerobic metabolism. Aerobic metabolism uses oxygen to break down glucose or fatty acids and produces ATP slowly but sustainably over long periods. Anaerobic metabolism does not use oxygen but produces ATP much more quickly, though it can only be sustained for short bursts of activity.

Normal Physiology and Excitation-Contraction Coupling

In normal physiology, all systems of the body — respiratory, circulatory, digestive, musculoskeletal, nervous, endocrine, and urinary — work together to maintain a state of equilibrium. The respiratory system brings oxygen into the body and eliminates carbon dioxide; the circulatory system transports blood, oxygen, and nutrients to cells while removing waste; the digestive system breaks down food into usable nutrients; and the musculoskeletal system provides support and movement. The nervous system coordinates all bodily activities, the endocrine system regulates metabolism, and the urinary system removes wastes. All these systems must function in harmony for the body to operate properly.

The muscles are the major body system affected by rigor mortis. They are composed of numerous muscle fibers bundled together by connective tissue, and their primary function is to generate force through contraction. In order to generate force, muscle fibers must be activated by motor neurons. This process is known as excitation-contraction coupling (ECC) — a process in which an electrical impulse (action potential) generated by a motor neuron causes the release of calcium ions from intracellular storage sites within the muscle fiber (Jungbluth et al., 2018). The calcium ions then bind to regulatory proteins located on the contractile filaments within the muscle fiber, activating the contractile process and generating force. ECC is vital to normal muscle function; without it, muscles would be unable to generate force or produce movement.

Conclusion

For a body to have normal muscle function, muscles require the presence of ATP, which is lost during rigor mortis and which normally enables muscle filaments to slide past each other and produce force. Because rigor mortis is a normal and inevitable consequence of death, it cannot be treated or wholly avoided. It can be slowed by environmental factors such as cold temperatures or by limiting movement of the body after death, but it is a condition inherent to the process of dying and is ultimately only delayed, not prevented.

References

Hall, J. E., & Hall, M. E. (2020). Guyton and Hall textbook of medical physiology e-Book. Elsevier Health Sciences.

Jungbluth, H., Treves, S., Zorzato, F., Sarkozy, A., Ochala, J., Sewry, C., ... & Muntoni, F. (2018). Congenital myopathies: disorders of excitation–contraction coupling and muscle contraction. Nature Reviews Neurology, 14(3), 151–167.

Vang, C., & Niznik, A. (2020). The effectiveness of isometric contractions compared with isotonic contractions in reducing pain for in-season athletes with patellar tendinopathy. Journal of Sport Rehabilitation, 30(3), 512–515.

Yan, T., Hou, C., Wang, Z., Li, X., Chen, L., Liang, C., ... & Zhang, D. (2022). Effects of chilling rate on progression of rigor mortis in postmortem lamb meat. Food Chemistry, 373, 131463.