The ancient Romans employed a wonderful invention in their architecture – it was called the arch. Arches allowed stresses built up from the weight of the Roman building materials (stones) to be distributed, which meant fewer stones were needed to support walls and domes.
Arches distribute stress. The same principle applies in our bodies. When you look at the body you never see a straight line. Everything is curved to a greater or lesser degree. Even the longest bone, the femur, has a curve to it. Curves are everywhere but probably the most noticeable curve is the spine. 
The spine has four curves. It forms a double S, with the curves in the neck and lumbar moving in opposite directions to the curves in the thorax and sacrum. The forward curve of the lumbar and cervical spine is termed “lordosis.” The backward curve of the thoracic spine is called “kyphosis.” These four curves are immensely important for an animal that walks upright; these four curves distribute the stress of keeping the torso vertical.
The spine, when healthy and possessing all its normal curves, acts like a spring. Every time we increase the pressure on our body – for example, by walking or running – the spine flexes. The curves deepen and then they release. If our spine were a straight rod, the stresses would fall in between the vertebrae, and the disks cushioning the vertebrae would wear out quite quickly. Of course, the ligaments wrapping the spine also take some of the strain, but these are more responsible for taking the strain of passive activities, such as sitting or standing. Our muscles support the dynamic movement of the spine.
For many people the curves in the spine become exaggerated in one direction or the other. Of most concern are changes in the lumbar and cervical spine. If the lumbar spine curves too far forward, hyperlordosis occurs. If the lumbar spine curves too far backward, hypolordosis occurs. These two conditions are arbitrary diagnoses; there is no standard that health care professionals apply to determine if an individual is hyper- or hypolordotic. The therapist has to decide, based on her own experience.
A study in 2002 at the University of Waterloo in Canada  noted, “Tissue failure can result from excessive strain.” The study measured the position of least strain for the spine and noted how people varied from that optimal position while walking, standing, and sitting. The authors noted, “Research supports the proposition that individuals with hypolordotic or hyperlordotic lumbar spine posture have more tissue strain and a smaller prefailure tissue safety margin when performing various tasks such as sitting, standing, and walking.” This simply means that if you have too little curve or too much curve in your lower back, you are at risk of lower back pain and more severe problems later in life.
The study also asked if the conditions of hyper or hypolordosis could be changed by exercise. The result of the study was a qualified yes. The authors also noted, “As this is the first study documenting whether lordosis should be and is trainable, no literature exists for comparison. People with hypolordosis appear to have greater posterior tissue strain when seated than do people with hyperlordosis The results indicate that a person with hypolordosis could be at greater risk for strain-related tissue failure when sitting than a person with hyperlordosis.”
This makes intuitive sense. If the normal ranges of motion for your back are reduced or exceeded significantly, you will have problems. A person who already has a significant forward curve in her lower back can cause hyperlordosis by standing for long periods, as this causes greater extension of the lumbar. On the other hand, someone who sits for long periods of time leaning against the back of a chair (the familiar slouch position) will tend to decrease lordosis leading to hypolordosis.
Sitting, it turns out, is one of the worst things we do to our spines, especially if we are already slightly hypolordotic.
The American Family Physician in its April 1998 publication stated, “Up to 90 percent of the U.S. population may have significant low back pain at some point. In 1984, it was estimated that over 5 million persons were incapacitated as a result of lower back pain. The financial impact in terms of health care dollars and lost work hours reaches billions of dollars each year in this country.”
That is a lot of people with lower back problems but when you watch virtually everybody who sits in a chair today, you can see one possible source of the problem. When people slouch in their couches, they rotate the top of their pelvis backward, flexing the spine and reducing the normal lordosis in the lumbar. Imagine doing this for many hours every day for decades (and for most of us, this is not hard to imagine!) Eventually the spine will lose that nice normal shape it had when we were young, and the damage from the constant strain upon the lower back will add up. Another problem from this posture is the weakening of the muscles of the back. If you rely upon the chair back to support your upper body, you rob your muscles of the chance to remain strong or grow stronger.
It is no wonder that all fitness coaches will advise you to bend your knees when you try to pick something up. Your back has gotten so weak from years of sitting that you had better baby your back or you will break it! Farm workers who spend hours every day working in the fields do not need to bend their knees.  They just simply bend over. But their backs are strong. Most Western backs aren’t.
One remedy – stop sitting in chairs! If you love your back, if you want to strengthen your spine and open your hips (another problem area for most Westerners), start sitting on the floor every chance you get. Give your chairs away. Live as close to the ground as you can get. Eat at your coffee table, read with books on the floor. Watch television while lying on the floor in seal pose. 
All forms of yoga can help strengthen the back. Yin Yoga can help reestablish the normal range of motion of the lumbar ligaments as well. But remember, there is no agreed-upon definition for when someone is hyper-this or hypo-that; everybody’s bones are different. When you practice moving your spine through its full and natural ranges of motion, be aware of going too far. Be aware of pain or its precursors, small tweaks. Don’t stay in a pose when the sensations of the poses are too great. The essence of the yin practice is to maintain a gentle, but persistent, pressure for a long period of time.
Our exploration of the physical body, the first kosha, has revealed many benefits of the Yin Yoga practice. The compression, twisting, and stretching of our body works deep into our connective tissues, and this frees up toxins and other materials that have become trapped. Yin Yoga helps to detoxify the deep tissues of our body. Additionally, Yin Yoga has many benefits for our joints: fighting contracture, fixation, and degeneration of the bones, while assisting in rehydrating the synovial fluids and moistening the ligaments. Understanding these physical benefits is useful. It is time now to look at the second kosha, the energy body, and discover what benefits Yin Yoga brings to us energetically.
- — Okay, we are deliberately ignoring a woman’s more obvious curves, which to men are far more noticeable. But even these curves are due partially to the spine’s shape.
- — Lumber posture – should it, and can it, be modified?
- — Which is a very inefficient way to pick something up, after all.
- — See the Asana section for details.