# Effect of zero gravity on spinal compression

BransonMO
I am trying to compare an experiment on spinal changes due to zero-g in astronauts. Researchers simulated zero-g spinal elongation by suspending human subjects so that no part of their body touched the ground.

This figure shows the % change in disc thickness for each subject compared to data obtained from astronauts in space.

I am trying to determine what is the best explanation for the significant difference in the spinal changes between the two groups:

1 - The mass of the volunteers did not decrease as it does in microgravity

2 - The weight of the body in the lab is still acting on the cartilage

3 - Volunteers were not suspended upside down to account for fluid accumulation in the head and neck

4 - Gravitational force is converted to tension in microgravity

Fnet = ma

## The Attempt at a Solution

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I think the best explanation is that suspension does not eliminate gravity, so there will be stretching forces at all places where the subjects were attached to suspension wires. This would cause an even greater elongation of the spine than just the absence of gravity, since now I have a tension (T) opposing the weight of the subject on their spine, and the net force will cause the tissue to deform.

Would this be as simple as Fnet = ma = T - mg?

so T = ma + mg = m (a+g) which is greater then the stretching force on spine that occurs simply by the lack of gravity in space. Howver, the net a on the body being suspended is 0, so that T = mg. I know that the lack of gravity is not the same as applying an opposing force (T) to the spine, but I am having a tough time using equations to prove it.

Is there a better way to calculate this, or to explain it?

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Homework Helper
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I would assume that in both graphs the % Change is relative to the people under normal Earth gravity. Contrary to the labeling on the graph, I can imagine why the closed circles represent the microgravity data and I can guess how the open circles might represent the lab data, but I don't see it the other way around.

BransonMO
I would assume that in both graphs the % Change is relative to the people under normal Earth gravity. Contrary to the labeling on the graph, I can imagine why the closed circles represent the microgravity data and I can guess how the open circles might represent the lab data, but I don't see it the other way around.
The increasing thickness (#s are > 0) represents spinal disc expansion. This would be expected in the weight of gravity were taken off the spine as in microgravity (white dots). My reasoning is that when suspended by rope/wires in normal G (black dots) the 2 opposing forces, weight and Tension, act to stretch the spine more than the lack of gravity alone.

The closest analogy I can come up with is a spring. Under normal G there is a weight pushing down on the spine, with normal force from the ground pushing up, so the spine/spring is compressed.

In micro-G, there is no net force on the spine so the "spring" elongates to some equilibrium position. where L 0g > L normal G

Finally, when suspended in air, the 2 opposing forces stretch the spine/spring, such that Lsuspended > L 0g

The slight curve of each data set (each individual vertebrae does not expand the same mount) would be due to the curved nature of the spine. Each spinal disc does not experience the same Force in each scenario.

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Here's the way I see it. First consider what happens to the spine of a subject standing on the surface of the Earth. The discs near the bottom will be compressed more than the ones near the head because they have to support the weight of the head plus the weight of the discs above them. In microgravity, I would expect the ones that are compressed more to expand more. Yet, the graph shows that the discs at the two ends expand less than the discs in the middle. That's what I don't understand. Maybe I'm trying to read too much in those plots and the question has to do just with what you mentioned, that the suspended subjects tend to stretch more because they are pulled by gravity hence their points are higher. Incidentally, do you know which end of the plot corresponds to the discs nearer the head, low cartilage position or high cartilage position?

BransonMO
Here's the way I see it. First consider what happens to the spine of a subject standing on the surface of the Earth. The discs near the bottom will be compressed more than the ones near the head because they have to support the weight of the head plus the weight of the discs above them. In microgravity, I would expect the ones that are compressed more to expand more. Yet, the graph shows that the discs at the two ends expand less than the discs in the middle. That's what I don't understand. Maybe I'm trying to read too much in those plots and the question has to do just with what you mentioned, that the suspended subjects tend to stretch more because they are pulled by gravity hence their points are higher. Incidentally, do you know which end of the plot corresponds to the discs nearer the head, low cartilage position or high cartilage position?
The vertebrae positions are in a legend below the figure. They are:

lumbar (1-5, lowest on the spine), thoracic (6-17, middle of spine) and cervical (18-25, highest on spine)

Thus, it shows that the spinal decompression/elongation is greatest in the thoracic discs in the micro-gravity environment, and are pretty consistent with some lows in the lumbar area in the suspension experiment. I think worrying about exact spinal locations is beyond the scope of the Q given the limited information I have.

Does my reasoning sound right? None of the other choices make any sense to me.