a head and neck restraint system includes a headpiece , a saddle , and tensioners connecting the headpiece to the saddle . the headpiece resides on top of a crew member &# 39 ; s head and the saddle includes a saddle strap residing under the crew member &# 39 ; s buttocks . the tensioners are pneumatic muscles which contract when a flow of gas is provided by gas generators . the gas generators are triggered by a g - sensor or by the initiation of a predetermined event . triggering signals pass through a manifold from an ejection seat , aircraft , or vehicle . contraction of the pneumatic muscles pulls the headpiece toward the saddle , thereby restraining the head , neck , and spine . the system is worn as an undergarment or incorporated into a garment . a manual adjustment provides head support during flight . the system is particularly useful for preventing head and neck injuries during ejection from or crash of an aircraft or vehicle