Posture is an active process and is the result of a great number of reflexes, many of which have a tonic character. The attitudinal, as well as the righting reactions, are involuntary.
(H. D. Denniston, 1935)
It is easy to overlook the fact that the human body is a mechanical system that obeys physical laws. Many of our postural and balance control mechanisms, essential for even the most basic activities, operate outside of conscious awareness. Only when these mechanisms break down – as in slipping or losing balance – are we reminded of our physical limitations. An understanding of these limitations is fundamental to practically all applications of ergonomics.
The skeleton plays the major supportive role in the body. It can be likened to the scaffolding to which all other parts are attached. The functions of the skeletal and muscular systems are summarised in Table 2.1.
Like any mechanical system, the body may be stable or unstable and is able to withstand a limited range of physical stresses. Stresses may be imposed both inter- nally or externally and may be acute or chronic. A useful starting point in the dis- cussion of mechanical loading of the body is to distinguish between postural stress and task-induced stress. According to Grieve and Pheasant (1982), postural stress is the term used to denote the mechanical load on the body by virtue of its posture.
Posture is defined as the average orientation of the average orientation of the body parts over time. Task-stress depends on the mechanical effort needed to perform the
Table 2.1 Functions of the skeletal and muscular systems
Skeletal system Muscular system
1. Support 1. To produce movement of the body or
2. Protection (the skull protects the body parts
brain and the rib cage protects the 2. To maintain posture
heart and lungs 3. Heat production (muscle cells produce 3. Movement (muscles are attached to heat as a by-product and are an
bone; when they contract, movement important mechanism for maintaining is produced by lever action of bones body temperature)
and joints)
4. Homopoiesis (certain bones produce red blood cells in their marrow)
Table 2.2 Task stress and postural stress Task stress
Postural stress High Low
High Digging a trench Painting a ceiling
Low Competitive Weightlifting Reading a book
Figure 2.1 The stability of the body parts depends on the shape of the base of support described by the position of the feet. (A) is unstable, (B) is fairly stable in all directions, (C) is stable antero-posteriorly, and (D) is laterally stable.
task. Task and postural stress can vary independently of each other (Table 2.2). Some tasks, such as lifting a barbell are high in task stress but can be performed in non- stressful postures. Painting a ceiling requires little effort to apply the paint but much effort to maintain the posture. Much biomechanical stress in unnecessary because it is postural and can be reduced by redesigning the task to improve the posture.
Postural stability
In order for the body to be stable, the combined centre of gravity (COG) of the various body parts must fall within a base of support (the contact area between the body and the supporting surface). In standing, the weight of the body must be trans- mitted to the floor through the base of support described by the position of the feet (Figure 2.1). The alignment of the body parts must be maintained to ensure continu- ing stability and it is in the maintenance of posture that much stress arises.
Some basic body mechanics
The basic limiting condition for postural stability in standing is that the combined COG of the various body parts is within the base of support described by the position of the feet (assuming no other external means of support). Ideally, the lines of action of the masses of the body parts should pass through or close to the relatively incom- pressible bones of the skeleton (Figure 2.2). The jointed skeleton thus supports the
Figure 2.2 The tent analogy. The skeleton is the tent pole, the muscles are the guy ropes and the soft tissues are the canvas.
body parts and is itself stabilised by the action of muscles and ligaments, which serve merely to correct momentary displacements of the mass centres from above their bony supports. Using a rather crude analogy, the skeleton can be likened to an articulated tent pole with guy ropes (postural muscles) on every side. The fabric of the tent corresponds to the soft tissues of the body. Any displacement of the COG of the structure in a given direction leads to tension in the guy ropes on the opposite side. Ligaments can be likened to the springs and rubber fittings that stabilise the articulations of the tent pole, and tendons to the ends of the guy ropes where they insert into the poles.
Postural stress can cause pain. Workers who have to work with the spine flexed forwards (by 60 degrees for more than 5% of the day or 30 degrees for more than 10% of the working day) or rotated (more than 30 degrees) suffer back pain (Hoogendoorn et al., 2000).
Demonstration
To demonstrate the ‘tent’ analogy, stand upright and relaxed with body weight equally distributed between the feet and neither on the heels or the balls of the feet. Place one hand on the low back muscles (you should feel a muscular ridge in the centre of your back). Place the other hand on your abdominal muscles. Palpate both sets of muscles, which should feel soft. Next let your weight move to the balls of your feet and lean forwards slightly. As you do this, tension should appear in the low back muscles as they act to maintain equilibrium. Repeat by swaying backwards, slightly, with the weight on your heels. Your abdominal muscles will begin to tense and your back muscles will relax as the weight moves onto the heels. In the middle, there will be a neutral position, where your upright stance can be maintained with minimal muscular load. This neutral position is one of low postural load.
Kyphosis
Lordosis
Lordosis Cervical
Thoracic
Lumbar
d c c
a b
Figure 2.3 The lumbar, thoracic and cervical spines and the pelvis (a) and sacrum (b). The weight of the upper body is transmitted through the lumbar spine, the iliac bones of the pelvis (c) to the hip joints (d) and legs.
Without its associated trunk muscles, the human spine is very weak – it buckles under a compressive load of only 90 N. Cholewicki et al. (1997) have shown that the function of the trunk muscles is critical in giving the spine its compressive strength.
They demonstrated that, although a neutral position of minimal postural stress does exist, it depends on low-level antagonistic co-contraction of the trunk flexors and extensors. This activity increases when the person carries a load and is an example of true postural muscle activity – the muscles act like guy ropes to stiffen the intervertebral joints. The main cost of the co-contraction is increased spinal loading. In upright postures, the benefits of increased spinal stability outweigh these costs (Granata and Marras, 2000).
Anatomy of the spine and pelvis related to posture
The spine and pelvis support the weight of the body parts above them and transmit the load to the legs via the hip joints. They are also involved in movement. Almost all movements of the torso and head involve the spine and pelvis in varying degree. The posture of the trunk may be analysed in terms of the average orientation and align- ment of the spinal segments and pelvis. Figure 2.3 depicts the spine and pelvis viewed frontally and sagittally.
The spine
Quadrupedal animals and human babies have a single spinal curve running dorsally from pelvis to head. The thorax and abdomen hang from the spine and exert tension that is resisted by the spinal ligaments, the apophyseal (facet) joints and the back muscles. In adult humans, the spine is shaped such that it is close to or below the
COG of the superincumbent body parts that are supported axially – that is, the effect of weightbearing in the standing posture is to compress the spine (Adams and Hutton, 1980). This compression is resisted by the vertebral bodies and the intervertebral discs. The cervical and lumbar spines are convex anteriorly – a spinal posture known as lordosis. It is the presence of these lordotic curves that positions the spine close to or directly below the line of gravity of the superincumbent body parts. The effect is to reduce the energy requirements for the maintenance of the erect posture and place the lumbar motion segments in an advantageous posture for resisting compres- sion (Klausen and Rasmussen, 1968; Adams and Hutton, 1980, 1983; Corlett and Eklund, 1986). The thoracic spine is concave anteriorly and is strengthened and supported by the ribs and associated muscles.
The term ‘spinal column’, although universally accepted, is something of a mis- nomer and ‘spinal spring’ might be more appropriate: the ‘S’ shape of the spine of a person standing erect gives the entire structure a ‘spring-like’ quality such that it is better able to absorb sudden impacts, such as the mechanical shock when the heel strikes the ground when walking (Schultz, 1969), than if it were a straight column.
The loss of the ‘S’ shape in sitting may be one of the reasons why drivers of trucks and farm vehicles who are exposed to vibration in the vertical plane are so prone to back trouble.
The cervical and lumbar spines are not fixed in lordosis. Each vertebral body is joined to its superior and inferior counterpart by muscles, ligaments and joints. The spine takes part in functional movements of the body – part of the postural adaptation required to carry out many activities takes place in the lumbar and cervical spines.
The spine can be considered simplistically to consist of three anatomically distinct but functionally interrelated columns (Figure 2.4). The anterior column, consisting of the vertebral bodies, intervertebral discs and anterior and posterior longitudinal liga- ments, is the main support structure of the axial skeleton. It resists the compressive stress of the superincumbent body parts. The two identical posterior columns are positioned astride the neural arch (which forms a bony cavity through which passes the spinal cord), and consist of the apophyseal (or ‘facet’) joints and the associated bony projections, ligaments and muscles. The posterior elements of the spine act as jointed columns that control the movement of the complete spine and provide
Figure 2.4 Function of (1) intervertebral disc and (2) facet joints. The disc resists the compressive load and the facets resist the intervertebral shear force. (From Kapandji, 1974, with permission.)
2
1
2
1
a
c
(A) b
(B)
Figure 2.5 Intervertebral disc and vertebral body. (a) In this view, the superior vertebral body has been removed to reveal the intervertebral disc below. a = the nucleus pulposus, b = the annulus fibrosus. At the rear can be seen the inferior facet joints (c). (From Kapandji, 1974, with permission.) (B) Detail of the structure of the annulus fibrosus.
The annulus consists of a number of layers of cartilage. The fibres in the layers run obliquely and in different directions somewhat like the layers of a cross-ply tyre.
The outer layers run perpendicularly to each other. (From Vernon Roberts, 1989, with permission of Churchill Livingstone.)
attachment points for the back muscles. The vertebral bodies and their related struc- tures increase in size from the top to the bottom of the spine in accordance with the increased load that they must bear.
The intervertebral discs act as shock absorbers and limit and stabilise the articula- tion of the vertebral bodies. Each disc consists of concentric layers of cartilage whose fibres are arranged obliquely in a manner similar to a cross-ply tyre (Figure 2.5). The layers of cartilage enclose a central cavity that contains a protein–mineral solution (‘proteoglycans’). Positive osmotic pressure ensures that water is always tending to enter the disc. Thus, the discs are pre-stressed to withstand loading (in a manner analogous to reinforced concrete beams used in the construction of modern buildings).
According to Kapandji (1974), the nucleus pulposus functions as a swivel joint.
Intervertebral discs exhibit viscoelastic behaviour. Forces of rapid onset are resisted in an elastic manner: the disc deforms initially then returns rapidly to its original shape when the force is removed. Under continuous loading, however, the disc exhibits a type of viscous deformation known as ‘creep’. Creep occurs as a result of loading above or below a threshold level. Under compressive loading, the disc narrows as fluid is expelled and the superior and inferior vertebral bodies move closer together (Eklund and Corlett, 1984). Under traction (‘stretching’ or ‘pulling forces’), fluid moves into the disc and the disc space becomes wider (Bridger et al., 1990).
The narrowing and expansion of the disc spaces is natural and occurs as a result of the forces exerted on the spine during daily living activities. Since there are 24 verte- bral bodies, all with discs between them, the shrinkage and expansion of the disc spaces results in measurable changes in stature: most people are about 1% taller when they wake up in the morning than when they go to bed at night for this reason (dePuky, 1935). Stature change varies exponentially with loading time: almost 50%
of the stature gained after a night’s sleep is lost in the first half-hour after rising.
Grieco (1986) suggests that, since the discs have no direct blood supply, the daily ingress and egress of fluid due to variations in loading is the mechanism whereby nutritional exchange with the surrounding tissues takes place. Postures that exert static loads on the body will interfere with this mechanism and are hypothesised by Grieco to accelerate the degeneration of the discs. Static compression of cells in the discs has been linked to an increase in the rate of cell death (Lotz and Chin, 2000).
Although it is too early to specify what the tolerance limits would be for safe exposure to static compression, there is some empirical support for the view that such loading should be avoided. Stressful postures adopted for 8 hours per day would be regarded as a health hazard according to this view.
Stature change also occurs with age; after about 30 years of age, the intervertebral discs degenerate, developing micro-tears and scar tissue, fluid is lost more readily and the disc space narrows permanently. At this stage, the spinal motion segments lose stability. It is not surprising then, that most occupationally induced low back pain occurs in middle-aged people. In the elderly, disc degeneration reaches a stage where, together with other degenerative processes, the spine is restabilised but with a cor- responding loss of mobility.
The pelvis
The pelvis is a ring-shaped structure made up of three bones: the sacrum and the two innominate bones. The sacrum extends from the lumbar spine and consists of a number of fused vertebrae. The three bones are held together in a ring shape by ligaments (Figure 2.6). The innominate bones are themselves made from the fusion of
b
d
a b
b
a
b
a b b
c c
c c
b a b
(A)
(B)
Figure 2.6 The pelvis as an arch: (A) The pelvis viewed from above: a = the sacrum, b = ilium, c = the pubis, d = position of the intervertebral disc between the first sacral and fifth lumbar vertebrae. Under load, the sacrum tends to move forwards, like an inverted keystone in an arch. It has to be held in place by strong ligaments. (B) The pelvis viewed from the rear: a = sacrum, b = ilium, c = ischium. The sacrum acts like a true keystone in this plane. (Redrawn from Tile, 1984.)
a
c
b
Figure 2.7 View of the sacroIliac joint from above: a, ligaments; b, sacrum; c, pelvis.
According to Tile (1984) the ligaments act like the cables of a suspension bridge preventing the sacrum from slipping forwards. If the joint is deformed by loading, the ligaments can be pinched by bone, causing pain in the very low back, usually on one side. (Adapted from Tile, 1984.)
three other bones, the ilium, the ischium and the pubis. The pubis lies at the anterior part of the pelvis. It joins the other bones together, completing the ring shape and acting like a strut to prevent the pelvis from collapsing under weightbearing (Tile, 1984). The posterior structures of the pelvis, the sacrum and the ilia carry out the actual weightbearing function.
The pelvis can be likened to an arch that transfers the load of superincumbent body parts to the femoral heads in standing and to the ischial tuberosities (part of the two ischia) in sitting (Figure 2.6).
When it is viewed from the rear (Figure 2.6), it can be seen that the sacrum resembles the keystone of the arch. The load from above is transmitted through the innominates to the femoral heads. However, when viewed from above, the sacrum has the wrong shape for a keystone – it tends to slide forwards, out of the arch (Figure 2.6). Under weightbearing, the tendency for the sacrum to slide forwards anteriorly is resisted by the strong ligaments between the sacrum and the ilia. It is these posterior sacroiliac ligaments that stabilise the joint between the sacrum and the ilia. DonTigny (1985) has pointed out that standing postures in which the person has to bend forwards slightly from the hip (such as when washing dishes at a sink) increase the tendency for the sacrum to be anteriorly displaced, thereby increasing the tension in the sacroiliac ligaments. Small displacements of the sacrum can occur, causing soft tissues to be ‘pinched’ and causing pain (Figure 2.7). This pain can be mistaken for low back pain.
The lumbo-pelvic mechanism
The lumbar spine arises from the sacrum and the degree of lumbar lordosis depends on the sacral angle, which, in turn, depends on the tilt of the pelvis (Figure 2.8). The relationship between the posture of the pelvis and that of the lumbar spine was eloquently described by Forrester-Brown (1930) as follows:
The simplest countryman understands that one cannot put on the top story of a house until one has built the ground floor and foundations; yet medical men are constantly trying to alter the position of the upper bricks of the spinal column without adjusting the base on which they stand.
Some ergonomists could be accused of making the same mistake when trying to design seats to prevent excessive lumbar flexion. Parents’ admonitions to young children to ‘sit up straight and don’t slouch’ may be equally mistaken.
When looking at the posture of the spine at work, it is necessary to consider the factors that determine the position of the pelvis, such as the slope of the seat or the position of the feet on the floor. The pelvis can be represented as in Figure 2.9, as
Figure 2.8 Relationship between sacral angle and lumbar angle.
Figure 2.9 Schematic representation of the muscular system of the pelvis (sagittal view).
When the abdominal or hip extensor muscles shorten, the pelvis tilts backwards.
The result is a flattening of the lumbar spine to maintain the trunk erect. When the hip flexors or erector spinae muscles shorten, the pelvis tilts forwards. This is accompanied by a compensatory increase in the lumbar lordosis.
40°
15° –5°
Abdominals Erector
spinae
Hip flexors
Hip extensors