impact injury to a body within or upon a rapidly moving object caused by the forces exerted when the object is brought to a sudden halt. Deceleration injury can occur in high-speed vehicles when they stop or slow down abruptly or when the occupants of the vehicle are propelled from it while it is moving. Most experiments in deceleration have been done in connection with air travel, in which the acceleration factor is usually much greater than in land vehicles.

Acceleration and deceleration forces can be measured in terms of gravitational acceleration (g). A force of three g, for example, is equivalent to an acceleration three times that of a body falling near the Earth. Factors that influence the effects of deceleration are the initial rate of speed, the distance covered and time consumed in deceleration, the direction of forces, and the area of distribution.

The best position for tolerance of deceleration seems to be for the pilot to have his back facing the line of acceleration, and with support from a firm metal seat lined with an energy-absorbing material such as a 0.5-inch (1.3-centimetre) cushion of felt. When deceleration occurs with the pilot in this position, the body is pressed against the seat and supported by the metal structure. When seated facing the line of acceleration, the pilot is pressed against the seat during acceleration but thrown forward upon deceleration.

Exposures to deceleration forces lasting longer than 0.2 second can cause fluid displacement or tissue deformation. If the duration of deceleration in a position facing forward is less than 0.2 second, the maximum endurable deceleration force is 30 g. This causes a drop in blood pressure, rise in pulse rate, weakness, and pallor of the skin. In the backward-seated position, forces up to 35 g can be tolerated with few apparent difficulties.

Windblast (wind) and wind drag can also cause injuries during deceleration. Deceleration from air resistance often causes more damage than mechanical deceleration, for it takes longer to stop by wind drag than by mechanical braking methods, and the pilot must endure the exposure in different body positions.

Injuries encountered in deceleration can range from shock, concussions, abrasions, sprains, skin tears, and internal-organ ruptures to fractured bones, respiratory and circulatory arrest, hemorrhages, and organ damage.