A motion restrictor device is operable to be worn with a protective helmet to reduce the risk of head or spine injury caused by injurious movement of the helmet. The device includes a helmet-engaging component supported on a harness. The component presents helmet-engagement surfaces positioned on opposite sides of the neck of the user. The component is operable to yieldably bias each of the helmet-engagement surfaces toward the helmet when the device is worn, such that contact with the helmet is maintained as the helmet-engagement surface shifts through a range of motion. The device also includes a brake assembly that restricts shifting of at least one of the helmet-engagement surfaces in response to injurious movement of the helmet.