It's often said that using a big arch while benching leads to back issues. A
takes a look at this assumption.
Shawn Lattimer provides a couple of very good and very technical responses:
the pressure on the spine itself doesn't change. There is no compressive force applied to the spine. Regardless of flat back or not, you still have the same surface area directly under the bar load, specifically your upper back.
When benching flat backed, only the center portion of your back takes the load, so less of your body is distributing the weight. When properly set up with the arch, shoulderblades pinched, traps retracted, you have much more body mass under the bar, which distributes the pressure of the bar over more tissues and bones. Remember, your body is 3-dimensional, you are talking about a flat plane here.
Therefore, if your back is healthy, there is no danger from benching with an arch, regardless of the popular "personal trainer" advice.