Cross bridge formation between myosin heads and actin molecules is caused by the elevation of calcium ion concentration in the cytosol. during rigor mortis, this elevation of calcium ion concentration in the cytosol is permanent because stop producing atp molecules required by the sarcoplasmic reticulum's calcium ion pumpstroponin molecules bind irreversibly to calcium ions to prevent them from being removed from the cytosoltropomyosin molecules bind irreversibly to calcium ions to prevent them from being removed from the cytosolacetylcholine continues to stimulate the release of calcium ions from the sarcoplasmic reticulum

by the sarcoplasmic reticulum's calcium ion pumps

Explanation:

Rigor mortis or postmortem rigidity, is a state caused by chemical changes in the muscles that occur post mortem and  cause the limbs of the corpse to stiffen.

This occurs because after death, there is no cellular respiration and consequently, no oxygen nor ATP (ATP is required for the muscle relaxation).

Calcium enters the cytosol after death because it is released due to the deterioration of the sarcoplasmic reticulum and due to the breakdown of the sarcolemma. Ca2+ ions activate the formation of actin-myosin cross-bridging necessary for the contraction. Muscles are unable to relax, because myosin heads continue to bind with the active sites of actin and there is no ATP to destabilize the myosin-actin bond.