Hi Kristina, 
The spinal column is a conduit for nerve fibers that travel to all parts of
the body. A lower back injury will commonly cause pain in a hip, the back of
a knee, a calf, or in the foot. Here is an excerpt from:
http://www.ninds.nih.gov/disorders/backpain/detail_backpain.htm
What structures make up the back?
The back is an intricate structure of bones, muscles, and other tissues that
form the posterior part of the body's trunk, from the neck to the pelvis.
The centerpiece is the spinal column, which not only supports the upper
body's weight but houses and protects the spinal cord - the delicate nervous
system structure that carries signals that control the body's movements and
convey its sensations. Stacked on top of one another are more than 30 bones
- the vertebrae - that form the spinal column, also known as the spine. Each
of these bones contains a roundish hole that, when stacked in register with
all the others, creates a channel that surrounds the spinal cord. The spinal
cord descends from the base of the brain and extends in the adult to just
below the rib cage. Small nerves ("roots") enter and emerge from the spinal
cord through spaces between the vertebrae. Because the bones of the spinal
column continue growing long after the spinal cord reaches its full length
in early childhood, the nerve roots to the lower back and legs extend many
inches down the spinal column before exiting. This large bundle of nerve
roots was dubbed by early anatomists as the cauda equina, or horse's tail.
The spaces between the vertebrae are maintained by round, spongy pads of
cartilage called intervertebral discs that allow for flexibility in the
lower back and act much like shock absorbers throughout the spinal column to
cushion the bones as the body moves. Bands of tissue known as ligaments and
tendons hold the vertebrae in place and attach the muscles to the spinal
column. 
 
 
As for your question about gluten, there are several ways in which it can
contribute to back pain. These include: weakening back muscles and a
resulting misalignment; inciting an autoimmune attack at the site of
previous damage through molecular mimicry ( a theory that is rapidly gaining
credence); induced mineral deficiencies resulting in mineral losses from the
lower back; exacerbation of inflammation at the site of a pinched nerve as
it exits the spinal column; etc. 
 
I hope this helps.   
best wishes, 
Ron