A seat assembly reclines during an under-vehicle explosion or other upward impact on the vehicle to mitigate spinal and lower leg damage to a human occupant. The seat assembly has a back frame portion and a lower frame portion. The lower frame portion has a pivotal connection with an energy absorbing mechanism mounted to the vehicle floor. The pivotal connection includes a stop mechanism to prevent seat assembly tilt during normal vehicle operation but allow tilt due to an explosion. A second mechanism is disposed between the back frame portion and the floor; this mechanism controls the seat assembly pivot and provides further absorbing of energy from the upward impact. Projections from the front of the lower frame portion toward the floor can be used to enhance seat assembly pivoting; specially designed seat engagement levers can be used for this purpose as well.