Final answer:
Rigor mortis is the stiffening of muscles after death due to myosin heads being unable to detach from actin-binding sites in the absence of ATP. Muscle contraction requires ATP, and when its production ceases post-mortem, the muscles remain in a contracted state, resulting in rigidity.
Step-by-step explanation:
The chemical changes that cause the limbs to stiffen after death are commonly referred to as rigor mortis. This process involves myosin heads within the muscle cells, which normally detach from the actin-binding sites during muscle contraction and relaxation cycles when ATP (adenosine triphosphate) is present. After death, when ATP production stops, these myosin heads are unable to detach from actin, resulting in the stiffening of muscles, as the cross-bridges formed during contraction remain in place.
In a live individual, ATP is continuously produced and used for muscle contraction. It powers not only the binding and unbinding of myosin to actin but also the reuptake of calcium ions into the sarcoplasmic reticulum. The absence of ATP after death, therefore, leads to a state where the muscles are locked into place. Unlike situations like 'writer's cramp' where temporary muscle fatigue and stiffness occur due to overuse and lactic acid build-up, post-mortem muscle rigidity is irreversible without the influence of decomposition processes.