Four series of sled tests were conducted with cadavers to analyse the response of the human head under different impact conditions (30 and 50kph) and with different types of restraint systems. The restraint systems used were two different force-limited shoulder belts (4 and 6kN), and an airbag coupled with a 4kN belt. The restraint systems and, to a lesser extent, the test conditions modify the behaviour of the head-neck system. Without airbag, the behaviours of the head-neck are driven by the load in the belt. The addition of an airbag influences the behaviour of the head and changes radically the behaviour of the base of the neck (T1). The rotation at the occipital joint in the transverse plane is not negligible. So, the injuries mechanisms of the head-neck could be very different, even if the maximum values of the resultant acceleration are similar for different restraint system.