The fundamentals of occupant protection in a crash involve vehicle crashworthiness and occupant restraint. Crashworthiness refers to implementing a strong occupant compartment that resists intrusion and to designing crushable front and rear structures that deform and perform work to dissipate the kinetic energy of the crash. This combination provides controlled vehicle deceleration and survival space in the occupant compartment. Occupant restraint refers to the use of lap-shoulder belts, airbags, and other systems to provide ride-down of the vehicle deceleration, containment on the seat, and distribution of forces on the pelvis, shoulder, chest, and other designated anatomical structures to decelerate the occupant. The effectiveness of current restraint designs for protecting the occupant and reducing the risk of serious injury and death in a crash are well documented. Lap-shoulder belts are approximately 40 % effective in preventing death, with the highest effectiveness of nearly 80 % in rollovers and the lowest of approximately 30 % in near-side impacts . In frontal crashes, the addition of the airbag to the three-point belt system raises the effectiveness level to approximately 50 %. Despite this impressive safety record, belt system performance is continually being refined. For example, recent papers have discussed the development of four-point harnesses for use in production vehicles [2, 3], and devices such as pretensioners and belt load limiters  are becoming common features in contemporary vehicles. A pretensioner, which is usually pyrotechnic, triggers from the deceleration characteristic imparted to the vehicle by an impact. The charge winds several centimeters of belt around its storage spool, thereby removing slack in the system, generating a decelerating force on the occupant, and reducing the time required to generate force (tension) in the belt (i.e., the response time). Load-limiters typically involve an element within the belt retractor that yields when a pre-determined belt tension level is reached. This tension may range from 2 to 6 kN, and in the most advanced systems can be transient and programmable. These refinements introduce significant changes in the shape of the restraining force profile (Fig. 5.1), which have been shown to enhance belt performance both in the laboratory and on the road [6–12]. As this technology continues to develop, the characteristics of the restraining force applied to the occupant are becoming increasingly controllable. Continued improvement in restraint performance may be possible by implementing active control as an integral part of the restraint system. Steps have been taken to develop active systems (sometimes referred to as “smart” restraints) that adapt based on various inputs. For example, dual-stage pretensioners may modulate the magnitude of belt retraction based on the severity of the collision. Other “smart” aspects of restraint systems have been discussed by several researchers [4, 13–16].
This chapter reviews the biomechanics of restraints. The discussion includes a description of occupant kinematics for belted and unbelted occupants in frontal impacts with and without an airbag. The synergistic integration of restraint system components is a particular focus. For example, the airbag’s role in facilitating force-limiting belts is discussed. Non-frontal impacts are then discussed, followed by some special restraint design considerations, such as the occupant’s age and body habitus.