Inversion helps in the relief of back pain in four ways:

• Discs hydrate: Clinical studies show that when inverted the separation between the vertebrae increases, this allows for moisture to be absorbed into the soft tissue of the discs, increasing the nutrient content as well as plumping the discs for better shock absorption and flexibility.
• Pressure is taken off of nerve roots: The height of the discs relates to the size of the passageway for the nerve roots to exit from the spinal column, so a plump hydrated disc creates maximum clearance, helping to alleviate any pressure or pinching of the nerve root.
• The spine is encouraged to realign: The traction applied also decompresses the spine to create an environment where misalignments can naturally fall back into place.
• Muscles are gently stretched as circulation is increased, helping to reduce tension.

The inner core of your discs is made of jelly-like material that provides the flexibility and "cushioning" in your back. When you are sitting, standing, exercising, or doing other weight-bearing activities, fluid is squeezed out of your discs and into adjacent soft tissue, just as moisture can be squeezed out of a sponge. As a result, your discs lose some of their height. To prove this fact, measure yourself in the morning and then again at night. You will lose half-an-inch to three-fourths-of-an-inch in height by the end of the day. (To see this change you can measure your true height or measure your waist. Height loss can be see in the size of the waist because although you are shorter your body mass is the same and therefore the 'column' of your body increases in diameter).

When you are lying down the compression in the spine is reduced enough to allow the discs to slowly reabsorb moisture and nutrition over the many hours you sleep. However, the discs may not always maintain their full height capacity, creating a total accumulation of height loss of up to two-inches in a life-time.

In fact, the only time in your life when you are giving your discs a break is when you are inverting. The Nachemson study1 provides some insight: A number of volunteers permitted a pressure sensor to be surgically implanted inside the third lumbar disc. The pressure inside the disc in the standing position was set at a base line of 100% and all other body positions compared to it. Interestingly one of the most compressive activities for the discs is sitting. The muscles in the stomach and back relax, but the pressure in the spine increases. If you are sitting in poor posture the pressure in the lumbar can climb as high as 250%. The real surprise occurred while lying down. The pressure inside the disc only lost 75% of standing body weight - it never went below 25%! This residual compression seems to be due to the hundreds of ligaments and muscles that encase the spine, holding it in compression like a mass of rubber bands. This study further indicated that the amount of traction force required to overcome the compression was a large number, approximately 60% of your body weight. Inversion to an angle of about 60° is the only practical way to offset that much gravity force while remaining relaxed. (Hanging by your arms will not create the same effect since it requires muscles to be engaged, plus the weight of the legs are much less then that of the torso and therefore the traction gained is not enough to bring the pressure to zero).

So many of our daily activities lend themselves to misalignments and possible permanent postural changes; sitting at the computer with rounded shoulders, carrying a heavy bag always on one shoulder, even wearing high heels. Also, many of our most popular sports are one-sided and rotational, like golf, squash, tennis, which puts significant stress on the spine as well as develops muscles on a single side of the body.

Misalignments mean that the body weight is no longer supported by an alignment of bones, and therefore the soft tissue of the body must resist gravity. Misalignments are not always felt on the inside but left alone they can cause visual changes to your posture, and those changes can be degenerative. If you want to test this as home, take an empty aluminium can and place pressure on the top. The can will be able to maintain its shape even with great force applied because the sides are in alignment, but add a small dent to the can and it will crumble under half the amount of pressure.

When a vertebra is bumped out of alignment the ligaments and muscles that support the spine can hold it in misalignment through the compression that they generate. Since these ligaments and muscles are engaged even when lying down, creating pressure as much as 25% compared to 100% standing, it can be difficult for the spine to naturally come back into alignment. When inverted to 60° the pressure in the spine drops to zero, as shown in the Nachemson medical study, with the pressure off of the vertebrae and with some gentle stretching the vertebra has the opportunity to move back into alignment.