The framework and moving parts of the human body are designed on mechanical principles, although far more elaborately so than any man-made structures such as an automobile engine or a cantilever bridge. They also are affected by the physiology as well as the pathology, which means that they have to work whether the tissues are healthy or diseased. The wonder is that they do not more often go to pieces altogether but just function poorly and, rather than creaking and rattling badly, merely send word to the brain of their worn parts and need of adjustment.
Also in considering low back pain one must remember that the place where discomfort is felt on many occasions is not the chief seat of the trouble. Indigestion may show itself as headache; a disturbance of the middle ear may result in vomiting. A breaking down of the arch of the foot or a badly set fracture of the leg frequently produces backache.
If the broken bone of the leg is set at a wrong angle, then the thigh bone does not rest properly on it. Also the muscles of the leg which are attached to the thigh bone do not pull in the direction that they should. So the mechanics are wrong clear up to the back. The back muscles and ligaments have to handle the whole weight of the body from the waist up. With the little leverage that they have and the rather insecure balance of the body upon the pelvis, which is the great girdle of bone to which the thighs are attached, they have a big job even when everything is normal. Put things out of plumb and their task is really tough.
But one does not have to have fallen arches, broken bones and such catastrophic happenings to develop backache. Nature rarely if ever makes a perfect piece of work in building us. It is common knowledge now that the two sides of one's face are not perfect twins. No more so are our lower extremities (legs to you) or, in technical anatomical terms, the combination of thighs and legs. (I seem to remember from my medical student days that the newly arrived Yiddish-speaking immigrants called the entire limb from the hip to the ground the "fuss" or foot.)
Careful measurements would probably show that your two legs are unequal in length. Most people compensate for this discrepancy with little trouble. An orthopedic friend of mine has told me of numerous patients of his who never got comfortable backs until he took measures to compensate for the unequal length of their "legs."
In fact the lower back is one of the most vulnerable parts of the body. Fortunately the overwhelming proportion of afflictions to it tests our philosophy and equanimity but does not endanger us. When, as and if you get lumbago, it might be worth while to have a good look over, and if it proves to be just "misery," try to be one of the groups, who grins and bears it. Lots of your friends are doing the same unbeknownst to you. And don't take kidney pills. Aspirin is cheaper and better. The kidneys are nestled in the small of your back, but, when they get diseased, they may cause you almost every kind of trouble except a backache.
The one chief cause for backache is man's decision to stand upright on two limbs. It may be difficult to determine with accuracy whether four-legged animals suffer much from this affliction, but I have seen no reason to suppose that they do. Few of us spend much more than a third of our time lying down, and when we are standing or even sitting, there is almost continuous strain on the muscles of our lower back.