As our tests of the iPhone 6, Samsung Note 3, and other smart phones a few months ago showed, keeping the devices in your back pocket may bend the device when you sit down. But storing your phone in your back pocket can also be bad for your back and butt.
Specifically, it can trigger sciatica, or pain that starts in your buttocks and shoots down the leg. I know that because, as a practicing neurologist, I have seen several patients complaining of the problem over the past few years. And I've been able to trace it back to the object tucked into their back pocket.