The Disc
The discs between your vertebrae are firm, water-rich cushions. Load them for half an hour and they lose a little height and volume, then recover when you lie down. This is one real disc, L4–L5, from my Duke study, scanned with MRI before and after 30 minutes of treadmill walking.
One disc, before and after
Drag the slider. The grey image is the MRI signal inside the disc. The line is the disc's outline, and the dashed line is where it started.
- Height (axial strain)
- 0.00%
- Volume change
- 0.00%
- Compression (mean Emin)
- 0.00%
Strain map: minimum principal strain, from 0% (no tint) to −10% or more (full red). Darker red means more compression.
The scans exist only at the two ends. In between, the page blends the two images, morphs one outline into the other and scales the measured strain, so the middle of the slider is an interpolation, not a measurement.
What to try
- Drag from before to after and watch the solid outline pull away from the dashed one. The change is small: about 2.3% of the disc's height, a fraction of a millimetre. That is why measuring it takes careful registration rather than a ruler.
- Turn on the strain map. Compression is not uniform. Some regions squeeze much more than others, which a single height number hides.
Where this disc sits
The same measurement for all 240 discs in the analysis: five lumbar levels in each of 48 before-and-after scan pairs. This disc, in red, is close to the middle.
Where in the disc does the volume go? Averaged over every disc, the water-rich nucleus at the center lost the most, and the tough outer ring barely changed.
- Nucleus
- −2.25%
- Inner annulus
- −1.19%
- Outer annulus
- −0.34%
Methods
Participants walked on a treadmill for 30 minutes at a constant speed, with 3D T2-weighted (SPACE) MRI of the lumbar spine before and after. Each disc was segmented, and an unsupervised deep-learning registration model learned a smooth, invertible deformation from the pre-walk disc to the post-walk disc. Strain is computed from the gradient of that deformation (Green-Lagrange strain, 0.875 mm voxels, Gaussian smoothing as in the study). The readouts and population plots are the study's own whole-disc and regional values. The strain map is recomputed from the same deformation with the study code. Its disc average (−4.69%) differs slightly from the table (−4.13%) because this page uses the outline from the raw scan. Only the disc itself is shown. No surrounding anatomy, participant codes or scan dates leave the lab's files. Papers are listed under Research, and the full story is in the case study.