Final answer:
Rigor mortis describes the sustained attachment of myosin to actin in the absence of ATP, leading to muscle stiffness. This occurs post-mortem as ATP synthesis stops, but it can also cause cramps in living beings when ATP is temporarily depleted.
Step-by-step explanation:
The phenomenon characterized by continued attachment of myosin heads to actin filaments due to a lack of ATP is known as rigor mortis. Without ATP, the myosin heads cannot detach from the actin-binding sites, causing muscle stiffness. Rigor mortis occurs after death because ATP synthesis ceases, which prevents the myosin heads from releasing from the actin. This binding is essential for muscle contraction and relaxation in living organisms, where ATP is continuously consumed and replenished.
The power stroke of muscle contraction involves myosin heads pulling the actin filaments toward the M-line, a process that depends on ATP. This process is a continuous cycle of attachment and detachment, which requires ATP both to enable the myosin heads to bind to the actin and to release after the power stroke. In a living organism, if ATP is depleted, it can lead to conditions like writer's cramp due to sustained contraction.