In biological terms, posture is constant, continuous adaptation . . . Standing is in real- ity movement upon a stationary base . . . From this point of view, normal standing on both legs is almost effortless.
(F. A. Hellebrandt (1938))
Humans are designed to stand on two legs, but they are not designed to stand still.
Standing is the position of choice for many tasks in industry but it can lead to discomfort if insufficient rest is provided or if unnecessary postural load is placed on the body. Some advantages of the standing work position are given in Table 4.1.
In everyday life, people rarely stand still for any length of time – if not walking or moving, they adopt a variety of resting positions that vary depending on the culture (Hewes, 1957; Bridger et al., 1994). Table 4.2 summarises some behaviours associ- ated with unconstrained standing. In most occupational settings where people engage in ‘industrial standing’, they are denied the opportunity to practise these behaviours by the design of their workspaces and the design of their jobs.
Short periods of walking and gross body movements are vital to activate the venous pump and assist the return of blood from the lower limbs (Cavanagh et al., 1987;
Stranden, 2000), so the idea that workers should stand still is physiologically and mechanically unacceptable. Anecdotal evidence across many cultures and over time tells us that people who do have to stand for long periods use standing aids such as the staff of the Nilotic herdsman or the spear of the sentry. An experiment on constrained standing by Whistance (1996) demonstrated that even unpractised users spontane- ously make use of such aids when they are provided.
Table 4.1 Some advantages of the standing work positiona 1. Reach is greater in standing than in sitting.
2. Body weight can be used to exert forces.
3. Standing workers require less leg room than seated workers.
4. The legs are very effective at damping vibration.
5. Lumbar disc pressures are lower.b
6. It can be maintained with little muscular activity and requires no attention.c 7. Trunk muscle power is twice as large in standing than in semi-standing or sitting.d
a Singleton (1972). c Hellebrandt (1938).
b Nachemson (1966). d Cartas et al. (1993).
Table 4.2 Some behaviours characteristic of unconstrained standing
1. Never stand still 10. Hang on elbows
2. Bear weight on one leg 11. Rest body on surface
3. Lean backwards against anything 12. Sit on heels against a wall 4. Lean sideways against a vertical surface 13. Use a footrest
5. Rest pelvis against counter 14. Sit down when tired
6. Maximise contact with fixed objects 15. Hang on to overhead objects 7. Use arms as props resting on a surface 16. Rest head on hand
8. Rest one foot on an object 17. Rest knee on something
9. Use thoracic support (e.g. lean on broom) 18. Rest hands on knees
Prolonged daily standing is known to be associated with low back pain. Where possible, jobs that require people to stand still for prolonged periods without some external form of aid or support must be redesigned to allow more movement or to allow the work to be done in a combination of standing and sitting postures.
Fundamental aspects of sitting and standing
Anatomy of standing
The pelvis is held in an anteriorly tilted position by the iliopsoas muscles and the hip joint is free to extend as happens during the stance phase of gait. The trunk and head are rotated until they are vertically above the legs. This is achieved by extension of the lumbar and cervical spines and is why the vertically held spine is ‘S’ shaped in humans whereas in quadrupeds it is ‘C’ shaped and held horizontal to the ground.
Bones and joints In the erect posture, the line of gravity of superincumbent body parts passes through the lumbar, sacral and hip joints and in front of the knee and ankle joints. This places an extension torque around the knee joint, which is resisted because the joint is already fully extended. The flexion torque around the ankle is resisted by the plantar flexors.
Muscles and ligaments A person standing erect under the influence of gravity is never in a state of passive equilibrium. The body can be conceived of as a pillar of segments stacked one on top of the other and linked by joints. It is momentarily balanced when the resultant of all forces acting on it is zero. The system is designed to minimise any displacement of the line of action beyond the base of support described by the position of the feet. Compensatory mechanisms come into play to maintain balance immediately this happens.
Muscles and ligaments play a stabilising role by means of the active and passive torques they exert around joints to correct small, fleeting displacements of the lines of action away from the joints. A ‘good’ posture may be defined as one in which destabilising moments are minimised and the posture is maintained by the resistance of the relatively incompressible bones (as well as interleaved soft tissues such as the intervertebral discs).
When the body is pulled ‘off-balance’ by the requirements of badly designed jobs or workspaces, the anti-gravity muscles come into play and a new equilibrium position is established but with the associated cost of isometric muscle activity.
The erector spinae muscles These are the main extensors of the trunk. They are also used to control flexion. In relaxed standing, very little muscle activity occurs since the lumbar lordosis minimises the trunk flexion moment. When the trunk is flexed even slightly forwards or when a weight is held in front of the body, the erector spinae muscles come into play. Work situations that set up static loading of these muscles include
• Working with the hands and arms held away from the body
• Holding a tool or a weight
• Standing with the trunk flexed to reach for work objects placed too far away or inaccessible owing to a lack of foot space
The leg muscles The soleus and gastrocnemius muscles are true postural muscles in the sense that they are always ‘switched on’ when standing. When a person is leaning forward, the activity of the gastrocnemius muscle increases. Prolonged standing causes significant localised leg muscle fatigue and is one of the causes of leg discomfort.
The abdominal muscles There is very little abdominal muscle activity in standing and even less in sitting (Burdorf et al., 1993). These muscles may help to maintain a proper relationship between the thorax and pelvis by preventing excessive anterior pelvic tilt and hyperlordosis. The abdominals can prevent trunk extension, caused, for example, by loads placed high on the back (or when putting on a backpack, for example) or when walking down steep hills.
The hamstring and gluteal muscles The hamstring and gluteal muscles are hip extensors. The gluteal muscles exhibit hypertrophy in humans and their function is to stop the trunk from ‘jack-knifing’ forwards over the legs, unlike in quadrupeds where the trunk is already ‘jack-knifed’ and the gluteals are used for locomotion. The gluteals are, however, used for locomotion in climbing ladders or stairs. Activity in the hamstrings is slight in the standing position but increases when the stander leans forward, holds a weight or pulls.
The iliopsoas muscles Psoas major and iliacus are hip flexors and are constantly active in normal standing as they prevent extension of the hip joint (the trunk ‘jack- knifing’ backwards over the legs, or loss of lumbar lordosis if the head position is maintained). The iliopsoas muscles act against the hip extensors.
The adductors and abductors of the hip When a person is standing on two feet, these muscles provide lateral stability, preventing translation of the pelvis in the frontal plane. When a person standing on one foot (and also during the stance phase of gait) the pelvis tends to tilt in the direction of the unsupported side. The hip abductors on the side of the supporting leg contract to keep the pelvis level.
Physiology of standing
The increase in energy expenditure when a person changes from a supine to a stand- ing position is only about 8% (Grandjean, 1980). However, erect standing imposes a hydrostatic handicap that makes humans liable to peripheral circulatory collapse.
Peak plantar (foot) pressures of 137 (kilopascals) kPa exceed the normal systolic
pressure of 17 kPa, resulting in occlusion of blood flow through the foot (Cavanagh et al., 1987). Walking and fidgeting temporarily reduce the pressure, allowing fresh blood to pervade the tissues. Venous and circulatory insufficiencies in the lower limbs also contribute to the discomfort that results from prolonged standing. It has been shown that venous reflux is more common in symptom-free surgeons (who stand for long periods) than in a comparison group who experience discomfort in standing.
Prolonged standing causes physiological changes including peripheral pooling of blood, a decrease in stroke volume and increases in heart rate, diastolic and mean arterial pressure, peripheral resistance and thoracic impedance. Standing up from a supine position is accompanied by an increase in the dimensions of the nasal passages (Whistance, 1996).
Constrained standing is particularly troublesome for older workers or for those with peripheral vascular disease because the ‘venous muscle pump’ that returns blood to the heart ceases to function. Fidgeting is a pre-conscious defence against the postural stresses of constrained standing or sitting. Its purpose is to redistribute and relieve loading on bones and soft tissues and to rest muscles.
Varicose veins and standing work
Varicose veins are superficial veins, often in the legs, in which the valves function ineffectively, resulting in pooling of blood and painful swelling. With deep veins, the problem is more serious and can cause blood to return along abnormal pathways, resulting in long-term health problems, including chronic oedema and leg ulcers. Risk factors include obesity, cigarette smoking, high blood pressure and lack of exercise.
The disease is one of the 10 leading causes of hospitalisation in Denmark. Occupa- tional standing is thought to be associated with varicose veins in the lower extrem- ities. Tuchsen et al. (2000) followed a sample of 1.6 million working Danes for three years from 1991. Men who worked mostly in a standing position were almost twice as likely to experience a first hospitalisation for varicose veins compared to all other men. Women who worked mostly in a standing position, were two and a half times more at risk than all other women. Tomei et al. (1999) compared the prevalence of chronic venous diseases in office workers, industrial workers and stoneworkers. The prevalence of the disorders increased with age and number of hours spent standing at work. Controlling for age, the prevalence was higher for workers who stood for 50%
or more of their shift. The findings suggest that standing work should be combined with other types of work in which sitting is possible.
Sitting
When a person flexes the hip and knee joint to sit down, the iliopsoas muscles immediately shorten and the hip extensors lengthen (Link et al., 1990; Bridger et al., 1992). The balance of antagonistic muscle forces, which keep the pelvis in its anteriorly tilted position, is changed and the pelvis tilts posteriorly almost immediately and continues in proportion to the flexion at the hip. In order to maintain the head erect, the lumbar spine flexes to compensate for the tilting pelvis and the lumbar lordosis diminishes and eventually disappears (Figure 4.1).
The spine of a person seated erect exhibits greatly diminished lumbar curvature (mean angle of curvature of 34 degrees compared to 47 degrees in standing, according
(A) (B)
(C) (D)
(E)
Figure 4.1 Posterior tilting of the pelvis and flattenting of the lumbar curve during the transition from standing to sitting.
Much of the postural adapt- ation to sitting takes place in the back, rather than the legs.
a
a
Figure 4.2 Anterior wedging of the intervertebral disc occurs in the slumped sitting position (a = posterior ligaments). Soft tissues between the anterior and posterior ele- ments of the spine may be pressurised, resulting in pain. (Adapted from Keegan, 1953, with permission of the The Journal of Bone and Joint Surgery, Inc.).
to Lord et al., 1997). The flexion moment about the lumbar spine is increased, putting the posterior spinal ligaments under tension and causing the intervertebral discs to be ‘wedged’ anteriorly and to protrude into the intervertebral foramen (Keegan, 1953; Figure 4.2). The mechanical effect of the flexion moment is an increase in the pressure in the intervertebral discs.
Disc pressures are lower in standing than in sitting and lower still when lying down. In standing, the load is shared between the facet joint. In sitting, the discs bear more of the load (Adams and Dolan, 1995), whereas when lying the absolute load on all structures is lowered. Rohlmann et al. (2001) found that disc pressure was lower in relaxed sitting than in standing, but higher when subjects attempted to extend the spine to sit erect. Both Nachemson (1966) and Rohlmann et al. report lower disc pressures when subjects recline against a backrest. This implies that seated workers should be able to adopt relaxed postures.
Brunswic (1984) investigated the relationship between lumbar curvature, hip flexion and knee flexion (Figure 4.3). Brunswic concluded that if the knees are flexed by 110 degrees, seat tilts between 5 degrees rearwards to 25 degrees forwards were accept- able. If the knees were flexed by 70 degrees or less, forward tilt of at least 10 degrees is required. Seated workers should not have to fully extend their legs to operate foot pedals, or, in the case of tall workers, workstations should be designed to provide space for the knees to flex.
Working posture
As far as the lumbar spine is concerned, a good working posture is one in which the spine is towards the mid-point of its range of movement and the trunk is unconstrained – free to move anteriorly and posteriorly.
85 80 75 70 65 60 55 50 45 40 35 30 25
Percentage of total lumbar flexion
110° 90° 70° 50° 30° 10° –25° –15° –20° –10° –5° 0° +5° +10° +15°
+20° +25°
negative neutral positive Seat angle of tilt:
Figure 4.3 Lumbar spine posture as a percentage of maximum lumbar flexion for differ- ent hip and knee angles. (From Brunswic, 1984, with permission.)
Spinal problems in standing
Low back pain is common in standing workers and a number of authors have sug- gested reasons for this. In extended postures (e.g. when standing with a pronounced lumbar lordosis) the facet joints may begin to take on some of the compressive load.
If the lumbar intervertebral discs are degenerated, the space between adjacent vertebrae decreases and the load on the facet joints increases even more. Adams and Hutton (1985) suggest that excessive facet joint loading may be a causal factor in the inci- dence of osteoarthritis. Bough et al. (1990) have shown that degeneration of the facet joints is a source of low back and sciatic pain.
Both Adams and Hutton and Yang and King (1984) suggest that excessive loading of the facet joints stresses the soft tissues around the joint and causes low back pain. For these reasons, excessive lumbar lordosis should be avoided when standing. Extrapolating from this, any workspace or task factors that require workers to arch the back more than they would normally do should be designed out.
Low back pain can also be caused by muscular fatigue if a standing person has to work with the trunk inclined forwards (when doing the washing-up or ironing, for example). In standing, the workspace must be designed to prevent workers from
having to stand with excessive lumbar lordosis or having to adopt forward-flexed working positions.
Spinal problems in sitting
Sitting has a number of advantages over standing as a working position. Static, low-level activity of the soleus and tibialis anterior muscles is required in standing and these muscles can fatigue. Because the lower limbs drain blood against gravity, pooling may occur when someone stands still for long periods, causing swelling at the ankles. In extreme circumstances, reduced return of blood to the heart may cause a drop in blood pressure and the person may faint. The hydrostatic head that has to be overcome to return blood to the heart from the lower limbs is less in sitting than in standing, if the seat is correctly designed.
Despite the popular myth that occupational sitting is a risk factor for low back pain, the evidence suggests that sitting at work, in itself, is quite harmless (Hartvigsen et al., 2000). However, prolonged sitting at work (more than 95% of the day) is associated with back pain (Hoggendoorn et al., 2000). Further, many people wth bad backs gravitate towards sedentary work. Poor design of workstations may then exacerbate their problems. In a study at the Eastman Kodak Company in New York, 35% of sedentary workers visited the medical department with complaints of low back pain over a 10-year period. People with existing low back problems often cannot tolerate the sitting position for more than a few hours over the workday.
It is generally accepted that flexed sitting postures should be avoided because of the posterior protrusion of the discs caused by anterior wedging strain. Mandal (1981, 1991) has criticised the ‘90-degree’ concept in seat design, stating that the slumped, flexed posture is inevitable if seats are designed in this way. Mandal has suggested that seats should be higher and should have a forward tilt to reduce the amount of hip flexion needed to use them.
An ergonomic approach to workstation design
Ergonomic workstation design encourages good posture. Figure 4.4 presents a frame- work for posture. It emphasises the role of three classes of variables (Table 4.3).
Clearly, ‘ergonomically designed’ furniture cannot be bought off the shelf. Deci- sions about the appropriateness and relative advantages of different designs can only
Task requirements
Working posture
Workspace design Personal factors
Figure 4.4 The postural triangle. A person’s working posture is a result of the requirements of the task, the design of the workspace and personal characteristics such as body size and shape and eyesight. Consideration of all three components is needed in posture analysis and workspace design.
Table 4.3 Examples of factors that influence working posture
Factor Example
1. User characteristics Age
Anthropometry Body weight Fitness
Joint mobility (range of movement) Existing musculoskeletal problem Previous injury/surgery
Eyesight Handedness Obesity
2. Task requirements Visual requirements
Manual requirements:
positional forces Cycle times Rest periods Paced/unpaced work 3. The design of the workspace Seat dimensions
Work surface dimensions Seat design
Workspace dimensions:
headroom legroom footroom Privacy
Illumination levels and quality
be made after considering the characteristics of users and the requirements of their jobs (Figure 4.5). Indeed, van Dieen et al. (2001) in a study of word processing, CAD work and reading found that trunk loading and trunk kinematics were more affected by the task carried out than the chair that was sat in.
Characteristics of users and workspace/equipment design
One of the most basic considerations in workstation design is the anthropometric fit between users and furniture (Figure 4.6). Designers typically design to ensure that 90% of users will be accommodated. Problems can therefore occur with extremely tall, short or obese individuals and special arrangements may need to be made to accommodate them. Much office furniture is designed around a desk height of ap- proximately 73 cm and assumes provision of a height-adjustable chair. This ensures, within limits, that
1. Short users can raise their chair heights such that the desk height approximates their sitting elbow height.
2. The desk is not so high that the chair height exceeds the popliteal height of a short person in order to achieve (1).