Spinal Biomechanics

Anatomy tells you what the spine is made of; biomechanics tells you what those parts do when you bend to tie a shoe, twist to look over your shoulder or lift a child from the floor. The same few principles, planes and axes, coupled motion, load sharing and stability, explain why some movements feel easy and others strain, why discs and facets fail in particular ways, and why advice about posture and lifting has changed over the years. This chapter turns the static spine of Chapter 1 into a moving, loaded structure you can reason about.

Section 1Planes, axes and coupled motion

The language of motion

To describe motion precisely, biomechanics uses three imaginary planes that pass through the body in anatomical position (standing upright, facing forward, arms at the sides, palms forward). Each plane has an axis perpendicular to it, and rotation in a plane happens around that axis, the way a door swings in a horizontal plane around the vertical axis of its hinges.

• The sagittal plane divides the body into left and right. Motion in it is flexion (bending forward) and extension (bending backward). It occurs around a frontal axis (also called the coronal or mediolateral axis) that runs side to side.
• The frontal plane (coronal plane) divides the body into front and back. Motion in it is lateral flexion (side bending) to the left or right, around an anteroposterior (sagittal) axis that runs front to back.
• The transverse plane (horizontal plane) divides the body into top and bottom. Motion in it is axial rotation to the left or right, around a vertical (longitudinal) axis.

Students often mix up the names of planes and axes because the sagittal plane rotates around a frontal axis and the frontal plane rotates around a sagittal axis. The trick is to picture the motion and ask what a skewer through the moving joint would look like: for nodding (flexion), the skewer runs from ear to ear, a side to side or frontal axis.

Six degrees of freedom

A vertebra moving on the one below can do more than rotate. It can also slide, or translate. Because there are three axes and the bone can rotate around each and translate along each, a motion segment has six degrees of freedom: three rotations (flexion and extension, lateral flexion, axial rotation) and three translations (forward and backward glide, side to side glide, and vertical compression or distraction). Research groups describe spinal motion in a three axis coordinate system with its origin in the vertebral body so that results from different laboratories can be compared. In real motion the rotations are much larger than the translations, which are usually a few millimeters at most. Translation becomes clinically important when it is excessive, as in the forward slip of one vertebra on another called spondylolisthesis.

Osteokinematics, arthrokinematics and the axis of rotation

Osteokinematics describes the motion of whole bones that you can see from the outside, such as "the lumbar spine flexes 50 degrees." Arthrokinematics describes what happens between the joint surfaces themselves: gliding, rolling and gapping. When the lumbar spine flexes, the inferior articular processes of each upper vertebra glide upward and forward on the superior processes of the vertebra below, and the facet joint surfaces partly separate. In extension the inferior processes glide downward and backward until their tips approach the lamina below, and the facet surfaces press together. Understanding these small glides helps explain why extension often provokes facet pain while sustained flexion stretches the posterior annulus and ligaments.

Because a vertebra both rotates and translates, the point around which it rotates is not fixed. At any instant there is a point that does not move, the instantaneous axis of rotation (IAR), and it migrates as the motion progresses. In the lumbar spine, the IAR for flexion and extension usually lies in the back half of the disc or near the upper endplate of the vertebra below. In degenerated or injured segments the path of the IAR becomes erratic and scattered, which is one way researchers have tried to detect abnormal motion. A related idea, the helical axis, combines rotation and translation into one screw motion and is used in three dimensional motion analysis.

Coupled motion

When the spine moves in one plane, it rarely moves only in that plane. Coupled motion is the consistent association of a primary motion around one axis with a secondary motion around another. The classic example is the pairing of lateral flexion with axial rotation. Coupling arises from the shape and orientation of the facets, the geometry of the curves, the discs and the ligaments, so it differs between regions.

• Lower cervical spine (C2 to C7): coupling is consistent. Lateral flexion is accompanied by axial rotation to the same side. When you side bend your neck to the right, the vertebral bodies also turn to the right, which means the spinous processes swing to the left (toward the convex side of the curve). The 45 degree facet slope makes this almost unavoidable: as one inferior facet slides down its sloped partner and the other slides up, the vertebra must turn.
• Upper cervical spine: the atlanto-occipital joint allows mostly flexion and extension, with small amounts of lateral flexion and rotation that tend to couple in opposite directions. At C1 to C2, rotation is coupled with a slight vertical translation, because the convex facets of the atlas ride down the convex facets of the axis as the head turns.
• Thoracic spine: the upper thoracic segments tend to follow the cervical pattern. In the middle and lower thoracic segments coupling is variable between studies and between people.
• Lumbar spine: coupling is small and inconsistent. Studies have found same side, opposite side and no coupling, and the pattern changes with posture, level and whether the movement starts from flexion, neutral or extension.

Fryette's laws and what research says

Early in the twentieth century, osteopathic physician Harrison Fryette proposed rules for coupled motion based on observations of models and patients. They are still taught in manual therapy programs, so it is worth knowing them accurately:

1. Law I (neutral): when the thoracic or lumbar spine is in neutral, side bending and rotation occur to opposite sides, and the pattern involves a group of vertebrae.
2. Law II (non-neutral): when the spine is substantially flexed or extended, side bending and rotation occur to the same side, and the pattern may involve a single segment.
3. Law III: introducing motion in one plane reduces the motion available in the other planes.

Law III is consistent with experimental findings and with everyday experience: you can rotate your trunk less when you are already bent far forward. Laws I and II have not held up well as universal rules. Systematic reviews of lumbar coupling studies, using cadavers, radiographs and three dimensional motion capture, have found that the direction of coupling varies between people, levels and starting postures, and no single pattern predicts it reliably. In the cervical spine below C2, by contrast, same side coupling is consistent in nearly all studies. A sound conclusion is that cervical coupling can be taught as a rule, while thoracic and lumbar coupling should be treated as individual and variable. This matters clinically because diagnostic or treatment approaches that depend on a fixed lumbar coupling pattern rest on an uncertain foundation.

Section 2Regional motion: cervical, thoracic and lumbar

What controls how much a region moves

Three features largely decide how much motion a region allows and in which directions. The first is facet orientation, introduced in Chapter 1: facets permit gliding along their surfaces and block motion that drives them into each other. The second is the disc height to body height ratio: a tall disc on a short body allows more tilt before the bones or ligaments stop the motion, the same way a thick rubber washer lets a bolt wobble more than a thin one. This ratio is about 2:5 in the cervical spine, 1:5 in the thoracic spine and 1:3 in the lumbar spine, which helps explain why the neck is the most mobile region and the thorax the least. The third is attached structures, above all the rib cage, which stiffens the thoracic spine, and to a lesser degree the large muscles and the pelvis.

The values below are representative segmental ranges from classic cadaver and imaging studies. They are useful for understanding patterns, but reported values vary considerably with age, method and individual, so treat them as approximate. Flexion and extension are given as a combined arc; lateral flexion and rotation are given to one side.

Cervical spine

The neck is built for mobility so that the eyes, ears and nose can be pointed wherever they are needed. The upper cervical complex (occiput, atlas and axis) and the lower cervical spine (C2 to C7) behave differently. The atlanto-occipital joints, deep cup-shaped joints for the occipital condyles, contribute a large share of nodding flexion and extension, while the atlanto-axial joints provide roughly half of all cervical rotation, about 40 degrees to each side. Rotation at C1 to C2 is limited mainly by the alar ligaments. Beyond about 40 degrees of head rotation, the lower cervical segments join in.

In the lower cervical spine, flexion and extension are greatest at C4 to C5 and C5 to C6, which is also where degenerative changes most often appear in middle-aged adults, possibly because these segments see the most motion. The uncovertebral joints guide flexion and extension and limit lateral translation. Commonly cited normal active ranges for the whole neck in young adults are roughly 45 to 60 degrees of flexion, 60 to 75 degrees of extension, about 45 degrees of lateral flexion and 70 to 80 degrees of rotation to each side. Range falls gradually with age, especially extension and rotation, so normal values must be judged against age.

Two terms describe head and upper neck positions in the sagittal plane. Protraction moves the head forward (lower cervical flexion with upper cervical extension, the chin poking forward) and retraction moves it back (lower cervical extension with upper cervical flexion, the "chin tuck"). These combined patterns show that the upper and lower cervical spines can move in opposite directions at the same time, a point that matters in posture assessment and exercise.

Thoracic spine

Although each thoracic segment allows only modest motion, there are twelve of them, so the region contributes meaningfully to trunk rotation, especially in the middle thoracic spine. The frontal plane facets permit rotation, but the rib cage stiffens the region: cadaver studies show that removing the ribs and sternum greatly increases thoracic motion. Flexion is limited by the posterior ligaments and by the ribs crowding together in front; extension is limited by contact of the long, overlapping spinous processes and laminae and by the anterior longitudinal ligament. Lateral flexion is limited by the ribs on the concave side.

The thoracolumbar junction (about T11 to L1) is a transition zone. The upper facets of T12 face like thoracic facets, while its lower facets face like lumbar ones, so rotation drops sharply from the thorax to the low back. A stiff region meeting a mobile one concentrates stress, which is one reason the thoracolumbar junction is the most common site of traumatic spinal fractures. The thoracic spine also moves with breathing: in inspiration the thoracic curve flattens slightly and in forced expiration it increases.

Lumbar spine

The lumbar spine is the region for flexion and extension. Segmental sagittal motion increases from L1 to L2 down to L5 to S1, so the two lowest segments together provide a large share of lumbar flexion and extension, and they are also the most common sites of disc herniation. Lateral flexion is moderate. Axial rotation is small, about 1 to 2 degrees to each side per segment, adding up to only about 10 degrees each way for the whole lumbar spine. The curved, sagittally oriented facets account for this: as a vertebra rotates, one inferior facet jams against its partner within a couple of degrees. Most of the trunk rotation people think of as "twisting the low back" actually happens at the hips and thoracic spine.

The lumbosacral junction combines large flexion and extension with a forward-tilted sacrum that creates anterior shear. The more coronal facets at L5 to S1, the strong iliolumbar ligaments connecting L5 to the ilium, and the disc and muscles resist this shear.

Lumbopelvic rhythm

Touching your toes is not just lumbar flexion. Forward bending combines lumbar flexion with anterior rotation of the pelvis at the hips, a coordinated pattern called lumbopelvic rhythm. In the classic description, the early part of the movement is dominated by lumbar flexion and the later part by hip flexion, and the order reverses on the way up: the hips and pelvis lead the return, and the lumbar spine extends last. In reality the two overlap throughout the movement. A typical adult might reach roughly 40 to 60 degrees of lumbar flexion and 60 to 80 degrees of hip flexion in full forward bending, though both vary widely.

Lumbopelvic rhythm explains several practical observations. Tight hamstrings limit pelvic rotation, so a person with short hamstrings must flex the lumbar spine more to reach the same point. A person with a painful or stiff low back often bends mainly at the hips and keeps the lumbar spine relatively straight. Fingertip-to-floor distance therefore measures combined hip and spine motion, not lumbar motion alone, which is why clinicians use other methods, such as the modified Schober test or inclinometers, to estimate lumbar flexion.

Section 3Loads on discs and facets

Basic load concepts

A load is a force or a moment applied to a structure. Five basic load types act on the spine: compression (pushing the vertebrae together along the long axis), tension (pulling them apart), shear (sliding one vertebra forward, backward or sideways on the next), bending (flexion, extension or lateral flexion, which compresses one side of the segment and stretches the other) and torsion (twisting about the long axis). Real activities combine several at once. Force is measured in newtons (N); a mass of 1 kg weighs about 10 N, so a 70 kg person weighs about 700 N. A moment (torque) is a force multiplied by its perpendicular distance from the axis, the moment arm, and is measured in newton meters (N·m).

When a tissue is loaded, it deforms. Stress is the load per unit area and strain is the deformation relative to the original length. Plotting load against deformation gives a load-deformation curve, and the slope of that curve is stiffness. Most spinal tissues are not linear springs: they are viscoelastic, meaning their response depends on how fast and how long they are loaded. Three viscoelastic behaviors matter in daily life:

• Creep: continued deformation under a constant load. A disc slowly loses height and a ligament slowly lengthens during prolonged sitting or standing.
• Stress relaxation: falling internal stress when a tissue is held at a constant length, as in a sustained stretch.
• Hysteresis: energy lost as heat during a loading and unloading cycle, seen as a loop on the load-deformation curve. It is part of how discs absorb shock.

Viscoelastic tissues are also stiffer when loaded quickly. A disc or ligament resists a sudden load more strongly than a slow one, but it may fail under a fast, high load in a way it would not under a slow one.

Compression and the disc

In upright postures, most of the compressive load on the spine passes through the vertebral bodies and discs. The body weight above the L3 to L4 level is roughly 60 percent of total body weight, but the compressive force on a lumbar disc in standing is usually larger than that, because the trunk muscles must also contract to keep the upper body balanced, and muscle forces compress the spine. During lifting, muscle forces dominate the total.

When a healthy disc is compressed, the incompressible nucleus pressurizes and pushes outward. The annulus fibers are pulled into tension around the circumference, much like the hoops of a barrel, and the annulus bulges slightly outward. The endplates bow toward the vertebral bodies. This system turns vertical compression into tension in the annulus, the tissue best suited to resist it. The weakest link under pure compression is the endplate, which typically cracks before a healthy annulus ruptures; reported failure loads for young adult lumbar segments are often in the range of about 5 to 10 kN, with much lower values in older people with low bone density. A degenerated disc, whose nucleus has lost water and pressure, transfers more load directly to the annulus and facets and distributes stress unevenly.

Intradiscal pressure in different positions

In the 1960s, orthopedic researcher Alf Nachemson measured pressure inside the third lumbar disc of volunteers with a needle transducer. His results, often shown as percentages of the pressure in relaxed standing, became some of the most reproduced figures in spine care: lying supine about 25 percent, side lying about 75 percent, standing 100 percent, sitting upright without support about 140 percent, sitting bent forward about 185 percent, and lifting with a flexed spine more than 200 percent. These values shaped decades of advice that sitting is "harder on the back" than standing.

In the late 1990s, Wilke and colleagues implanted a pressure sensor in the L4 to L5 disc of a healthy volunteer and recorded pressure through a full day. Many findings matched Nachemson: lying produced the lowest pressure (about 0.1 MPa), relaxed standing about 0.5 MPa, standing bent forward roughly doubled it, and lifting a 20 kg weight with a rounded back produced the highest values (about 2.3 MPa) compared with lifting the same weight close to the body with bent knees (about 1.7 MPa). The main difference was sitting: relaxed or slouched sitting produced pressures similar to or lower than standing, not higher. Pressure also rose overnight as the unloaded disc reabsorbed water. The newer data come from very few subjects, so neither set is the final word, but together they support a balanced message: forward bending and lifting with load far from the body raise disc pressure most, while ordinary sitting is not dramatically worse than standing.

Bending, torsion and shear

In flexion, the front of the disc is compressed and the back is stretched, the nucleus pressure rises, and the posterior annulus, posterior longitudinal ligament, ligamentum flavum, interspinous ligaments and facet capsules are tensed. The facet joints unload as their surfaces slide apart. Laboratory studies in which spinal segments were flexed and compressed thousands of times, as in repeated bending and lifting, produced progressive tears in the posterior annulus and eventually herniation of nucleus material, often after the inner annulus had already fissured. This is the mechanical basis for the idea that repetitive loaded flexion is a risk factor for disc injury, although living tissue also adapts and repairs, so laboratory cycle counts cannot be applied directly to people.

In extension, the posterior annulus is relaxed, the anterior longitudinal ligament and anterior annulus are stretched, and the facets carry more load, as their surfaces press together and the inferior articular tips approach the lamina below. Extension also narrows the intervertebral foramina and the central canal slightly, which is why people with spinal stenosis often feel worse standing upright or leaning back and better sitting or leaning forward.

In torsion, only about half the fibers of the annulus, those running in the direction of the twist, are stretched at a time, because the lamellae alternate. The lumbar facets protect the disc: as a lumbar vertebra rotates, its inferior facet on the side opposite the direction of rotation presses into the superior facet below, while the facet on the other side gaps. In right rotation of a lumbar segment, the left facet joint is compressed. This bony block stops lumbar rotation within a few degrees, before the annulus is strained to failure. Combined twisting and bending, however, increases annular stress and is a common component of disc injury.

Shear is especially important at the lower lumbar segments, where the lordosis and the forward-sloping sacrum create a tendency for the upper vertebra to slide forward. The facets (particularly their more coronal parts), the disc, the iliolumbar ligaments and the muscles resist it. If the pars interarticularis fractures, the facet block is lost and the vertebra may slip forward, producing isthmic spondylolisthesis, most often at L5 on S1.

How the disc and facets share load

The disc and facets form a load-sharing partnership. In upright standing with a normal lordosis, the disc carries most of the compression and the facets carry a smaller share, commonly cited in the range of about 10 to 20 percent. The facet share increases with extension, with prolonged standing (as the discs lose height through creep) and with disc degeneration, and in some studies of narrowed discs it rose substantially higher. The facets carry a large share of the resistance to shear and torsion at all times.

Section 4Stability and the neutral zone

Why the spine needs muscles to stand

A spine stripped of its muscles is surprisingly fragile. In classic experiments, an isolated cadaver thoracolumbar spine fixed at the base buckled under a vertical load of only about 20 N, and an isolated lumbar spine under roughly 90 N, far less than the weight of the upper body. In life the same spine carries loads of thousands of newtons. The difference is made by muscles and the nervous system that controls them. The ligamentous spine works like a flexible mast; muscles act like guy wires that stiffen it in every direction. Stability, in this mechanical sense, is the ability to return to a balanced position after a small disturbance rather than buckling or sliding away.

Panjabi's three subsystems

In 1992, biomechanist Manohar Panjabi proposed a model of spinal stability that remains the standard framework. It describes three interdependent subsystems:

• The passive subsystem: vertebrae, discs, facet joints, joint capsules and ligaments. It provides stiffness mainly near the end of range, when ligaments become taut and bones make contact. Its ligaments and capsules also contain mechanoreceptors that report position and load.
• The active subsystem: the muscles and tendons around the spine. They generate force and stiffness throughout the range of motion.
• The neural control subsystem: the nervous system, which receives information from receptors in the passive and active subsystems, determines what stability is needed, and sets muscle activation patterns, both in anticipation of a load and in response to it.

The subsystems can compensate for one another. A person with lax ligaments may stay well by relying on muscle control, while a person with good passive structures but poor control can still have trouble. Dysfunction that exceeds the compensating capacity of the other systems produces instability, symptoms or adaptations such as altered movement.

The neutral zone and elastic zone

If you apply a small moment to a motion segment in the laboratory and plot rotation against moment, the curve is not a straight line. Near the neutral position the segment moves a fair amount with very little resistance; further out it stiffens steeply. Panjabi named these regions:

• The neutral zone (NZ): the part of the range near neutral where motion meets minimal internal resistance. Here the segment is lax and stability depends heavily on muscles.
• The elastic zone (EZ): from the end of the neutral zone to the end of the physiologic range, where passive tissues resist increasingly.
• Together they make up the range of motion. Loading beyond it enters the plastic zone, where tissues are permanently deformed, and finally failure.

Laboratory studies found that the neutral zone increases with injury and disc degeneration, sometimes more than the total range of motion does, and that simulated muscle forces decrease it. Panjabi therefore suggested that an enlarged neutral zone, rather than an enlarged total range, may be a more sensitive marker of a segment that has lost stability, and that muscles help by keeping the segment within a safe, stiffer zone. These ideas are well supported in cadaver models; directly measuring the neutral zone in living patients remains difficult.

Muscles, co-contraction and motor control

Because the ligamentous spine is unstable on its own, stability in life comes from coordinated activity of many trunk muscles at once, a strategy called co-contraction. Modeling studies suggest that modest co-contraction, often only a small percentage of maximal effort, is enough for most daily tasks, and that higher levels are needed for heavy or unexpected loads. More co-contraction raises stability but also raises spinal compression and energy cost, so the nervous system balances the two.

Research in the 1990s reported that in people with low back pain, the transversus abdominis switched on slightly later than normal when they moved an arm quickly, a delay in its usual feedforward (anticipatory) activity. This finding inspired motor control exercise, which trains people to activate deep trunk muscles and then integrate that control into functional movement. Later studies showed that altered trunk muscle control in back pain is real but highly variable: some people brace and stiffen more rather than less. Systematic reviews of trials have found that motor control exercise helps people with chronic low back pain about as much as other forms of exercise, but not clearly more. The practical lesson is that many types of exercise help, and no single muscle needs to be isolated for most people.

Clinical instability

The word "instability" is used in two quite different ways. Clinical instability in the classic definition by White and Panjabi is the loss of the spine's ability, under physiologic loads, to maintain its normal relationships between vertebrae so that there is no initial or later damage to the cord or nerve roots, no major deformity and no incapacitating pain. Fractures, ligament ruptures, tumors, infections and some spondylolisthesis can produce this kind of instability, which often requires immobilization or surgery. For the cervical spine, commonly cited radiographic warning signs are more than about 3.5 mm of horizontal translation or more than about 11 degrees of angulation between adjacent segments on flexion and extension films.

In manual therapy and exercise settings, "instability" or "functional instability" is also used for segments thought to have poor control within their normal range, sometimes described as aberrant motion: catching, painful arcs or sudden shifts during movement. This concept is plausible and has guided treatment classification systems, but its signs are harder to measure reliably. It should not be confused with structural instability. Its opposite, a segment that moves too little (hypomobility), is a key idea behind joint manipulation and is discussed in later chapters.

Section 5Posture and load in daily life

The line of gravity in standing

Posture is the position of the body's parts relative to each other and to gravity at a given moment. Standing posture is often assessed with a plumb line, a vertical reference representing the line of gravity. In the traditional description of ideal standing alignment seen from the side, the line passes through or near the earlobe (external auditory meatus), the bodies of the cervical vertebrae, the shoulder joint, roughly through the lumbar vertebral bodies, slightly behind the hip joint (near the greater trochanter), slightly in front of the center of the knee and slightly in front of the lateral malleolus at the ankle.

Wherever the line of gravity falls in front of or behind a joint, it creates a moment that must be balanced. Because the line passes in front of the thoracic spine, gravity tends to flex it, and posterior ligaments and back extensors resist. Because it passes behind the hip, gravity tends to extend the hip, resisted largely by the strong iliofemoral ligament, so standing costs the hip muscles little. In front of the knee it tends to extend the knee, which locks it straight passively. In front of the ankle it tends to dorsiflex the ankle, so the calf muscles, especially soleus, stay lightly active and make constant small corrections. This is why quiet standing involves gentle postural sway and low but continuous muscle activity. The arrangement is efficient: in good alignment, much of the work is done by ligaments and bony stacking rather than effortful muscle contraction.

Moment arms: why distance matters more than weight

The most useful single idea for understanding spinal load is the moment arm. The back extensor muscles attach close to the spine, with an effective moment arm of only about 5 to 6 cm from the center of the disc. Any weight held in front of the body, including the weight of the head and trunk themselves, acts at a much longer moment arm. To balance the moment, the extensors must pull with a force many times larger than the weight, and that muscle force compresses the spine.

A simple worked example shows the scale. Suppose a person leans forward so that the 400 N of upper body weight acts 20 cm in front of the L5 to S1 disc, and holds a 100 N box (about 10 kg) 40 cm in front of it. The flexion moment is (400 N × 0.20 m) + (100 N × 0.40 m) = 80 + 40 = 120 N·m. If the extensors work at a 0.05 m moment arm, they must generate 120 / 0.05 = 2,400 N. Adding the roughly 500 N of weight above the disc gives a compressive load near 2,900 N, about six times the weight lifted plus body weight. If the box is held close, at 20 cm, its moment drops to 20 N·m, the extensor force falls to 2,000 N and the estimated compression falls by about 400 N. Bringing the load closer reduces spinal load more than almost any other change. This is a simplified two dimensional model that ignores abdominal pressure, co-contraction and ligament forces, but it captures the main relationship. For comparison, an occupational safety guideline from the 1980s set an action limit of about 3,400 N of compressive force at L5 to S1, with about 6,400 N considered a level that most workers could not tolerate.

Forward head posture and sitting

The same lever logic applies to the neck. An adult head weighs roughly 4.5 to 5.5 kg. When it is balanced over the cervical spine, the moment is small. When it drifts forward, as in looking down at a phone, the moment arm lengthens and the neck extensors must work harder. One widely cited computer model estimated that tilting the head forward 60 degrees raises the effective load on the neck to the equivalent of about 27 kg. That model is a useful illustration of the lever principle, but it was not a measurement in people, and studies have not shown a consistent link between how far a person's head sits forward and whether they have neck pain.

Sitting changes the lumbar curve. When a person sits, the pelvis tilts backward and the lumbar lordosis flattens, so slouched sitting places the lumbar spine in flexion, stretching the posterior annulus and ligaments over time through creep. A lumbar support and a reclined backrest of about 110 to 120 degrees reduce disc pressure and back muscle activity in laboratory studies. On the other hand, the in vivo pressure data above show that ordinary sitting is not dramatically more loading than standing, and population studies have not found that any particular sitting posture reliably causes back pain.

Creep, time and the "next posture"

Time matters as much as position. Holding the lumbar spine in full flexion for even 10 to 20 minutes causes creep in the posterior ligaments, and in laboratory studies the ligaments and reflex muscle protection took much longer than the loading period to recover. Near full forward flexion, the back extensors often fall electrically quiet as ligaments and fascia take the load, a pattern called the flexion relaxation phenomenon; many people with chronic back pain lose it and keep the muscles active. In the morning, discs are fully hydrated and stiffer in bending, so the posterior annulus is under more strain at a given flexion angle. In one randomized trial, people with chronic low back pain who avoided early morning lumbar flexion for six months reported less pain than a control group, a small example of biomechanics translating into simple advice.

These observations support a modern view summarized by the phrase "the best posture is the next posture." No single sitting or standing position has been shown to prevent back pain, and systematic reviews have found little or no association between posture measures and pain. Prolonged, unchanging positions are uncomfortable for most people, and variety appears more important than achieving an ideal shape. Similarly, reviews of workplace training in lifting technique have not shown that it prevents back pain, even though lifting with the load close and avoiding combined twisting and bending clearly reduce mechanical load. The message for students is to separate two questions: what increases load on tissues (well established) and what causes pain in people (much more complex, involving tissue tolerance, conditioning, sleep, stress and beliefs, as later chapters on pain explain).

Words worth knowing