Final answer:
Rigor mortis is the stiffening of joints and muscles in a body that occurs a few hours after death. It happens due to the inability of the myosin heads to detach from actin filaments in the absence of ATP. This leads to muscle stiffness and prevents movement.
Step-by-step explanation:
Rigor mortis is the stiffening of the joints and muscles of a body a few hours after death, usually lasting from one to four days.
Micro-contraction cycles of actin sliding along myosin continue as long as ATP is available. In a live person, the detachment of myosin heads from the actin-binding sites results in muscle relaxation. However, after death, in the absence of ATP, myosin heads remain bound to the actin filaments, resulting in muscle stiffness and causing rigor mortis to occur.
For example, when a person dies, the depletion of ATP stops the micro-contraction cycle, causing all myosin heads to remain bound to actin filaments and muscle cells to become rigid. This rigidity is what is known as rigor mortis.