The belt winds onto and off a reel (18) which in turn deforms an energy absorbing element (24) when the reel is exposed to rotary force in the belt wind-off direction, notably in sudden braking and in this way guards against any increase in belt tension. A pre-tensioner join (64,80,86,94) joined to the reel on excessive braking causes the reel to turn in the wind-on sense. A release sector (24D) between reel and pre-tensioner cancels the reel rotation in the wind-off sense once the pre-tensioner has operated. The release sector also cancels out reel rotation whenever the absorber is exposed to excessive deforming force. The release sector forms a break section joined to both reel and pre-tensioner to part the reel-connected side from the other side when the release sector (24D) shears. The absorber presents a torsion bar (24) joined at one end to the reel and joined at the far end to the inertial locking mechanism which responds to any acceleration. The release section (24D) joined to the pre-tensioner extends integrally from one end of the bar (24) so as thus to form a sector exposed to torsional breaks.
