Final answer:
Rigor mortis is due to myosin heads remaining bound to actin because there's no ATP available postmortem to facilitate their detachment, causing muscle stiffness.
Step-by-step explanation:
Rigor mortis after death is caused by myosin crossbridges that fail to dissociate from the actin thin filaments because of lack of ATP (Adenosine Triphosphate). During life, ATP supplies the energy for muscle contractions and is necessary for myosin heads to detach from actin-binding sites. After death, ATP generation ceases, leading to a state where the myosin heads cannot disengage from actin, resulting in the stiffening of muscles known as rigor mortis.
The myosin and actin filaments are part of the sarcomere, which is the basic unit of muscle contraction. The continuous cycle of myosin heads pulling on actin to contract the muscle can only proceed when there is sufficient ATP. Without ATP, the detachment of myosin from actin is prevented, which would otherwise lead to muscle relaxation during life.