Lower back pain in runners is a fairly common occurrence. It is usually not hard to pinpoint where the pain is, but the interesting question is why.
One of the most fundamental contributors is simply how you carry yourself as you run. Many runners are very aware of posture these days and try to remain very upright as they run, but often by keeping their shoulders pinned back and their spine in what they perceive to be a straight position, they actually block the normal rotational motion of the spine which would help to dissipate impact forces on the spinal structures.
Carrying yourself buoyantly against gravity does not have to be a fight. Try standing quietly, soft and relaxed for a moment, feeling the floor pressure under your feet. Now imagine a small helium balloon attached to the top of your head, just slightly behind the centre point. Make it real and give it a colour. Helium rises, and the balloon’s job is to take the weight of your head off your spine, allowing it to expand and lengthen. Feel how your shoulders drop, and how your lower belly draws in without conscious effort.
This is as much effort as it should take to support yourself while running, so use your balloon to create a relaxed self carriage which can be sustained over distance without over-stressing other structures.
You want to go forward, so don’t spend your energy straining upward!
For more information on moving with less stress, see Stability, Sport and Performance Movement: Great Technique without Injury by Joanne.