Low Back Stiffness
Both the hamstrings and the hip flexors can impact your posture. It is important to stretch these muscles out in order to maintain a balance between the front and back sides of the body. Inflexibility in these muscles can produce an uncomfortable pull on the spine or pelvis, which you will experience as back stiffness or back pain.
You will find a good hamstring stretch in one of the other articles on this website. Link to Knee stretch - hamstrings.
Click on this YouTube link for a demonstration of a hip flexor stretch.