In rigor mortis, the muscles contract hours after death due to a breakdown of the sarcoplasmic reticulum. this is because the sarcoplasmic reticulum stores acetylcholine, which triggers muscle contraction calcium, which triggers muscle contractions atp, which prevents myosin from detaching from actin tropomyosin, which prevents myosin from detaching from actin

The muscles of a dead person typically contract hours after death due to a breakdown of the sarcoplasmic reticulum, and fails to relax; this is known as rigor mortis. The contraction happens because the sarcoplasmic reticulum stores calcium, which triggers muscle contractions. Relaxation fails to occur because cross-bridges form, but can't release due to lack of ATP formation.