How to Arch Your Back?

In general, you don’t want to over arch your lower back during any strength training exercise. Your lower back already has a natural curve. Over-arching creates tremendous pressure to the discs in the lower back and creates a high risk of injury. In particular, you don’t want to arch your back when you do the bench press. Note that when you do a back extension exercise, the curvature of the back should be distributed along the entire length of your spine rather than focused in the lower back.

Bench-pressing the wrong way
Some weight lifters think that anything they do to pile on poundage — including arching their back and squirming around — is fair game. In reality, how much weight you hoist above your chest isn’t necessarily related to how strong your chest muscles are. When you arch your back, you simply increase your mechanical advantage (and your injury risk); more muscles pitch in to move the bar upward. We know one guy who convinced himself that arching was an essential part of the bench press. We produced several anatomy textbooks before we convinced him that we hadn’t fabricated this bit of information.

Bench-pressing the right way
Keep your back in contact with the bench throughout the exercise. You don’t need to force your back into an unnaturally flat position — it’s okay to have a small, natural arch under your lower back. If you can’t plant your feet flat on the floor because the bench is too high, place your feet on the bench.