Improved Ergonomics for Standing Work
Is standing a safe posture? What can we do to design the workplace to minimize the risks of standing work?
- By Joy M. Ebben, Ph.D, CPE, CHFEP
- Apr 01, 2003
STATIONARY standing is a posture often taken by workers when performing their job duties. There are a number of applications where standing is the recommend working posture, as opposed to sitting. Standing work, compared to sitting when working, is recommended when the task cannot be performed with the employees keeping their arms comfortably at their sides. An excellent example is the research comparing standing versus sitting workstations for checkout cashiers (Lehman, Psihogios, & Meulenbroek, 2000). Another appropriate standing working posture is when assembling, testing, or repairing larger products (i.e., greater than 6 inches high).
Standing is recommended when the work area is too large to be comfortably reached when seated. The maximum reach envelope when standing is significantly larger than the corresponding reach envelope when sitting for both men and women (Sengupta & Das, 2000). Too-large work areas frequently are found in assembly environments, including auto assembly, packaging, welding, sheet metal, and paint lines. A similar application is when the person works in more than one workspace to perform her job duties.
Standing is preferred when the work surface does not allow the person to comfortably position her legs under the surface because of an obstruction. This is often the case when working on a conveyor or a progressive assembly line, working in a kitchen, using a workstation with a drawer located underneath the work surface or a wide front beam, working at a retail counter, or using specialized equipment. Standing is sometimes recommended to maximize grip forces (Catovic, Catovic, Kraljevic & Muftic, 1991) and static or dynamic lifts (Yates, & Karwowksi, 1992). In general, more strength can be exerted while standing (Mital & Faard, 1990).
There is also an extensive lists of jobs that are most appropriately done standing, including construction workers, highway flaggers, medical personnel, painters, electricians, plumbers, loggers, firefighters, plant inspectors, and maintenance personnel.
Is standing a safe posture? Is it better than sitting? What can we do to design the workplace to minimize the risks of standing work? A good understanding of standing work can be obtained from the published research. Researchers have studied standing and body movement; blood pressure; leg and feet swelling; weight load distribution; spine shrinkage; musculoskeletal discomfort; reach; lifting and grip strength; muscle activity; heart rate; association with heart disease and varicose veins; footrests; floor coverings; work/rest cycles; and worker preference.
To summarize the literature, neither static standing nor sitting is recommended. Each position has its advantages and disadvantages. Ideally, the worker is provided with a workstation and job tasks that allow the employee to frequently change his working posture, including sitting, standing, and walking. If either posture is feasible and you must choose one, I recommend sitting using a properly designed chair. (Unsupported sitting, for example, results in disk pressures 40 percent greater than during standing work (cited by Yates & Karwowski, 1992)). However, in many situations, sitting is just not a feasible option. The research provides us with a number of workstation and workplace design features that can help minimize the risk factors associated with standing work.
Footrests, Floor Mats, Shoes
Is standing truly static? Not really. An erect person is somewhat unstable, given an average of only 100 mm between the insides of the feet and a person's average height (Konz & Johnson, 2000). A standing person is like a tall building swaying in the wind.
According to research cited by Whistance, Adams, van Geems & Bridger (1995), individuals required to stand for prolonged periods adopt asymmetrical standing attitudes four times more often than symmetrical attitudes. Shifting the weight from foot to foot provides an important relief mechanism. People tend to stand with one foot forward, which increases their stability and also can reduce twisting stress if the person turns to the side opposite of the forward foot.
The body of standing person also rocks forward and backward and from side to side. The anterior-posterior sway is greater than the lateral sway. Compensatory muscle activity is required to counterbalance the continuously moving gravitational moments acting around the joints.
One recommendation is to provide a footrest at standing workstations. The use of a footrest reduces intravertebral disc stress by preventing excessive lordosis (Whistance, Adams, van Geems & Bridger, 1995). In a study cited by Rys and Konz (1994), subjects were allowed to stand with no footrest or stand and use one of three different footrests: a flat platform, 15-degree angled platform, or a 50 mm bar. They used the footrest options significantly more than standing without a footrest. They used the bar significantly less than the other two footrest options. The bar was used 59 percent of the time, and the other two platforms were used approximately 80 percent of the time. Subjects switched their foot from the floor to one of the footrests once every 90 seconds.
The idea behind floor mats is to encourage the body to naturally and imperceptibly sway, encouraging subtle movement of the calves and leg muscles (King, 2002). Research supports that various floor mats are preferred over concrete floors for standing work. In one of the more recent articles, Cham and Redfern (2001) reported that "floor mats characterized by increased elasticity, decreased energy absorption, and increased stiffness resulted in less discomfort and fatigue" (p. 381). Interestingly, they found that differences in reported musculoskeletal discomfort could not be significantly detected until the third hour of testing.
A field study investigated floor mats and shoe insoles for reducing standing fatigue (King, 2002). Four conditions were studied, each including a hard wood-block floor as the base: a) standing on a hard wood-block floor, b) standing on a floor mat, c) wearing shoes with insoles, and d) standing on a floor mat wearing shoes with insoles. Subjects were exposed to each condition for one work week (40 hours). Fatigue and discomfort were measured using questionnaires. In general, the mat, insoles, and combined condition of insoles and mat were more comfortable than standing on the hard wood-block floor. However, there was no significant difference between the conditions of using insoles or using both insoles and a mat. More research is encouraged in this area using more reliable and sensitive measures.
What about shoes? Konz & Johnson (2000) offer the following advice: Use shoes that mold to your feet and cause a minimum of pressure on all parts of your foot. Because of the swelling during standing, purchase shoes one-half to one size larger than you normally would. A good time to buy work shoes is right after you have been standing for an extensive length of time. Most of the swelling takes place in the midfoot, lowering the arches and increasing the width of the foot (Rys & Konz, 1994). Buy shoes that allow you to wiggle your toes. People who stand for long hours should buy shoes that have laces at the ball and the top of the foot, to allow for swelling and also to make adjustments for two different-sized feet. There can be a 5-10 percent difference in the size of a person's feet (Rys & Konz, 1994).
Workstations used in a standing position require sufficient foot clearance. When there is insufficient foot clearance, the worker must stand farther away from the workspace and tends to lean forward in an unhealthy, awkward posture. Recommended foot clearance space is 150 mm deep, 150 mm high, and 500 mm wide (Rys & Konz, 1994).
Leg and Foot Swelling
Metabolic wastes tend to accumulate in the muscles during isometric work because of minimized blood flow. Body sway affects the pressure distribution on the foot, resulting in occlusion of blood flow from the foot. If the legs don't move, the blood from the heart tends to go down to the legs and remains there--a phenomenon called venous pooling. This creates more work for the heart; the heart must increase its beat rate to provide the same amount of blood as prior to venous pooling. Venous pooling causes leg swelling, edema, and varicose veins. The mean venous pressure in the ankle in one study for sitting was 56 mm and for standing, 87 mm. In another study, the values were 48 mm and 80 mm, respectively (cited by Konz & Johnson, 2000). Walking drops the ankle venous pressure to 21 mm and 23 mm, respectively, in about only 10 steps, after which time the drop in pressure is stabilized (cited in Knoz & Johnson, 2000). Walking two to four minutes for every 15 minutes of standing work is more comfortable than standing without walking (cited in Knoz & Johnson, 2000).
Chester, Rys, and Knoz (2002) studied constrained sitting and standing. The largest increase in leg volume was found using sit/stand chairs and the least with sitting. Standing fell in between.
Standing work is associated with subsequent hospitalization due to varicose veins (Tuchsen, Krause, Hannerz, Burr, & Kristensen, 2000). Also found is a relationship between standing at work and carotid atherosclerosis, particularly for men with carotid stenosis or ischemic heart disease (Krause, Lynch, Kaplan, Cohen, Salonen & Salonen, 2000). In other words, a consideration is the negative impact of standing on employees with pre-existing medical conditions.
Many studies gather discomfort data for sitting or standing when performing work tasks. In one study (Chester, Rys & Konz, 2002), constrained (i.e., no movement) standing had the poorest discomfort ratings for the upper legs, knees, lower leg, ankles, and feet compared to sitting and sit/stand. A sit/stand condition caused the most discomfort in the hips.
Spine Shrinkage
Spinal shrinkage, using a simulated nursing task, was found to be significantly less when sitting than when standing (Beynon & Reilly, 2001). There was less shrinkage over a four-hour test session when the subjects sat during their 20-minute mid-session break, as opposed to standing during their break. During their break, heart rates were higher for the standing individuals compared to the sitting individuals.
Maximum Strength When Standing
Standing postures can be rated according to how long a person can hold an object in a specified position (Miedema, M.C., Douwes, M., & Dul, J., 1997). The posture that represents 75 percent of the person's standing height and 50 percent of his arm length has the highest holding time when standing. The worst holding times are when the task is performed at 25 percent of standing height (near the floor).
Work/Rest Schedules
Member of the safety and ergonomics community agree on the importance of breaks to reduce the occurrence of musculoskeletal disorders. Breaks, appropriately structured, can allow an employee to recover from job stressors. Recovery time can be calculated as a ratio of recovery time to exposure time. If a person stands a total of five hours in an eight-hour day, the employee has three hours of recovery time with five hours of exposure time.
However, the above approach to quantifying recovery time assumes the rate of recovery is constant over time. This is not the case with some of the risk exposures when standing. The rate of recovery from fatigue is exponential (Konz & Johnson, 2000). Recovery is much quicker during the first few minutes of a break but as recovery time increases, the rate of recovery decreases. If recovery is complete in 60 minutes it would take only four minutes to drop from 100 percent fatigue to 75 percent fatigue; but it would take 42 minutes to drop from 25 percent fatigue to no fatigue (Konz & Johnson, 2000). In other words, for fatigue recovery, a number of short (five-minute) breaks are more beneficial than one 15-minute break. Using the short-break approach, more recovery is accomplished over three breaks and less fatigue is created during the shorter work periods (Konz & Johnson, 2000).
Three recommendations for reducing the risk of musculoskeletal disorders are to reduce the cumulative load, provide more variation, and provide recovery. Van Dieen and Vrielink (1998), addressing these three recommendations, investigated work-rest schedules for standing workers. Studying poultry conveyor line inspectors, they used four different work-rest schedules: 60 minutes work with 15-minute break (i.e., 60-15), 45-15, 30-15, and 30-30. They concluded that the 60-15 should be avoided. This schedule resulted in the highest back discomfort ratings. The optimal schedule depends upon whether you want to recover from time-dependent back discomfort or leg swelling.
Dieen and Vrielink (1998) write that recovery from back shrinkage is fairly quick, making it desirable to have frequent short breaks (less than 10 minutes of recovery time). Recovery from leg swelling is a slower process, so a longer recovery time is recommended. Increasing the recovery time to 15 minutes could minimize swelling. Increasing the shift time beyond 30 minutes could increase swelling. The use of a sit/stand chair was not recommended to help increase the task time to reduce leg swelling because sit/stand chairs are associated with increased leg swelling (Chester, Rys & Konz, 2002).
Summary
Standing while working is often the most appropriate working posture. However, there are a number of serious considerations regarding employees' health and comfort. Frequent and sufficient rest breaks are required to reduce the hazards of standing when working. Employees should sit or lie down during their breaks.
Providing changes in posture and encouraging walking also will reduce hazards. Never require the employee to stand in a constrained posture. Proper shoes should be worn and footrests provided. Finally, special consideration should be given to employees with pre-existing health conditions that may accentuate the negative consequences of working in a standing posture.
References
1. Beynon, C., Reilly, T. Spinal shrinkage during a seated break and standing break during simulated nursing tasks, 617-622; Applied Ergonomics 32/6, December 2001.
2. Catovic, E., Catovic, A., Kraljevic, K., Muftic, O., The influence of arm position on the pinch grip strength of female dentists in standing and sitting positions, 163-166; Applied Ergonomics 22/ 3, June 1991.
3. Cham, R., Redfern, M.S., Effect of flooring on standing comfort and fatigue, 381-391; Human Factors 43/3, Fall 2001.
4. Chester, M.R., Rys, M.J., Konz, S.A., Leg swelling, comfort and fatigue when sitting, standing, and sit/standing, 461-468; Advances in Occupational Ergonomics and Safety, 2001.
5. King, P.M., A comparison of the effects of floor mats and shoe in-soles on standing fatigue, 477-484; Applied Ergonomics 33/5, September 2002.
6. Konz, S., Johnson, S., Work Design 1-629; 2000.
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13. Tüchsen, F., Krause, N., Hannerz, H., Burr, H., Kristensen, T.S., Standing at work and varicose veins, 414-420; Scandinavian Journal of Work, Environment and Health 26/5, October 2000.
14. Van Dieën, J.H., Oude Vrielink, H.H.E., Evaluation of work-rest schedules with respect to the effects of postural workload in standing work, 1832-1844; Ergonomics 41/12, December 1998.
15. Whistance, R.S., Adams, L.P., van Geems, B.A., Bridger, R.S., Postural adaptations to workbench modifications in standing workers, 2485-2503; Ergonomics 38/12, December 1995.
16. Yates, J.W., Karwowski, W., An electromyographic analysis of seated and standing lifting tasks, 889-898; Ergonomics 35/7, July/August 1992.
This article originally appeared in the April 2003 issue of Occupational Health & Safety.