During a forward acceleration of at least 8g on the seat supporting structure, the rearward angular displacement of the head reference line shall be limited to 45° from the torso reference line, or
Head restraints must be at least 700 mm (27.6 in) above the seating reference point in their highest position and not deflect more than 100 mm (3.9 in) under a 372 N⋅m (3,292 in⋅lbf) moment. The lateral width of the head restraint, measured at a point either 65 mm (2.56 in) below the top of the head restraint or 635 mm (25.0 in) above the seating reference point must be not less than 254 mm (10.0 in) for use with bench seats and 171 mm (6.73 in) for use with individual seats. The head restraint must withstand an increasing rearward load until there is a failure of the seat or seat back, or until a load of 890 N (200 lbf) is applied.An evaluation performed by NHTSA in 1982 on passenger cars found that "integral" head restraints—a seat back extending high enough to meet the 27.5 in (698.5 mm) height requirement—reduces injury by 17 percent, while adjustable head restraints, attached to the seat back by one or more sliding metal shafts, reduce injury by 10 percent. NHTSA has said this difference may be due to adjustable restraints being improperly positioned.
The focus of preventive measures to date has been on the design of car seats, primarily through the introduction of head restraints, often called headrests. This approach is potentially problematic given the underlying assumption that purely mechanical factors cause whiplash injuries — an unproven theory.