Head restraint evaluations are based on two criteria. The first is the distance down from the top of the head of an average-size male to the top of the restraint. A head restraint should be at least as high as the head's center of gravity, or about 3½ inches below the top of the head.
The second criterion is backset, the distance from the back of an average-size male's head to the front of the restraint. Backsets of more than about 4 inches have been associated with increased symptoms of neck injury in crashes.
Each head restraint is classified into one of four geometric zones — good, acceptable, marginal, or poor — according to its height and backset (see diagram). Marginal head restraints have the minimum height necessary to protect an average-size male from whiplash injury. Acceptable and good restraints are high enough to protect taller occupants as well as people of average height and shorter. Good and acceptable head restraints also have smaller backsets, which benefit occupants of all heights.
The rating for a fixed head restraint is straightforward. The zone into which its height and backset place it also defines its rating.
Rating adjustable head restraints that don't lock in their adjusted positions is equally straightforward — the rating is defined by the zone for height and backset in the down and/or rear position. For adjustable restraints that lock in position when adjusted, the rating is based on the midpoint of the best (highest and closest) and worst (lowest and farthest) positions in relation to an average-size male.
This measurement and rating system is an international protocol available from the Research Council for Automobile Repairs.