Various embodiments of seat harness pretensioning devices for use in land, air, and sea vehicles are described herein. In one embodiment, a seat unit for use in a military land vehicle or helicopter includes a stroking device that enables the seat to move downwardly in response to sudden movement. The seat unit also includes a seat belt or harness that extends around the occupant in the seat. Sudden movement of the seat in response to an explosion or hard landing causes a tensioning system to automatically pretension the seat harness.