Another interesting article in this vein. Have suspected for awhile that it is not ipso facto harmful. If the good Lord didn't intend for us to round the lower back, why did he make it to bend ? http://breakingmuscle.com/strength-conditioning/embracing-the-flexed-lumbar-spine-in-lifting In any case, I've been doing rock-bottom goblet squats with a heavy dumbbell for awhile now. And I know that my lumbar spine is in flexion. No ill effects.