BACK CARE FOR SITTING WORK

Professor Alan Hedge, PhD Director Human Factors and Ergonomics Laboratory Cornell University Department of Design and Environmental Analysis Ithaca, NY http://ergo.human.cornell.edu

Sitting has become a way of life for many Americans. We sit in cars, buses, trains or planes when we travel, we sit to eat meals, we sit and watch TV, we sit in classrooms, and for many of us, we sit most of the day at work, often in front of a computer all day. Studies of sedentary workers show that low back problems are associated with poor chair design and inappropriate sitting posture.

Although for many activities we could stand just as easily as sit, for example, we could stand to watch TV, stand to eat, and stand in front of a computer all day. There are at least three good reasons why we prefer sitting.

1. Sitting uses about 20% less energy compared with standing to do the same work, so comfortable sitting helps to relieve fatigue.
2. Sitting helps to reduce the strain on our back muscles and on the intervertebral discs of the lumbar spine, providing that we sit back in a supported, reclined posture.
3. Sitting gives us greater postural stability for performing fine manipulative tasks, such as eating with a knife and fork, sewing, writing, etc.

(Image reproduced courtesy of Humanscale)

Anatomical Changes During Sitting

When we change from a standing posture to a sitting posture anatomical changes occur. The lumbar spine changes shape depending on what we sit on and how we sit. If we sit on a flat surface, such as a bench, bleachers or stool, without any back support we tend to hunch the body forwards for support, often resting our arms on our legs to reduce fatigue. As we hunch forwards, the lower back curves outwards into a kyphotic shape. This is generally regarded as an unhealthy posture if sustained for a prolonged period. With nothing to lean back on, the upper body becomes fatigued and we typically regard this as an uncomfortable way to sit. However, this is the way that many people end up sitting over the course of each workday, so it isn't surprising that studies of sedentary workers, such as office workers, have frequently reported high levels of postural discomfort. [2] What we want is to be able to sit and maintain the lumbar spine in a posture called lordosis.

Ergonomic Chair Design

If we sit on a good ergonomic chair, the seat pan is curved from back to front to encourage the pelvis to rotate forwards, and this helps the lumbar spine to maintain lordosis. As we sit back the lower back should be well supported by a contoured lumbar support. The average preferred height for a good lumbar support is abut 7.5 -inches above the compressed seat surface (19 cm), and the mean preferred seat depth (horizontal distance from front of seat to lumbar support point) is about 15.25-inches (38.7 cm). Ideally, the chair should have a backrest that is sufficient high to provide support to the thoracic region as well. [3] Also, make sure that the seat is height adjustable (15" - 22"), the backrest angle is adjustable (100° - 120°), the armrests have a minimum 16.1” width and have adjustable height (7.1”-10.6” above compressed seat height). [4] Studies of office chairs show that users often cannot correctly identify or operate chair controls [5], so make sure that the chair has user-friendly controls.

(Image reproduced courtesy of Humanscale)

Safe Sitting

Keeping the spine healthy requires periodic changes in posture, and dynamic movement helps to promote circulation and reduce muscle fatigue. Sitting in any static posture for a prolonged period eventually will become uncomfortable. This means that there isn't a fixed posture that's best for everyone, all of the time, whatever the task at hand. Rather, there is a desirable range of movement that works well for most people doing most of the kinds of tasks performed when sitting.

The preferred way of sitting involves the following:

• Make sure that the seat height is correctly adjusted so that your feet are on the ground or on a solid surface like a good footrest.
• Recline back in the chair, with the chair backrest angled between 100-110-degrees, so that the chair back can help support the weight of the torso.
• Make sure that the chair has good lumbar support in the right area for your shape and size of back. If there is an adjustable support, use this to get the best position. If not, use a rolled towel or a cushion to improve your lower back support.
• Make sure that the seat pan is the right size for you and doesn't press behind your knees.
• Look for a chair that doesn't tip up the seat pan when you recline because this can put pressure under the thighs and behind the knees.
• If the chair has arms, make sure that these are correctly adjusted for height so that your shoulders are relaxed, not hunched or raised when you rest on the armrests.
• If the chair has a high neck/headrest make sure that this can be used in different sitting positions.

Now that your body is in a good, supported posture you should be ready to work without any discomfort, but remember to take periodic breaks, and to change posture, get up and move around.

(Image reproduced courtesy of Humanscale)

Tips for Choosing the Best Ergonomic Chair

When choosing a chair ask yourself the following questions:

1. Is the chair comfortable to sit in for the way that you work?

• a) Does the shape of the seat fit you and let your legs move freely?
• b) Is the cushioning comfortable and made of a breathable material?
• c) Do you have at least one-inch free space on either side of your hips and thighs?
• d) Do you have at least one-inch free space between the edge of the seat and the back of your knees?
• e) Can you sit comfortably with your feet on the floor or a footrest?

2. Can you easily adjust the important features of the chair?

• a) Can you adjust seat height while you are sitting in the chair?
• b) Is the range of height adjustment of the chair adequate?
• c) Can you adjust the position of the lumbar support and is this comfortable?
• d) Can you recline the chair back to a comfortable position?
• e) Are the controls easy to understand and use?

3. Is the chair stable when you sit on it?

• a) Does it have a 5+ pedestal base so it won't easily tip over?
• b) Does the chair move easily when you need to?
• c) Can you swivel easily so that you don't have to twist your back to turn?

4. Does the chair have comfortable armrests?

• a) Are the armrests broad, contoured, and adequately cushioned?
• b) While sitting can you adjust the height of the armrests?
• c) Can you easily move the arms out of the way if you need to do this?

If you answered "No" to any of the questions then the chair you are considering might not be right for you. If you answered "Yes" to all of the questions, then this chair will work for you. Remember, you will probably sit for a large part of your life, so make sure that your chair is a source of comfort and pleasure, not discomfort and pain.

[1][1] Wilke, H.J., Neef, P., Caimi, M., Hoogland, T., and Claes. L.E. (1999) New In Vivo Measurements of Pressures in the Intervertebral Disc in Daily Life, Spine, 24 (8), 755-762.

[1][2] Lueder, R. and Noro, K. (eds.)(1994) Hard facts about soft machines: The ergonomics of seating. Bristol, PA., Taylor & Francis.

[1][3] Coleman, N., Hull, B. P. and Ellitt, G. (1998) An empirical study of preferred settings for lumbar support on adjustable office chairs. Ergonomics, 41 (4), 401-19

[4] Human Factors Engineering of Visual Display Terminal Workstations (HFS 100-1988). HFES, Santa Monica.

[5] Helander, M.G., Zhang, L. and Michel, D. (1995) Ergonomics of ergonomic chairs: a study of adjustability features, Ergonomics, 38 (10), 2007-2029.

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