The epistemology of lower back pain

Or: trying to learn to stop worrying and love spine neutrality

I’ve had pretty bad lower back pain—specifically, sciatica—for about two months now. For readers who have never experienced sciatica: I hope you never do. For those who have, you already have a good sense of what I’m referring to: persistent, sharp pain that begins in the lower back¹ and radiates down through one’s hamstring and calf, occasionally even reaching the toes. The leg pain is worst in the mornings for me, and it’s also perhaps the strangest part of the whole experience: I’ve seen it described as “a knife sawing through your muscles” or “something continually pulling on your calf”, both of which ring true. For some, stretching those muscles might provide momentary relief, but it doesn’t address the root cause, which is that there’s something pinching the sciatic nerve, disrupting the normal signals that the nerve carries between the leg and the brain.

As anyone who’s experienced it knows, sciatica is both very painful and very frustrating in how it limits one’s ability to partake in normal daily activities.² In an effort to stay sane and also hopefully get better, I’ve tried to learn as much as I can about the underlying mechanisms. I’ve read academic papers, Stuart McGill’s Back Mechanic, and more r/sciatica posts than I care to admit. This has led to mixed results so far: the pain’s the same, but I know much more about spinal anatomy than I did before. It’s also clearer to me now how epistemologically murky this whole space is. You’d think lower back pain is straightforward—the spine’s pretty mechanical, after all—but it turns out not be as simple as something like a broken leg. This uncertainty can be overwhelming for lots of people, since it makes it harder to know when or whether one will recover, and what course of action is most likely to facilitate that recovery.

Here, I want to focus on what I’ve learned and what is still unclear. And while spine neutrality and back pain might seem far afield from the usual stuff I write about, there’s a common theme: fundamentally I’m interested in what we know and how we go about learning it—epistemology—and it’s been striking to see how many epistemological gaps recur across Cognitive Science and the study of sciatica. The world, including our bodies and our minds, is vast and complicated, and we must make sense of it with limited tools.

Separately, lower back pain is also just an important public health issue. It’s pretty prevalent (consistent point estimates tend to be about 7-10%, but some range closer to 40% depending on definitional boundaries). It’s one of the leading causes of disability worldwide and as such, it’s also a major economic drain. Many people struggle to find relief, and even standard treatment regimes don’t necessarily involve the patient in understanding the root causes. It’s likely that a high proportion of people reading this article have experienced lower back pain or will experience it at some point in their lives—and the experience can be psychologically distressing even in the best of times. Hopefully this article provides some useful information to those readers.

Obligatory caveat: this article is very much not exhaustive, I am very much not an expert in the spine, and none of this is intended as medical advice. Further, if any readers find any errors in the article, I’d love to know and correct them.

Spine anatomy, briefly

To understand why we get back pain, we need to know a little about how the back works. Here, we’re on epistemologically firmer ground, since we’re dealing with questions of anatomy and mechanism.

The spine runs from your neck (the cervical portion) along your upper back (the thoracic portion) down through your lower back (the lumbar portion) to your sacrum and tailbone. Each of these portions contains vertebrae (bones), which in turn are separated by discs—structures consisting of a collagen ring and a gel-like nucleus. Notably, there’s also the spinal cord, which runs from the brain down the spine—through each of the vertebrae—and serves to collect signals about sensations (e.g., in our back or legs) and also send motor signals (e.g., control movements). The part about the spinal cord is especially important, because it means there are nerve roots exiting the spine at each joint level, which can potentially come into contact with a bulging disc—leading to nerve compression and pain.

Depiction of two vertebrae (gray/beige) separated by a disc (light blue). Image from Wikipedia.

Overall, it’s a remarkable feat of biological engineering: the spine has to be flexible enough to allow bending forward (flexion) and backwards (extension), but it also has to be capable of stiffening sufficiently such that it can bear heavy load. Picking up a heavy load with your spine in a bent position can be dangerous, as the force of the load won’t be dispersed across your spine but will rather be concentrated in a specific area. That increases the risk of something like a herniated disc, and it’s also why you should bend from the knees and hinge at the hips when picking something up—kind of like doing a squat or deadlift.

Sciatica: common causes

Lower back pain can present in all sorts of ways. One problematic presentation that I’ll focus on here is called sciatica, which is associated not just with pain in the lower back but also with pain (or numbness or tingling) that radiates down the leg, sometimes all the way to the toes or soles of the foot. This is a form of radiculopathy, or “pinched nerve”: specifically, the sciatic nerve, which extends from the lower lumbar portion of the back through the sacrum.

As a medical condition, sciatica has a long and storied history: the Greek physician Hippocrates (~460-360 BC) allegedly coined the term, which is based on the Greek ischios (“hip”), as Hippocrates believed the condition arose from disorders of the hip. As this article makes clear, early physicians described the condition in much the same ways as modern accounts—emphasizing pain extending from the buttock to the lower leg or foot—and tended to advocate treatments like bed-rest, massage, or in some cases spinal surgery.³

On some level, the cause of sciatica is straightforward in the sense that the condition is defined as an impingement of the sciatic nerve. Yet this explanation is virtually tautological: the key question—both for diagnosis and treatment—is what’s causing that impingement. Sciatica is a symptom, not a diagnosis in itself, which means we need to know what’s contributing to it.

Here’s where the epistemological territory gets slightly murkier. Most sources, from McGill’s Back Mechanic to review articles on sciatica, suggest that the most common cause of sciatica is some form of disc herniation. Recall that discs sit between the vertebrae of the back, absorbing shocks and facilitating movements. Herniation means that part of a disc—sometimes the gel-like nucleus, sometimes just the outer layer—is protruding outward. If that protrusion presses on the sciatic nerve, it can cause sciatica.⁴ This review article concludes that about 90% of sciatica cases are caused by disc herniation, which makes sense given the basic mechanics of the spine. There are other potential causes, such as cysts, tumors, or a strained piriformis muscle, but these presumably make up a smaller proportion of cases.

The reason I say it’s epistemologically murky is that there are also plenty of cases of disc herniation that don’t present with sciatica symptoms. That same review article suggests that anywhere from 20%-36% of people without sciatica have at least mild disc herniation—a statistic which comes from an imaging study conducted on individuals without lower back pain. That means there’s a good number of people walking around with disc herniation that don’t have any lower back pain.

Based on the evidence, then, it seems reasonable to say something like: given that someone has sciatica, there’s a good chance they have some kind of disc herniation. But disc herniation doesn’t appear to necessarily cause sciatica. In principle, even the fact that sciatica co-occurs with disc herniation is not itself evidence that the latter causes the former. That said, the strong empirical correlation coupled with a plausible mechanistic account (as well as stronger causal evidence from animal models) seems like a reasonable basis for asserting a causal relationship.

In my case, an MRI scan revealed evidence of disc herniation at the left L5/S1 site, which impinges on the sciatic nerve. Given that my pain radiates down my left leg, I think (and my doctor and physical therapist agree) it’s fair to assume the herniation is the cause of the sciatica.

My MRI results: protrusion at the L5/S1 site, which impinges on the sciatic nerve.

However, medical imaging is not the only determining factor for a diagnosis. Precisely because herniation occurs in asymptomatic patients (see above), it’s also important to undergo an extensive behavioral assessment. A good physical therapist (or doctor, etc.) will likely perform an evaluation to assess the presentation of symptoms for each individual. It’s possible to perform some of these evaluations yourself: McGill’s Back Mechanic discusses some of the methods for doing so, which help probe whether the pain increases with flexion vs. extension, sitting vs. standing, and so on. These assessments are crucial for determining a treatment plan.

This question of assessment is again one of epistemology. You might imagine that an MRI result should be privileged, epistemologically speaking, given that it’s showing “what’s really going on”. But pain is by definition an experiential phenomenon, which is why a behavioral assessment that identifies which postures cause or relieve pain is an important component of the diagnostic process. At the same time, an MRI is important too, especially if a doctor is going to recommend surgery.

But what causes disc herniation?

Disc herniation has all sorts of causes. The prototypical case that might be familiar to many readers is improper form when lifting something heavy. For example, maybe you were moving a heavy couch and bent too much at the spine—placing excessive strain at the site of flexion and causing a disc to bulge.

But herniation can also occur in slower, more gradual ways. Another common cause is sitting or standing with poor posture. Many people sit all day in front of a computer or driving a car. Without sufficient lumbar support, this position can place strain on the discs in the lumbar region, leading to gradual degeneration.

In my case, I’ve had chronic lower back issues for at least a few years ever since I injured myself rock climbing. In past occurrences, the pain was often triggered by something simple like bending down to pick something up off the floor, and it tended to dissipate within a week or two with light activity. This latest time, the “triggering” event was even more innocuous: I was leaning against the kitchen counter and used the lumbar portion of my back to push myself gently forward, and I felt the telltale “tweak” in my sciatic nerve indicating that something had gone wrong. I continued light activity for a couple weeks, but the pain—far from dissipating—continued to grow in intensity until it reached the point it’s at now. My somewhat informed guess is that I herniated a disc a few years ago, and a lifetime of poor posture and working hunched over on my laptop has only served to make the discs more susceptible to injury.

What to do about it

Unfortunately for people suffering from sciatic pain, the area of treatment and prognosis is murkiest of all. Broadly, you can think of treatments as falling into a few buckets: behavioral and “natural” recovery; pharmaceutical; and surgical.

The best-case scenario is that the pain from the herniated disc (or other source of nerve impingement) heals on its own. Sometimes this happens in a couple days, and sometimes it takes a couple weeks. As time goes on, pain sensitivity can actually increase, and one’s range of motion becomes increasingly impaired. Historically—all the way back to Hippocrates—bed rest was recommended as a treatment for this kind of sciatic pain. The logic, presumably, is that rest is important for breaking the cycle of repeated nerve inflammation.

More recently, however, physical therapists working in rehabilitation tend to emphasize the importance of getting up and moving around as much as possible (i.e., without exacerbating the underlying pain). The argument here is that it’s crucial to expand the repertoire of pain-free movements, and also that a sedentary lifestyle may well compound the underlying problems that led to disc herniation in the first place. Additionally, discs rehydrate while lying down, which means that this can—at least in theory—increase the chance of inflammation, since the discs will be bulging more than usual when you eventually stand up.

Yet even within the set of physicians recommending movement over rest, there are different approaches. The McKenzie method, for example—which is widely practiced among physical therapists—emphasizes increasing mobility, including of the spine. The core idea is that the body can learn to heal itself through movement, so McKenzie-trained physical therapists encourage movements that gradually restore the back’s ability to engage in flexion and extension. Stuart McGill (author of Back Mechanic) also argues that it’s important to expand mobility, but focuses more on increasing core stability with the “Big 3” exercises. And when it comes to the spine specifically, McGill argues that most people benefit from a stiffer spine, which proves more robust to bearing load.⁵

Naturally, the disagreements here can be frustrating to someone trying to address their sciatic pain through a conservative, non-surgical approach. That said, the McGill and McKenzie approaches seem relatively united in their emphasis on finding pain-free movements and postures; the key gap seems to be between those who recommend rest and those who don’t.

If these more conservative approaches don’t work, there’s also the possibility of pharmaceutical interventions or surgery. In terms of drugs, doctors might recommend anything from ibuprofen (to reduce pain and inflammation) to gabapentin (a nerve blocker). A more invasive pharmaceutical treatment might involve an epidural steroid injection (ESI), in which an anti-inflammatory steroid is injected directly into the epidural space where the nerve is inflamed. Finally, there’s discectomy: surgical removal of the portion of the bulging disc that’s pressing on the nerve. All these approaches come with their own complications—surgery, after all, involves cutting open your back and removing tissue from your body—which is why doctors tend to recommend starting with more conservative treatments. (Note that for a single ruptured disc, the surgery options can be relatively straightforward and safe, and can be done on an outpatient basis.⁶)

How do these different approaches compare in terms of efficacy? According to this review article, there’s pretty good evidence that activity is better than rest, some evidence that physical therapy helps, some evidence that injections (like ESI) can provide short-term relief, and pretty good evidence that surgery works better at providing faster relief but that the results even out after a few years. Regarding surgery specifically, the authors write:

From these randomised controlled studies, we can conclude that surgery provides faster relief for patients with sciatica, whereas the results of surgery and conservative care are similar at 1 year and beyond…following 6–8 weeks of symptoms that have failed to respond to conservative treatment, if a DH is found to be the cause of sciatica then patients may become surgical candidates.

On the other hand, back pain researchers like Stuart McGill caution against opting for surgery. For one thing, the surgery may not work if the disc herniation isn’t actually causing the sciatica—as such, it’s important that doctors demonstrate to patients that they understand the underlying mechanisms of their pain first. Additionally, any invasive surgery will involve some risks: the surgeon might damage the nerve or other tissue, resulting in even more back problems. McGill’s not against surgery, but he encourages patients to work first on core stability, which will be important for avoiding future back injuries (even if they do opt for surgery).

The difficulty of knowing things

As I mentioned at the start of this article, the spine is pretty mechanical. And yet it seems there’s still so much we don’t understand about the causes of back pain—much less how to treat it. It’s easy to feel epistemically nihilistic about it all: if something as apparently mechanical as this is so hard to fix, what are we meant to do about more opaque medical problems—especially those concerning complex systems like the brain?

Part of the challenge is methodological. Take, for instance, the question of whether and to what extent surgery is efficacious. One could, in principle, compare outcomes across patients who received surgery and those who didn’t. However, those samples aren’t drawn from the same population (i.e., individuals who received surgery might have had worse pain in general). As this review article describes, randomly assigning patients with similar levels of pain to receive surgery (treatment) or not (control) is one way to solve this problem—but in practice, some of the control group will probably end up needing surgery, and some of the individuals in the treatment group will end up opting out.

Perhaps a more fundamental challenge is that the nature of reality—even with something as mechanical as the spine—is pretty much always more complicated than the theories and experiments we can devise to describe it. Abstraction and simplification are key parts of the scientific process, but the challenge is figuring out the right abstractions for the things we care about. This is a problem that’s familiar to my own experience in research, and it’s why I spend so much time writing about the epistemological challenges facing Cognitive Science and LLM-ology. That doesn’t mean it’s impossible or hopeless. At least for the time being, I’m still excited to be an LLM-ologist. But it probably should give us some pause in terms of our sense of how tractable it will be to construct accurate, interpretable mental models about the complex world we inhabit.

1. Left side, in my case.

2. Even writing Substack posts is challenging, since the pain is at least somewhat present whether I’m sitting, standing, or lying down, and it’s hard to focus on writing. Things like putting on my own socks are out of the question!

3. Sciatica treatments will be discussed in more detail in the section below.

4. Protrusions higher up on the spine can impinge the femoral nerve, which leads to pain radiating through the thigh.

5. Note that if you listen to McGill talk about his research (e.g., here), he’s quite careful to be nuanced and point out that some individuals benefit from spinal mobility training moreso than stability.

6. This note was added on January 9, 2025 to reflect a helpful comment from a reader.