Why is your lower back sore?
Simple answer? You are not moving enough. You are not moving correctly. If you think about our spine, there are many beliefs that throughout exercise and training we need to keep a neutral spine. This is both correct and misleading. The spine is strongest when it is in a neutral position in the lower back. However... We also need to train our spine in flexion and extension. Have you got a really strong deadlift but somehow you manage to put your back out when bending over to pick something up off the floor? I wonder why... It is because you probably never put work in the gym when your spine is flexed or extended. Think about it. If you never ever trained your bicep at full extension and then all of a sudden did something challenging in an extended position, your body is not going to like it.