Unless they are talking about robot exploration of Mars traveling on these fast fusion engines, how will the inertia from that speed not kill the human passengers?

In Star Trek they had to invent a BS device called “inertial dampners” to explain why the high speed they were traveling at did not crush them into jelly in their chairs or fling them into the viewing screen when they stopped (Newton’s Law of motion) and why they did not need seat belts (and why chairs were not bolted to the floor).