Final answer:
Rigor mortis is a biological occurrence that happens after death, where muscles stiffen due to myosin heads being unable to detach from actin-binding sites in the absence of ATP. This is distinct from living muscle contractions like in 'writer's cramp'.
Step-by-step explanation:
The statement that rigor mortis happens 3-4 hours after death, and muscles begin to stiffen is True. This process occurs due to the cessation of ATP production upon death. During the life of an organism, ATP is responsible for the detachment of myosin heads from actin-binding sites, allowing muscles to relax after contraction. After death, when ATP synthesis stops, myosin heads remain bound to actin, preventing muscle relaxation and resulting in stiffness, also known as rigor mortis. This biological phenomenon significantly contrasts with muscle contractions during life, such as in 'writer's cramp', which results from temporary fatigue and excessive use of muscles, rather than a permanent lack of ATP.
Rigor mortis is a clear sign of muscle status post-mortem and can help forensic scientists estimate the time of death. Over time, rigor mortis diminishes as enzymatic reactions cause the muscles to soften again.