Pain •Sensible advice for aches, pains & injuries

Sciatica Tutorial

A basic tutorial about buttock and leg pain

updated April 17 2011
4,250 words
by Paul Ingraham, Vancouver, Canadabio
I am a science writer, the Assistant Editor of, and a former Registered Massage Therapist with a decade of experience treating tough pain cases. I’ve written hundreds of articles and several books, and I’m known for sassy, skeptical, referenced analysis and a huge bibliography. I am a runner and ultimate player, and live in beautiful Vancouver, Canada. • full bioabout

If you mainly have low back pain, please change articles: you need to be reading Save Yourself from Low Back Pain!

Sciatica is a symptom — shooting pain in the buttock and down the back of the leg, which is caused by irritation of the sciatic nerve by one of several mechanisms. It is often associated with back pain, but it can happen independently. There are several related scenarios that can cause buttock and leg pain, and a sciatic nerve pinch is much less common than simpler muscle pain. Even when the sciatic nerve is being pinched, this is often a relatively trivial side effect of muscle pain — help the muscle, and the pinch goes away.

Here are several ways that you can have sciatica:

  1. By far the most common is muscle knots, more technically known as trigger points. Muscle pain in the low back and gluteal musculature routinely causes symptoms that spread impressively down the back of the leg. This is often mistaken for impingement of the sciatic nerve. It is not true sciatica.
  2. Also allegedly common is impingement of the sciatic nerve by the piriformis muscle deep in the buttock, the elusive “piriformis syndrome,” which is as unproven as Bigfoot, but with a lot more credible “sightings.”1 A piriformis muscle contracted enough to cause this problem invariably contains trigger points that also radiate pain down the back of the leg, and so the two problems often overlap. It can be difficult to tell the difference between symptoms of the nerve pinch, and symptoms of piriformis and other muscular trigger points.
  3. In an unlucky few, the sciatic nerve, or part of it, actually passes through the piriformis muscle, rather than underneath it — one of the most problematic common anatomical variations.2 This probably results in a much greater vulnerability to sciatic nerve irritation — a kind of “super piriformis syndrome,” like piriformis syndrome with a hair trigger. In this case, sciatic nerve impingement is usually a more significant factor, and harder to resolve.
  4. By far the least common cause of sciatica is an intervertebral disc herniation pressing on the roots of the sciatic nerve in the low back. As with piriformis syndrome, the symptoms may be caused by muscular dysfunction. Disk herniations are often asymptomatic — they only tend to cause problems when certain other unknown factors are present, one of which is may be the presence of severe trigger points in the lumbar muscles.
  5. Hamstring syndrome, in which fibrotic bands (from trauma, or born-with-’em) can trap and irritate the sciatic nerve where the hammies attach to the sit bones.3

Each of these scenarios involves muscle knots. To a great degree, muscle knots are either a cause and/or a complication of sciatica. Trying to treat them is often a good way to intervene — to break the grip that piriformis has on the sciatic nerve, or to improve the soft tissue environment of unhappy lumbar nerve roots. By no means is it guaranteed to work, but it’s quite easy, inexpensive, and safe to make the attempt.

Rarely is the problem “mechanical” in nature, despite the popularity of this view among virtually all health care professionals. Chiropractors are particularly prone to diagnose a sciatica problem as a symptom of some kind of joint dysfunction, alignment, or postural problem. The sacroiliac joint is often diagnosed as being “out,” and the lumbar joints are portrayed as being fragile and vulnerable when quite the opposite is true. Although chiropractors are most likely to diagnose in this way, physicians, physiotherapists and massage therapists are all equally prone to this kind of “structural” diagnosis…missing the most straightforward explanation and treatment opportunity.

How can I tell what kind of sciatica I have?

It can be difficult to tell the difference between the pain of trigger points and true sciatica nerve or nerve root impingement. And bear in mind that, even when it is possible to identify actual nerve impingement, the main problem is probably still muscular. The two problems are generally treated in much the same way.

The clearest sign of nerve impingement is significantly altered sensation below the knee, either:

Another relatively clear sign of nerve impingement is:

Nerve impingement pain is technically called “lancinating” pain. It is typically hot or sharp, instead of diffuse and aching. Most people strongly agree that it feels “electrical.”

by “debivort”

A disk herniation

Only one possible source of sciatic. This is cervical vertebra, but the principle is the same: the disc bulges outwards and presses on a bundle nerves. Illustration by “debivort,” Creative Commons License

by “debivort”

A disk herniation

Only one possible source of sciatic. This is cervical vertebra, but the principle is the same: the disc bulges outwards and presses on a bundle nerves. Illustration by “debivort,” Creative Commons License

However, do not be too convinced by the quality of the pain alone: trigger points are quite capable of causing pain that feels exactly like nerve pain.

Trigger points rarely causes a “pins and needles” sensation, so if you have pins and needles, that is a pretty strong indication of nerve impingement. But even then, you can’t be sure, because trigger points do occasionally cause a very nervy tingling.

The only symptom that is virtually guaranteed to be caused by nerve impingement alone is true tactile numbness. If you have a “dead” patch of skin, then you almost certainly really do have a pinched nerve.

However, even then, some uncertainty remains. Although trigger points cannot cause a truly numb patch of skin, they can (and routinely do) cause a feeling of “dead heaviness.” People routinely start an appointment for sciatica by telling me that their leg is numb — but after a little discussion, it becomes clear that they mean that the leg feels sick, heavy, weak and useless … but not actually numb to the touch. Without numbness to touch, nerve impingement cannot be diagnosed, and by far the more likely cause of the symptoms are a batch of nasty trigger points in the low back and hips.

This difference between nerve impingement and trigger points can be extremely difficult for patients to wrap their heads around, so I’ll go into more detail in the next section. But first, a delightful patient quote of the day from Dr. Grumpy:

“The pain went down through my legs. Not all my legs, I mean, but just the ones on the bottom of my body.”

And if you think that’s weird, you should see my inbox.4

The fear of nerves

I came across this full-page advertisement in National Geographic magazine:

“Do you feel burning pain in your feet?” the ad asks. “Or uncomfortable tingling, numbness, stabbing, or shooting sensations? If so, you may have nerve pain.”

Yes, you might. But it’s not bloody likely! The clinical reality is that neuropathy is a lot less common than nearly all patients and most doctors believe.

Nerves have a mystique

Nerves make people nervous! So to speak. The whole idea of nerves gets people anxious. Could it be a nerve? people ask. Is this a nerve problem? What if it’s a nerve? Is something pinching my nerve? Something must be pinching a nerve.

The kind of advertisement with the stinging ants you see above greatly aggravates our society’s general anxiety about nerves. Pfizer and other pharmaceutical companies spend about a gazillion dollars on marketing that can create more worry about nerve pain in a year than I can counteract in an entire lifetime of low-budget public education! Bummer.

Nerve root wiggle room

It’s amazingly difficult to actually pinch most nerves, or nerve roots (as they exit the spine). In general, nerves have extremely generous “wiggle room.” For instance, in the lumbar spine, the holes between the vertebrae that the nerve roots pass through can be more than a couple centimetres at their widest, while the nerve roots themselves are only about 3-4mm thick.5 If you stretch or compress the spine, the holes do change size a little — as much as 70–130% in the looser neck joints,6 a little less in the low back.7 But even at their smallest, there’s still plenty of room.


Schematic of nerve root wiggle room

On the left are the approximate proportions of a healthy nerve root and the hole it passes through (intervertebral foramen). When the spine is pulled or compressed, the holes get a little larger or smaller, as shown on the right…but there’s still lots of nerve root room.

There’s so much space for nerve roots that even dislocations routinely fail to cause impingement.8 Once again, I invoke the example of a patient with a severe lumbar dislocation … and no symptoms at all, not even symptoms of pinched nerve roots. Her nerves seemed to be fine, even in an anatomical situation most people would assume to be extremely dangerous.

A young woman with “sciatica” (hint: not actually sciatica)

Yesterday I was working with a young woman who had “sciatica.” Allegedly, her sciatic nerve was pinched by her piriformis muscle — a reasonably common scenario — and sending hot zaps of pain down her leg. She came to me with this diagnosis already in place. She also had some tingling in her feet. The description of her symptoms did, indeed, sound a lot like a nerve impingement problem. On the face of it, it did seem likely that her sciatic nerve was being pinched.

However, a couple things didn’t add up. For instance, she had no numbness at all — no dead patches of skin, which are highly characteristic of true nerve impingement. Instead, she had a lot of “dead heaviness” in the leg, a different kind of numb feeling that is much more closely associated with muscle knots than nerve pinches — and a lot more common.

I quizzed her carefully about the quality of her pain. She assured me it was “zappy” and “electrical” … just as you would expect of nerve pain. Yet something didn’t seem quite right. I couldn’t shake the impression that she was interpreting an intense non-neurological pain as a “zappy” pain simply due to her strong belief that she had a nerve problem. When you think a pain is nervy, you’re going to interpret, feel and describe it in nervy terms.

So I did some experimenting, and clinched the case:

This young woman’s “nerve” pain could be perfectly reproduced by prodding muscle tissue that was nowhere close to the sciatic nerve. Pressing on the side of her hip, on the gluteus medius muscle, several centimetres away from the sciatic nerve, she reported the same “electrical” pain flowing down her leg. It even stimulated the weird, tingling sensations in her foot.

This largely eliminates a diagnosis of sciatic nerve impingement.

A more likely story

In spite of spending most of my career trying to explain to people that this sort of thing is common, I was surprised myself — fooled, really. Muscle knots are always fooling patients and professionals alike. Vastly more common than nerve problems, and often more painful, they nevertheless get upstaged and misdiagnosed by another phenomenon.

Do you feel burning pain in your feet? Or uncomfortable tingling, numbness, stabbing, or shooting sensations? If so, you are more likely to have “muscle knots” than nerve pain.

The take home message of this section is: do not underestimate the power of trigger points to cause pain that seems like a nerve pinch.

What’s the worst case scenario for sciatica?

Most cases of sciatica will resolve on their own, with no special treatment, within 3–6 weeks, just like low back pain or a crick in the neck. Most people will never have the problem again, or only once or twice more in their lives.

The worst cases are completely debilitating for brief periods, but most of the time cause only extremely annoying pain that makes daily activities frustrating, but not actually impossible.

Unfortunately, a minority — perhaps 20% of patients — will become chronic and/or recurrent sufferers.9 An even smaller, unluckier minority of sciatica sufferers face a lifetime of pain that never or rarely leaves, or episodic pain that inevitably returns. Stubborn cases may be at least partially explained by genetics,10 and this one of the important reasons why patients need to be wary of therapeutic wild goose chases looking for the cause of their pain.

Once any kind of pain has been around for a while, it has the potential to actually damage the way the nervous system interprets pain — this is known as “central sensitization,” meaning the central nervous system has become oversensitive to pain.11

What can I do?

The next several sections go over some of the treatment options for sciatica. In summary, most people should avoid surgery, as it simply does not provide that much benefit. However, it may be a worthwhile option for very painful cases of sciatica. For everyone else, the best therapy is to try to “act normal” as much as possible — reduce fear and anxiety, move as much as reasonably possible, stretch and wiggle, keep the surrounding muscles happy with heat, and so on.

What about surgery?

Surgery is only an option when it is related to a herniated disc, as opposed to when the sciatic nerve is being impinged by the piriformis muscle, or (obviously) if sciatica-like symptoms are being generated by muscle knots alone.

Even when there is a herniated disk, surgery should only be seriously considered in the worst, most painful cases.

For the rest of us, sciatica surgery is barely worthwhile. In 2008, a group of Dutch researchers published the results of an excellent study of surgery for sciatica.12 Although not perfect, it is certainly the best such experiment yet performed, and will be more or less the “last word” on the subject for a while, the new-and-improved conventional wisdom.

(It is also quite readable for a scientific paper, and keen patients or professional readers might want to browse the full paper, which is freely available on the British Medical Journal website.)

Peul et al found that operating relatively quickly on a herniated disc causing sciatica “roughly doubled the speed of recovery from sciatica compared with prolonged conservative care.” However, that sounds a lot better than it is, because a year later there is basically no difference between people who had surgery, and people who didn’t. In fact, “These relative benefits of surgery, however, were no longer significant by six months’ follow-up, and, even at eight weeks, the statistically significant difference between treatment groups in primary outcome scores was not sufficient to be clinically meaningful.”

In other words, the effect of surgery is pretty underwhelming. A graph shows this very clearly:

That pretty much says it all, doesn’t it? For your trouble of getting cut open, you get a pretty modest dip in pain in the early days, but it’s not very long at all before you’re pretty much back in the same boat as the fellow who didn’t bother. “Neither treatment is clearly preferable,” the researchers concluded. They argued that it might be time to stop recommending surgery based on physician preferences, and start asking patients what they think of the options.

It is worth noting that this study is considerably less optimistic about operating than some previous studies. Previously, the conventional wisdom was that operating relatively soon could result in more significant pain relief in the short term. However, this new study was better-designed, much more “scientific” — more of a carefully controlled test of the theory, so it gives us many reasons to put more stock in its results.

In fact, the results of the research are so conclusive that authors wonder “whether surgery has any effect at all on the natural course of sciatica.”

I have been arguing for years that back surgeries need to be compared to sham surgeries if we are ever to really know if they work, so I was particularly pleased that these scientists suggest exactly that — a somewhat radical proposal, virtually unheard of in this field. There are problems and challenges with comparing real surgeries to sham surgeries, but it can and has been done, with fascinating results.

Does surgery have any effect at all on the natural course of sciatica?

Slightly faster recovery and relief of leg pain might be worthwhile for some patients. However, much of this eagerness (and probably some of the pain) is driven by fear — back pain and sciatic have a unique ability to scare the pants off patients. The fear around these conditions is in large part due to the ignorant fear-mongering of doctors who are too quick to scary expensive tests (MRI) and recommend surgery, when they should know full well that the vast majority of people recover quickly with no treatment at all!

The risks of surgery for sciatica are relatively low — but even when the risks are low, there is still a considerable financial, personal and social “overhead” any time people get cut open. We should generally avoid any kind of invasive medical procedure unless the benefits are extremely clear.

Like a charge of murder, it should probably be proven “beyond a reasonable doubt” that surgery is worthwhile. Such proof is simply not present in this case of surgery for sciatica!

So, if you’re not going to get operated on … what else can you do?

Relax the area with heat and vibration

Whether the pain is caused by the crushed sciatic nerve itself, or just by tight muscles, the muscles need to relax in either case. Hot tubs, with jets, are ideal for all kinds of sciatica.

Due to the thickness of the tissue in the buttocks, the heat will not have any circulatory effect on the nerve or the piriformis muscle, but it will be neurologically sedative. The vibration of jets will amplify that effect. Muscles relax when they are vibrated — a neurological effect known as “proprioceptive confusion.”

(If you live in Vancouver, see the footnote for a great local tip.13)

To get the most out of using any hot tub, see A Better Hot Bath.


When stretching for sciatica, please stretch very gently and calmly: the piriformis muscle, which is producing the pain directly or indirectly, tends to be reactive in character. It needs to be gentled. The focus of the stretching should be neurological, not mechanical — that is, slowly get the muscle “used to” a greater length and lower tone. There are at least two stretches that are particularly useful in this scenario.

Piriformis stretch (seated version) — starting from a seated position, place your ankle (on the side you’re stretching) over the opposite knee. Let your lifted knee relax downwards for a moment, and then begin to lean forward from your pelvis. Avoid simply slumping forward, which is useless. The image that is the key to this stretch is to “push your belly button between your legs.”

Piriformis stretch (prone version) — lie down on your belly. Lift your lower legs straight into the air, and then let them fall out to the sides. Relax them there, but don’t let them fall out of position. This is a very low-intensity stretch — you probably won’t even feel it. This stretch works best if you sustain it for several minutes at a time, or even long periods. You can intensify it by wearing heavy boots or ankle weights, or by getting someone to help you by gently pressing your legs out to the sides.

Use it or lose it

Mobilize your hips. If you creatively explore your pain-free range of motion, it will gradually increase. If you don’t, the muscles in your hip will tend to remain as they are.

Apply a tennis ball

The muscles of the hip and buttock are one of the few places in the body where it is possible to effectively treat your own muscle knots with a tennis ball. Simply lie on a tennis ball such that it presses on deep, aching sore points — and wait for the sensation to fade. See Tennis Ball Massage for Myofascial Pain Syndrome for more information. I do have one caution about treating yourself in this particular case: the piriformis muscle is so unusually reactive, in my experience, that you must be particularly gentle and conservative in your approach.

No bed rest — it doesn’t work!

Bed rest has been a popular treatment for sciatica for the better part of the last century. It’s more or less dying — most doctors know that it doesn’t work these days, and don’t prescribe it. But you still run across this myth from time to time.

In a 1999 sciatica study in New England Journal of Medicine,14 researchers “randomly assigned 183 subjects to either bed rest or watchful waiting” for two weeks and found that “bed rest is not a more effective therapy than watchful waiting.” Nor is less effective. The results were exactly the same. If that sounds like no big deal, consider the difference in the lives of those patients! Two weeks of bed rest? Compared to two weeks of going about your business!

Posture and ergonomic improvements

Sciatica of these types are most commonly related to excessive sitting, and sometimes to poor ergonomic design of the chair, work station, and postural dysfunction as well. It is worth experimenting with these factors. The most obvious and relevant option is to buy a timer and use it to remind yourself to get our of your chair every fifteen minutes. For more ideas, see Does Posture Correction Matter? and Unconventional Ergonomics.

About Paul Ingraham

I am a science writer, former massage therapist, and assistant editor of I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook and Google, but mostly Twitter.


  1. Halpin RJ, Ganju A. Piriformis syndrome: a real pain in the buttock? Neurosurgery. 2009 Oct;65(4 Suppl):A197–202. PubMed #19927068. “There is no definitive proof of its existence despite reported series with large numbers of patients.” BACK TO TEXT
  2. Natsis K, Totlis T, Konstantinidis GA, et al. Anatomical variations between the sciatic nerve and the piriformis muscle: a contribution to surgical anatomy in piriformis syndrome. Surg Radiol Anat. 2014 Apr;36(3):273–80. PubMed #23900507.

    This dissection study of 275 dead buttocks found that 6.4% of them had variations of sciatic nerve and piriformis muscle anatomy, with considerable variety in the variation. They found several different arrangements, and concluded: “Some rare, unclassified variations of the sciatic nerve should be expected during surgical intervention of the region.” Prepare to be surprised, surgeons!

    All of these differences are potentially clinically significant, probably especially in the cases where the nerve (or part of it) passes right through the muscle. For a couple case studies, see Arooj 2014 and Kraus 2015.

  3. Migliorini S, Merlo M. The hamstring syndrome in endurance athletes. Br J Sports Med. 2011;45(4):363. PainSci #54078. BACK TO TEXT
  4. And that is why I shy away from email diagnosis and consultations, despite being more or less constantly asked to diagnose and consult. Readers send me “descriptions” of their pain problems, routinely with the considerate disclaimer that they “don’t expect a diagnosis,” when what they really mean is, “I realize I can’t hold you to it, but I want a diagnosis anyway.” Many of them are articulate and give perfectly nice descriptions. Many more are sworn enemies of articulate, and my chance of having a clue what they are talking about based on their email is approximately zero percent. And so it goes. BACK TO TEXT
  5. Torun F, Dolgun H, Tuna H, et al. Morphometric analysis of the roots and neural foramina of the lumbar vertebrae. Surgical Neurology. 2006 Aug;66(2):148–51; discussion 151. PubMed #16876606. This was exasperatingly hard data to find for some reason, and the paper abstract begins by saying so: “There have been few anatomic studies on the foramina and roots of the lumbar region….” This is in a 2006 paper! Hardly ancient. BACK TO TEXT
  6. Takasaki H, Hall T, Jull G, et al. The influence of cervical traction, compression, and spurling test on cervical intervertebral foramen size. Spine (Phila Pa 1976). 2009 Jul;34(16):1658–62. PubMed #19770608. BACK TO TEXT
  7. Sari H, Akarirmak U, Karacan I, Akman H. Computed tomographic evaluation of lumbar spinal structures during traction. Physiother Theory Pract. 2005;21(1):3–11. PubMed #16385939. BACK TO TEXT
  8. Ebraheim NA, Liu J, Ramineni SK, et al. Morphological changes in the cervical intervertebral foramen dimensions with unilateral facet joint dislocation. Injury. 2009 Nov;40(11):1157–60. PubMed #19486975.

    Researchers dislocated neck joints in corpses to measure the effect on the size of the intervertebral foramina. (Interesting chore!) Dislocation made the spaces quite a bit larger, indicating that any nerve root pain associated with these injuries “is probably due to distraction rather than due to direct nerve root compression.”

  9. Peul WC, Hout WB, Brand R, Thomeer RT, Koes BW. Prolonged conservative care versus early surgery in patients with sciatica caused by lumbar disc herniation: two year results of a randomised controlled trial. BMJ. 2008 May 23. PainSci #56216. The authors of this paper write, “At the two year assessment only 80% of all patients reported that they had recovered. Some patients who had reported complete recovery within a year of randomisation later apparently had recurrent symptoms of leg or back pain and, at two years’ follow-up, experienced no improvement or even deterioration compared with their pre-randomisation status. Physicians guiding patients with sciatica should remember that the long term prognosis may be less favourable than is suggested by the first impression after successful treatment.” BACK TO TEXT
  10. Costigan M, Belfer I, Griffin RS, et al. Multiple chronic pain states are associated with a common amino acid-changing allele in KCNS1. Brain. 2010 Sep;133(9):2519–27. PubMed #20724292.

    Mark your calendars: 2010 was the year researchers confirmed a gene as “one of the first prognostic indicators of chronic pain risk,” doubling or tripling the odds that a low back pain patient will recover in a timely fashion from nerve root injury. Screening for this gene is not yet something that is clinically available, but it probably will be someday, and then you will know: the universe really does hate you.

  11. Woolf CJ. Central sensitization: Implications for the diagnosis and treatment of pain. Pain. 2010 Oct;152(2 Suppl):S2–15. PubMed #20961685. PainSci #54851.

    Pain itself often modifies the way the central nervous system works, so that a patient actually becomes more sensitive and gets more pain with less provocation. That sensitization is called “central sensitization” because it involves changes in the central nervous system (CNS) in particular — the brain and the spinal cord. Victims are not only more sensitive to things that should hurt, but also to ordinary touch and pressure as well. Their pain also “echoes,” fading more slowly than in other people.

    For a much more detailed summary of this paper, see Pain Changes How Pain Works.

  12. Peul WC, Hout WB, Brand R, Thomeer RT, Koes BW. Prolonged conservative care versus early surgery in patients with sciatica caused by lumbar disc herniation: two year results of a randomised controlled trial. BMJ. 2008 May 23. PainSci #56216.

    OBJECTIVES: To evaluate the effects of early lumbar disc surgery compared with prolonged conservative care for patients with sciatica over two years of follow-up.

    DESIGN: Randomised controlled trial.

    SETTING: Nine Dutch hospitals.

    PARTICIPANTS: 283 patients with 6-12 weeks of sciatica.

    INTERVENTIONS: Early surgery or an intended six months of continued conservative treatment, with delayed surgery if needed.

    MAIN OUTCOME MEASURES: Scores from Roland disability questionnaire for sciatica, visual analogue scale for leg pain, and Likert self rating scale of global perceived recovery.

    RESULTS: Of the 141 patients assigned to undergo early surgery, 125 (89%) underwent microdiscectomy. Of the 142 patients assigned to conservative treatment, 62 (44%) eventually required surgery, seven doing so in the second year of follow-up. There was no significant overall difference between treatment arms in disability scores during the first two years (P=0.25). Improvement in leg pain was faster for patients randomised to early surgery, with a significant difference between "areas under the curves" over two years (P=0.05). This short term benefit of early surgery was no longer significant by six months and continued to narrow between six months and 24 months. Patient satisfaction decreased slightly between one and two years for both groups. At two years 20% of all patients reported an unsatisfactory outcome.

    CONCLUSIONS: Early surgery achieved more rapid relief of sciatica than conservative care, but outcomes were similar by one year and these did not change during the second year. Trial Registry ISRCT No 26872154.

  13. The Vancouver Aquatic Centre (downtown on Beach Avenue, beside the Burrard Street Bridge) has a hot tub with unusually powerful jets. They are really incredible! In fact, they may be too incredible for a bad case of sciatica: but once it starts healing, the strength of these jets is very helpful. BACK TO TEXT
  14. Vroomen PC, Wilmink JT, Kester AD, Knottnerus JA. Lack of effectiveness of bed rest for sciatica. N Engl J Med. 1999 Feb 11;340(6):418–23. PubMed #9971865. PainSci #56953. BACK TO TEXT