What it means
A low back with no natural curve, tiring quickly on standing.
Where it comes from
A posterior pelvic tilt from short hamstrings with weak back extensors.
How it is measured
It is measured on the side view, in the sagittal plane. The defining measurement is Sway back (Lumbar lordosis), and it is reported as a finding from below 20°. The number is computed from landmarks on that photograph — not estimated by eye, and not compared against a stock example.
What the numbers mean
| Measurement | Grade |
|---|---|
| 20° and above | Within normal range |
| 15–20° | Mildly flattened |
| 10–15° | Moderately flattened |
| 10° and below | Severely flattened |
Source of the bands: Conventional 20-45° lumbar window; surface measurement
What is usually seen alongside it
- Pelvis tipped forward or back (Sagittal pelvic tilt)
None of these is a diagnosis on its own. Together they corroborate the finding; apart, they do not refute it — a measurement nobody took counts neither for nor against.
What confirms it
Short hamstrings rotate the pelvis backwards; the popliteal angle and the straight-leg raise measure that.
Flat back — and what fifty-four studies said about curve shape
A flat back means the lumbar curve sits below its reference range. Everyone's next question is "so will my back hurt?" The honest answer is more complicated than yes or no.
A systematic review that asked exactly this
Christensen and Hartvigsen [1] reviewed 54 epidemiological studies to see whether the sagittal curves of the spine are associated with health. Their finding:
"We found no strong evidence for any association between sagittal spinal curves and any health outcomes including spinal pain." They add that the included studies were generally of low methodological quality.
⚠️ In fairness, the other half belongs here too: they found moderate evidence of association with four outcomes — temporomandibular disorders, pelvic organ prolapse, daily function, and death. But they write that these associations are unlikely to be causal.
So what is the number for
Comparison with yourself, not prediction of pain. If you start a programme, this is the baseline and the same measurement can be taken again in eight to twelve weeks. What it is not is a verdict on the future of your back.
⚠️ If you have back pain, its source is found by examination. A page telling you that your back is flat and that is why it hurts has said something this review does not support.
References
- Christensen ST, Hartvigsen J. Spinal curves and health: a systematic critical review of the epidemiological literature dealing with associations between sagittal spinal curves and health. J Manipulative Physiol Ther. 2008;31(9):690–714. PMID 19028253 · DOI 10.1016/j.jmpt.2008.10.004
The evidence on this
Study summaries that measured this finding — with their numbers and their limits.
- Which exercise for chronic back pain, and how much — Systematic review and network meta-analysis, 24 randomised controlled trials
The exercise programme
The pelvis is rolled back and the lumbar curve is flattened. The protocol: (1) stretch the short hamstrings that pull the pelvis back; (2) strengthen the quadriceps and hip flexors — the straight-leg raise works rectus femoris, the one quadriceps head that also crosses the hip and tips the pelvis forward; (3) strengthen the lumbar extensors; (4) hold a neutral pelvis over the thigh in the hinge and in standing. Anything that tucks the pelvis further — a bridge cued to tuck first, a dead bug pressing the back flat — is left out.
The stages advance on a standard, not on the calendar:
- Stage 1 — release and wake up — weeks 1–4: Hamstring stretch, Supine hamstring stretch
Move on: When the movement is done without pain and without compensating, and the stretch is no longer what limits it. - Stage 2 — control — weeks 3–8: Hamstring stretch, Supine hamstring stretch, Straight leg raise — quadriceps, Standing hip flexor lift, Prone back extension
Move on: When the target muscle is felt through the whole range and the last set looks like the first. - Stage 3 — load and integration — weeks 6–16: Straight leg raise — quadriceps, Standing hip flexor lift, Prone back extension, Hip hinge pattern, Standing posture reset
Move on: When the corrected pattern holds in everyday movement and under fatigue.
Each session 15–20 minutes, 6–7 times a week, for at least 16 weeks. (from a study)
This example is for a moderate grade; each stage runs shorter or longer with the measured grade, and a person's own programme is written after assessment, around their other conditions and limits. If a movement causes pain, stop it and contact the clinic. Muscle colours in the animations: orange working, blue stretching, amber holding still.
Every number on this page is measured on the skin surface in a photograph. That is not the same as a measurement taken on a radiograph: it correlates with one and does not replace it. Wherever the decision depends on imaging, this page and the report itself say so.
What happens at the clinic
The assessment starts with three standing photographs — front, side and back. Every angle is measured on those photographs, the report prints the number and the grade for each, and the exercise programme is built from those numbers rather than from a template. Posture analysis explains the whole path, and the online assessment can be started from your own photograph now.
Frequently asked questions
At what point is flat back reported?
When sway back (lumbar lordosis) is below 20°, it is reported as a finding. That is where the "Mildly flattened" band begins. The bands come from: Conventional 20-45° lumbar window; surface measurement
Can flat back be assessed from a photograph?
A photograph gives a number, which most assessments do not have. But it is a surface measurement and it does not replace imaging. Short hamstrings rotate the pelvis backwards; the popliteal angle and the straight-leg raise measure that.
What is the most severe grade?
The worst band in this table is "Severely flattened". The grade is printed beside the measurement itself in the report, so it is always clear which number produced the word.