While designing for dynamic environments, like those found on high-speed vessels, engineers are constantly challenged with problems of acceleration. Acceleration is often measured in units of “g”, with 1 g equal to what is experienced due to gravity at earth’s surface (9.8 m/s^2).Acceleration imposes strict limitations on safety, and under severe exposure humans and hardware can quite literally break.
The human response to acceleration can include all sorts of adverse reactions to the body. Prolonged exposure can affect circulation and rupture blood vessels and endanger skeletal structures. At high accelerations, G-LOC, redding-out, and greying-out are among the possibilities. For comparison's sake, astronauts can sustain 3 to 4 g during launch, but the limits of human tolerance depend on multiple factors, including magnitude, direction and rate of change. What a person can withstand in the forwards-facing direction might be entirely different from what they will tolerate in the vertical.