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Protect the Disks in Forward Bends and
Twists
What causes disk trouble? What is sciatica? How can
you protect your students from a disk injury, or help an injured student heal?
Read on for answers to all of these questions as well as specific asana
suggestions.
With a basic knowledge of the anatomy of the spine,
it's much easier to learn how to teach students healthy habits that will protect
their disks. To get specific advice, asana instructions, and cautions for
teaching students with existing injuries, continue to
Practical Ways to Protect the Disks.
By Roger Cole
Practicing asanas is one of the best things your students can do to maintain
healthy backs. However, there are a few mistakes in practice that can seriously
injure their backs. One of these is the improper practice of forward bends and
twists, which can damage the disks near the base of the spine. Every yoga
teacher should know how to prevent this.
Luckily, most back injuries are not disk injuries, but disk injuries are serious
because they are so debilitating and long lasting. Many of the things you teach
your students to help them avoid disk injuries will also protect them from other
types of back injuries, especially torn muscles, tendons, and ligaments caused
by excessive bending of the lower spine.
Sciatica: A Pain in the . . .
A student with a disk injury may have severe pain and muscle spasms in his back,
but other back injuries can cause the same symptoms. The symptom that sets disk
problems apart is radiating pain, that is, pain that feels like it's coming from
a location distant from the injury. The most common type of radiating pain from
a disk problem is called sciatica, because it follows the course of the sciatic
nerve. This nerve, and its branches, runs through the buttock, down the outer
back thigh and outer calf, and ends at the top of the foot between the first and
second toes.
A student with a minor disk problem may only feel a dull ache deep in the fleshy
part of the buttock, and it might occur only during forward bending or prolonged
sitting. (Although the buttock is the most common location, the pain sometimes
feels as if it is coming from deep in the hip, and it may be accompanied by
muscle spasms there.) A student with a severe disk problem is likely to feel
sharp, "electric" pain, tingling sensations, or numbness all the way from the
buttock down the thigh and calf to the foot, even during simple movements. In
serious cases, nerve damage can also cause weakness in leg muscles, such as the
hamstrings or the shin muscles that flex the foot upward at the ankle joint.
The Root of the Problem
All these symptoms are caused by pressure on the roots of spinal nerves where
they exit the vertebral column. The pressure may come from a bulging disk, a
herniated disk, or a narrow disk space.
It's easy to see how these problems occur once you understand the basic
structure of the spine. The spinal column is made of bony vertebrae separated by
flexible disks. The vertebrae surround and protect the spinal cord. At regular
intervals along its length, the spinal cord sends out long nerve fibers to
various parts of the body. These nerves exit the spine between adjacent
vertebrae. The part of the nerve near the spinal cord and vertebrae is called
the nerve root. Adjacent vertebrae are matched in shape so that, when the disks
separate them properly, they form holes (foraminae) through which the nerve
roots pass freely. As the nerves exit these holes, they pass very close to the
disks.
An intervertebral disk is composed of a tough, fibrous ring (the annulus
fibrosus) wrapped around a jelly-like center (the nucleus pulposus). The whole
disk is attached firmly to the main, cylindrical part (the bodies) the vertebrae
above and below, so the nucleus is completely enclosed. (Note that the
attachment is so strong that the disks cannot slide, so the term "slipped disk"
is a misnomer.) When the spine bends, the bodies the adjacent vertebrae pinch
closer together on one side and draw farther apart on the other side. This
squeezes the disk that lies between them on one side and widens the disk space
on the other, pushing the disk's soft nucleus toward the open side. This is
usually not a problem; in fact, it's necessary for normal, healthy movement of
the spine.
However, forcing the bend can push the nucleus pulposus so hard against the
annulus fibrosus that the annulus stretches or tears. If it stretches, the disk
wall bulges out, and may press on the adjacent nerve (especially in forward
bends; see below). If it tears, some of the nucleus can leak out (herniate) and
press very strongly on the nerve. Another, often-related disk problem is simple
deterioration over time. As disks lose their plumpness, the vertebrae draw
closer together. This narrows the foraminae through which the nerves pass,
thereby squeezing the nerves.
The five mobile vertebrae of the lower back are called the lumbar vertebrae, and
they are numbered, from top to bottom, L1 through L5. Below L5 lies the sacrum,
a large bone composed of five vertebrae fused together with no disks between
them (nerves exit the sacrum through holes in the bone). Although the sacrum is
a single bone, the top vertebra of the sacrum is still called S1. So the disk
between lumbar vertebra 5 (L5) and sacral vertebra 1 (S1) is called the L5-S1
disk. The next disk up, between lumbar vertebrae 4 and 5, is called the L4-5
disk, and so on.
Nerve fibers that exit the spine below vertebrae L3, L4, L5, S1, and S2 combine
to form the sciatic nerve. This means that many of the fibers that contribute to
the sciatic nerve pass directly over the L3-4, L4-5, and L5-S1 disks. If these
disks are injured in a way that presses on the overlying nerve roots, it can
cause sensations (pain, tingling, numbness) that the brain thinks are coming
from the sciatic nerve. This is why students with sciatica often feel more
symptoms in the buttock or leg than in the back. Some don’t even realize they
have a back injury.
Of all the disks in the entire spine, the L5-S1 disk is subject to more
mechanical stress than any other, so it is injured most often. The L4-5 disk is
subject to the second-largest amount of mechanical stress, so it is injured the
next most often. The reason these disks take such a beating is that they lie at
the "bottom of the totem pole," the base of the vertebral column. This increases
mechanical stress in two ways.
First, it makes them bear more weight than other disks. The compressive force of
this weight flattens and spreads the nucleus pulposus, pressing outward on the
annulus fibrosus on all sides. This pressure not only stretches the annulus, it
also tends to slowly squeeze fluids out of the disks, narrowing the space
between vertebrae.
Second, and probably more important, the entire vertebral column acts as a long
lever that exerts its greatest leverage on the lowest lumbar disks. How much
leverage? Imagine a pair of pliers with handles as long as your spine. Now
imagine putting your finger between the jaws and having a friend squeeze the
handles together. When we hold the sacrum fixed and bend the spine, we exert
similar leverage on the L5-S1 disk, and nearly as much on the L4-5 disk.
Although this leverage effect occurs in backbends and side bends, it is most
likely to cause injury in forward bends, especially when they are combined with
a slight twist. In backbends, the nucleus pulposus shifts forward, but the disk
wall cannot bulge forward because it runs up against a broad, strong ligament
(the anterior longitudinal ligament) that runs vertically along the front of the
vertebrae and disks for the entire length of the spine. In side bends, the bone
structure of the spine itself makes it difficult (but not impossible) to bend
the spine too far.
In forward bends, however, the lumbar bone structure offers no significant
resistance, so the nucleus pulposus shifts freely backward, where it presses the
disk wall against the narrow, relatively weak posterior longitudinal ligament.
This ligament runs vertically up the back of the vertebral bodies and disks.
Although it helps prevent the disk from bulging straight backward, it does allow
it to bulge (or herniate) diagonally backward and to one side. This aims the
protruding disk wall or herniated nucleus exactly at the point where the spinal
nerve crosses the disk. We amplify this diagonal action if we twist slightly
while bending forward. Twisting not only directs the bulge of the disk toward
the nerve, it also adds its own compressive force to the nucleus and its own
extra stretch to the disk wall. Therefore, forward bends in general, and twisted
forward bends in particular, pose the greatest risk to the lumbar disks and
nerves.
Among the forward bends, it is the seated ones that are most likely to cause
trouble. In reclining forward bends (for example, Supta Padangusthasana, or
Reclining Big Toe Pose), gravity does not compress the disks. In standing
forward bends (for example, Uttanasana, or Standing Forward Bend), if the sacrum
is tilted far enough forward to allow the spine to hang down, then gravity
actually elongates the spine, widening the disk spaces. Only in seated forward
bends does gravity compress the disks.
The erector spinae muscles that run vertically up the back exacerbate this
compression, especially in seated poses. Although these muscles tend to bend the
spine backward, and therefore help prevent excessive flexion, they also pull the
vertebrae closer to one another, which puts additional pressure on the disks.
When reclining, the erector spinae muscles are relaxed. In standing forward
bends, they may be relaxed or moderately active. But in sitting forward bends,
unless the hamstrings are very loose, the erector spinae muscles must contract
very strongly to tilt the pelvis forward. This adds a very strong compressive
force to the disks. Combined with the force of gravity and the effects of
leverage, this puts enormous pressure on the lower lumbar disks in seated
forward bends.
Although seated forward bends are the worst, simply sitting upright is also hard
on the lumbar disks. Whenever we sit, the top of the pelvis tends to tilt
backward, bringing the sacrum along with it. This causes slight to moderate
flexion of the lumbar spine, so the disks' nuclei push backward somewhat. The
erector spinae muscles contract to prevent the pelvis from tilting farther back
and to keep the spine from slumping. This limits flexion, but adds more vertical
pressure. Meanwhile, gravity compresses the disks more strongly when the spine
is upright than when it is tilted forward. So sitting upright puts more downward
pressure but less backward pressure on the disks than bending forward does.
We tend to sit upright for long periods of time, so the effect on the disks is
cumulative. The disks gradually lose fluids, and the spine becomes measurably
shorter. As any person who suffers from sciatica can tell you, prolonged sitting
(for example, in an office chair, in a car, or on a meditation cushion) can
really make symptoms worse. Although not so prolonged, seated twists can also be
hard on the disks because they combine the effects of upright sitting with the
effects of twisting. Rounding the lower back in twists makes them much worse.
Whether sitting upright or bending forward, the position of the pelvis is
crucial. The pelvis holds the sacrum in place. If the top of the pelvis tilts
backward while sitting, or if it fails to tilt forward in a forward bend, it
forces flexion at the L5-S1 and L4-5 joints. Tight hamstrings or hip rotator
muscles are usually to blame for holding the pelvis back. For this reason,
students who are inflexible in these areas are more prone to disk injury than
those who are flexible there.
With a basic knowledge of the anatomy of the spine, it's much easier to learn
how to teach students healthy habits that will protect their disks. To get
specific advice, asana instructions, and cautions for teaching students with
existing injuries, continue to Practical Ways
to Protect the Disks.
Roger Cole, Ph.D. is an Iyengar-certified yoga teacher
(www.yogadelmar.com), and
Stanford-trained scientist. He specializes in human anatomy and in the
physiology of relaxation, sleep, and biological rhythms.
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