To
understand just how dramatically gravity affects your spine, try this exercise
at home: Measure your height in the morning and then measure your height again
at night. You will be 1/2" to 3/4" shorter at the end of the day
because of the compressive effects of gravity on your spine. During weight
bearing activities (sitting, standing, exercising), fluid is squeezed out of
your discs and into the adjacent soft tissue. This lost of moisture results in
height loss by the end of the day!
During non-weight bearing activities (sleeping), the discs expand as they soak up fluid increasing the length of the spine. However, you will not gain the full 100% of the height lost back. This is evident in the fact that as a baby, your discs are 90% water. By the time you reach 70 years of age, your discs are only 70% water! Over a lifetime, this loss of moisture causes thinning discs and you will lose ‡" to 2" in height!
Loss of height due to gravity would be easy to live with if it was the only result of thinning discs. However, another result of thinning discs is worn facet joints due to increased pressure. The joints can become irritated and inflamed, and the capsule of lubricating fluid that surrounds each joint might swell and press on a nerve root.
How do activities affect the amount of pressure on your discs?
Our
daily activities (running, sitting, weightlifting) and the force of gravity
places pressure on your discs. For example, sitting places 50% more pressure on
your discs than standing. Sitting and leaning forward (i.e., typing at a
computer) places 150% more pressure on discs than standing. The average person
will spend 16 hours (2/3 of your life) day sitting or standing.
What evidence shows that gravity is a major cause of thinning discs? Astronauts grow in space! After 84 days in space (a gravity-free environment) astronauts grew 2" in height. During twelve weeks in orbit, their discs had continued to take moisture from the blood stream, but with no gravitational pull to squeeze moisture out, the discs remained plump, making their spines longer, and themselves taller. In fact, space suits are designed to accommodate the extra 2" spinal stretch. After a few days on earth (a gravity environment), they returned to their normal height.
How can Inversion reduce the effects of gravity?
When you invert and relax, your body can stretch up to 2 inches. "Muscles relax quite quickly in the fully inverted position, and the length of the spine measurably increases after only a few minutes. Some of the lengthening effect is gained from re-absorption of fluid into the center of the disc. Used over a longer period, this may delay the degeneration process that occurs due to 'drying out'" (Beating Back Pain, Tanner).
Image 1. Supine - Lumbar curve maintained, posterior erector muscles contracted (shortened), intravertebral disc pressure, and psoas exerts pressure on spine.
Image 2. Pre-Inversion - Lumbar curve reduced, muscular relaxation, and pressure of psoas muscle decreases.
Image 3. Seated Inversion - Joint spaces widened, pain relief, intravertebral disc pressure reduced, and musculature stretching.
Summary