Final answer:
Without ATP production after death, myosin heads cannot detach from actin, causing muscles to stiffen in a state called rigor mortis, with sarcomere length becoming fixed.
Step-by-step explanation:
After death, since ATP is no longer produced, the cross-bridges that form between myosin heads and actin filaments within the sarcomeres of muscle fibers cannot detach. This is because the detachment of myosin heads from actin requires energy from ATP. Consequently, the length of the sarcomere becomes fixed in the state it was in at the moment of death, unable to contract or relax further. This results in the stiffening of the muscles, known as rigor mortis, where muscles become rigid due to the myosin heads remaining permanently attached to actin as all the energy sources for detaching them have been depleted.