In Podiatric Medical School we were taught that about 20% of low back pain was caused by foot problems – specifically excessive foot pronation. An article in September 17, 2013, medical journal Rheumatology online says that women “with flat feet are 50% more likely than those with normal or high arches to have low back pain.” 
In school, I suspected that the flat feet / low back pain relationship was perhaps exaggerated in order to make flat feet seem more important. After all someone once said that 90% of people excessively pronate – and those that don’t wish they could! (OK- a bored and cynical podiatry student said that – but it wasn’t me.)
Why do flat feet tend to cause lower back pain? Without getting too technical, when your arch is too flat (pronation) your leg turns inward and as you walk the forces going up your leg pass through the ankle, knee and hip in a crooked way (trying not to be technical!) Ultimately the pelvis is forced to compensate. The abnormal movement of the pelvis causes pain where the lower spine (lumbosacral area) rests on the pelvis.
People with a pair of flat feet technically have a change in the position of their femoral heads which causes an increase in lumbar lordosis, lumbosacral angle, and jamming of the L4-5 and L5-S1 facets. When you see them walk they often have a sway back and their tush looks like it’s larger than it really is!
The Rheumatology article speculates that women may be more susceptible to the flat feet / low back pain problem than men because women’s pelvic bones are wider and not as flexible as men’s, and women move their upper bodies more when they walk. (Does that make them better dancers?)
When people come to see me thinking that their feet may be implicated in their lower back pain, the first thing I do is a complete biomechanical examination of their feet. We look at the structure, measure ranges of motion of key joints, do a careful gait analysis (medical talk for watching you walk) and check your leg lengths. If you have a short leg we try something in your shoe to balance your leg lengths. We control your excessive pronation temporarily with a special strapping of your foot. Simple, simple things – and you would be amazed at how many people tell us their back pain improves! I’ve seen the flat feet / low back pain relationship for thirty years in practice and strongly suspected that flat feet were implicated in over 20% of lower back pain. It’s great to see a prestigious medical journal to confirm my experience. Now if they could just get the word out to those people with lower back pain – we could make a lot of people happy!
Source: http://rheumatology.oxfordjournals.org/content/early/2013/09/17/rheumatology.ket298.abstract, Rheumatology, online September 17, 2013.