A protective helmet, which includes a motion restrictor device, is disclosed which has at least one strut member associated with the helmet and a harness assembly, and the at least one strut member includes a locking assembly associated with the strut member, which upon a predetermined force being sensed by a force sensor or a predetermined amount of or rate of acceleration being sensed by an acceleration sensor, stops substantially all relative motion between the ends of the strut member and the predetermined force is substantially transferred from the helmet to the harness assembly.