Final answer:
During rigor mortis, muscle stiffness occurs due to the myosin heads not detaching from actin-binding sites, as no ATP is produced postmortem to facilitate this detachment. This results in the muscular rigidity associated with rigor mortis.
Step-by-step explanation:
During rigor mortis, the muscles stiffen due to the absence of adenosine triphosphate (ATP). Normally, ATP is necessary for myosin heads to detach from the actin-binding sites, which allows muscles to relax after contraction. However, after death, ATP production ceases, myosin heads remain attached to actin, and the muscle cannot transition back to a relaxed state. Consequently, the muscles become rigid, which characterizes rigor mortis. Contrary to options 1 and 3 provided in the question, the myosin heads do not detach from actin, and tropomyosin and troponin do not disengage during rigor mortis. Option 4 is not directly related to the cause of rigor mortis; while lactic acid may be present due to prior anaerobic metabolism, it is not the immediate cause of muscle stiffness in rigor mortis. The correct understanding pertains to the absence of ATP and the resultant inability of myosin to release actin.