Final answer:
Rigor mortis is the stiffening of muscles after death due to the absence of ATP, resulting in the inability to move body parts. In a live person, a similar condition is referred to as "writer's cramp".
Step-by-step explanation:
Rigor mortis is the stiffening of muscles after death due to the absence of ATP, which normally allows the myosin heads to detach from the actin-binding sites, resulting in muscle relaxation. In a live person, this process is called "writer's cramp", while in a recently deceased person it results in rigor mortis. ATP is necessary for the myosin heads to detach from the actin-binding sites, and once ATP is depleted after death, the muscle becomes rigid.