Final answer:
The head restraint should be positioned at or above the top of the ears and closely behind the head, aligning the head's center of mass with support points to avoid neck injuries and support good posture.
Step-by-step explanation:
If the head restraint in a vehicle is adjusted properly, it should be positioned directly at or above the top of the person's ears and against the back of the head. This setup is crucial because if an accident were to occur, an improperly positioned head restraint could lead to increased risk of neck injuries such as whiplash. Properly adjusting the head restraint enhances protection from nervous system injuries and supports a good posture by aligning the center of mass of the head with major points of support such as the vertebrae and the atlanto-occipital joint.
The muscles at the back of the neck are responsible for keeping the head erect. When a person is awake and alert, these muscles compensate for the fact that the head's center of mass is not directly over the principal point of support. However, when asleep, these muscles relax, which is why a person's head often falls forward, illustrating the importance of ensuring correct head support both when awake (in terms of posture) and when seated in a vehicle.