Lumbar Spine, Sacrum, and Coccyx
Meet your lower back, the part that causes pain for the vast majority of people. But before you curse the day you were born with it, know that those five lumbar vertebrae (L1–L5) have a mighty big job to do: they support most of the weight of your body. As you can see in the illustration, being the largest of the vertebrae, they're highly qualified for the job.
These bones are indeed made for walking, running, sitting, and lifting. All these activities, of course, have a potential injury risk—which you can reduce by keeping your back and abdominal muscles strong and maintaining proper flexibility throughout your back and body. Good muscle conditioning lends support to your lower back (and other parts of the spine), and with proper stretching, you can keep the area flexible as well.
Lumbar Lordosis or "Swayback"
An excessive curve in the lower back is called lordosis, also known as swayback. This curve puts way too much pressure on your lumbar vertebrae. Lordosis can be caused by disease, a movement of the spine such as bending the back, or bad posture. Think of the final pose of gymnasts when they dismount from the parallel bars. The chest is thrust forward, shoulders back, and the lower back arches. This is what extreme lordosis in the lower back can look like. Of course, gymnasts do it on purpose. Although it's not a disease when they perform these contortions, they can end up with back problems because of it.
For the rest of us mortals, simply sitting incorrectly can cause too much pressure on the lumbar spine. That's why knowing how your vertebrae should be aligned and taking appropriate steps to make that happen can go a long way toward relieving back issues.
Sacrum and Coccyx
You might think that spinal fusion is something only surgeons do, but nature actually does this too and if you're over 30 years old, it's already happened to you. Your sacrum (from the Medieval Latin os sacrum, meaning holy bone), the flat triangular bone situated between your hips, is actually five fused vertebrae. This fusion isn't complete until you're about 25 or 30. This part is the lowest and last curve in your spine. The curve, called the lumbosacral curve, helps support body weight.
Below the sacrum is the tail end of your spine, called the coccyx or tailbone. Again, several fused vertebrae (generally 3–5) form the coccyx. Injury to this area can lead to coccydynia, which is a real pain in the you-know-what.
Coccydynia is a painful condition involving swelling around your tailbone. When the ligaments and tendons in this area become inflamed, it hurts to sit. You can also get this pain from a fracture of the coccyx, which can happen if you fall and land on your tailbone.
Jason Highsmith, MD is a practicing neurosurgeon in Charleston, NC and the author of The Complete Idiot's Guide to Back Pain. Click here for more information about the book.