Cornell University Ergonomics Web
DEA 3250/6510 CLASS NOTES
Sitting and Chair Design
1. Introduction - Anestimated 50% of people in the industrialized world suffer some form ofback complaint and many of these are related to poor seat design. How wesit and what we sit on affects the health of the spine. The lumbar regionis the most frequently damaged (L4 and L5). The vertebrae in the lumbarregion are the largest in the spine.
- Definition - Sitting is a body position in which the weight ofthe body is transferred to a supporting area mainly by the ischial tuberositiesof the pelvis and their surrounding soft tissue.
- Purpose - to remove weight from the feet and maintain a stableposture so muscles not directly involved with the work can relax.
- Ideal - There is no single ideal sitting posture. Illustrated 90-degree person sitting posture is for anthropometric reference only. Can't design a chairfor the best single way to sit. We need a variety of chairs that allow different users to each sitin a variety of postures.
2. Posture - the relative orientation of parts of the body in space.
- Best Posture - imposes the least postural stress. Muscles mustdo work to counteract the effects of gravity and other forces as the bodystands or moves through space.
- Postural Strain - adverse consequences of more than a few minutesof postural stress.
- Fidgeting - is the bodies defense against postural stress of whichdiscomfort is a sign.
- Rate of Fidgeting - can be used as an index of chair discomfort.Higher fidgeting rates correspond to higher discomfort rates.
- Crossing and Uncrossing the legs - is a characteristic way ofre-distributing pressure on the buttocks and also helps to pump blood throughthese tissues.
- Postural Comfort - is defined as the absence of postural discomfort,it is therefore a neutral state that we cannot sense.
3. Biomechanics of Sitting - depending on chair and posture, someproportion of total body weight is transferred to the floor via the seatpan and feet, armrests, and backrests.
- Lumbar Region - is normally lordotic (concave, toward the stomach).This reduces the pressure between the vertebrae. The region is normallylordotic for two reasons:
- Thickness - Vertebrae and discs are thicker anteriorly than posteriorly.
- Sacrum - Upper surface of sacrum is at an angle to the horizontalplane.
- Pelvis - The sacrum is fixed to the pelvis, so rotational movementof the pelvis affects lumbar vertebrae.
- Forward rotation - of the pelvis leads to increased lordosis ofthe lumbar spine, helping to maintain an upright trunk position.
- Backward tilt - of the pelvis leads to increased flattening ofthe lumbar spine and eventually increases kyphosis.
4. Sitting Postures - Sitting with the knees and hips flexed, pelvisrotated backward leads to minimize lordosis, flat, or even kyphotic lumbarspine. Three types of sitting postures normally distinguished:
- Anterior (forward leaning) - center of mass in front of the ischialtuberosities. Floor supports more than 25% of body weight. Common posturefor desk work.
- Middle (relaxed, unsupported) - center of mass directly aboveischial tuberosities. Floor supports 25% of body weight. Straight or slightlykyphotic lumbar spine.
- Posterior (backward leaning) - center of mass behind ischial tuberosities.Floor supports less than 25% of body weight. Common for chairs with large,inclined, backrests. Preferred for resting.
5. Factors Influencing Posture -
- Hamstring muscles - also influence sitting posture and configurationof the lumbar spine. These muscles cross from lower leg to pelvis, crossingthe hip and knee joints.
- Location and slope of work area - have a major influence on posturesof neck, shoulders, and upper extremities and therefore also must be consideredalong with the design of the seat, and the sitting habits of the person.
6. Preferred Sitting Angle - Sitting leads to 40 - 90% more stresson the back (disc pressure) than standing posture.
- Studies - From a study of college students the preferred seatback angle for comfort is 15-degrees. This is in keeping with other studies by EtienneGrandjean where VDT operators have preferred 13-15-degree backward incline. Atthis angle the pressure on the intervertebral discs is minimal.
- "Keegan's normal posture" - X-ray studies on 5 peoplelying on their sides (1955, 1960) show that stresses on the spine are mostevenly distributed when legs are at 135-degrees from torso, i.e. 135-degrees hip angle and45-degrees knee angle.
7. Seat Design Criteria
- Seat Height - Optimum seat height is controversial.
- Traditional Criterion - Seat height should be adjusted to supporta knee angle of 90-degrees to prevent leg swelling. However 75% of leg swellingmay be due to low leg muscle activity rather than chair.
- Minimum Height - should be 15" (38cm) which designs to the5th percentile of women with 1" heels. The seat should adjust 9"(23cm).
- Fixed Height - should be about 17" (43cm). This is a compromise.A chair that is too high leads to increased pressure at the popliteal fold(underside of knees), decreasing blood circulation and increasing pressureon the nerve. A chair that is too low increases weight on the ischial tuberosities.
- Seat Pan -
- Seat Depth - recommended is 16.5" for fixed seats and 14-18.5"for adjustable seats. If the seat depth is greater than the buttock-popliteallength (fifth percentile woman is at 17") then the user won't be ableto use the backrest.
- Seat Pan Contours - Half body weight is supported by an 8% areaunder the "seat bones" (ischial tuberosities). If the seat ishard and flat the pressures can be 85-100 p.s.i. Seat contouring and cushioningcan be used to distribute pressure over a larger area and rotate the pelvisforward the promote better posture.
- Seat Cushioning - recommended thickness at 1.5-2". Cushionshould be firmer in back and thicker while less firm and thinner at front.Too much cushioning can cause the body to sink into a chair constrainingmovement. A soft chair may be comfortable at first, but as the body sinksblood circulation lowers, skin temperature rises in affected areas, andcompression under thighs increases. These factors combine to increase discomfort.
- Cushion Compressibility - Compressibility is termed indentationload deflection (ILD) or indentation force deflections (IFD). An ideal combinationis a soft top layer (25% ILD) over a firm bottom layer (65% ILD). Increasedratios between the two, greater than 2.6, leads to better quality support.
- Seat Width - around 20 - 22" to accommodate clothed persons.If seat has armrests then elbow to elbow breadth may be more relevant.
- Seat Angle - Positive seat angle helps user to maintain good contactwith backrest. For most purposes a 5 - 10 angle is recommended.
- Armrests - give additional postural support and aid in standingup and sitting down. Armrests should be padded and engage the fleshy partof the forearm. They should not engage the bony parts of the elbow wheresensitive ulnar nerve is close to the surface so a gap of approximately4" between the armrest and seat back is recommended. Cantilevered elbowrests should be 8-10" above the seat surface height. Armrests shouldbe at least 17.2" apart to exceed thigh breadth of 95th percentilefemales. Finally, armrests shouldn't limit chair access if it is to be usedat a table.
- Backrests -
- Height - Higher backrests give better trunk weight support. Threecategories:
- Low-level backrest - supports the lumbar region only. Depth ofthe lumbar curve of the backrest should be 0.6 - 2.0". Backrest heightsof 5, 7, and 9" seem equally effective.
- Medium-level backrest - gives full shoulder support (e.g. carseat, office chair) and may need to be about 26" high to accommodatethe 95th percentile man.
- High-level backrest - full support of head and neck (e.g. planeseat) and may need to be about 36" for a 95th percentile man.
- Angle - Optimal angle seems to be between 100-110-degrees.
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FAQs
Adjust the height of your chair so that your feet rest flat on the floor or on a footrest and your thighs are parallel to the floor. Adjust armrests so your arms gently rest on them with your shoulders relaxed.
Why is sitting ergonomics important? ›
Implementing ergonomic solutions can make employees more comfortable and increase productivity. Why is ergonomics important? Ergonomics is important because when you're doing a job and your body is stressed by an awkward posture, extreme temperature, or repeated movement your musculoskeletal system is affected.
What is 3 best ergonomic positions when you sit for long time? ›
Best sitting position
keeping the feet flat or resting them on the floor or a footrest. avoiding crossing the knees or ankles. maintaining a small gap between the back of the knees and the chair. positioning the knees at the same height or slightly lower than the hips.
What are some examples of proper ergonomics? ›
Posture And Positioning
Avoid twisting or bending your trunk or neck. Frequently used items should be positioned directly in front of you and angled upward on a copyholder when working. Keep your shoulders relaxed with your elbows close to your sides. Avoid resting your elbows on the hard surface or edge of your table.
What are the 5 points of ergonomics? ›
While the principles below may not be applicable in some projects, the concepts can be adapted to both physical and digital projects.
- 1- Neutral Postures. ...
- 2- Reduce Excessive Force. ...
- 3- Keep Things Easy to Reach. ...
- 3- Work in Power or Comfort Zone. ...
- 4- Reduce Excessive Motion. ...
- 5- Reduce Static Load. ...
- 6- Minimize Pressure Points.
There are five aspects of ergonomics: safety, comfort, ease of use, productivity/performance, and aesthetics.
What are the 4 principles of ergonomics? ›
Prominent ergonomic principles include: Use of neutral posture, rotating tasks to avoid overwork of muscles/other tissues, use of proper handholds and proper gripping technique, and proper lifting/carrying/pushing/pulling procedures.
What is the purpose of sitting? ›
Definition - Sitting is a body position in which the weight of the body is transferred to a supporting area mainly by the ischial tuberosities of the pelvis and their surrounding soft tissue. Purpose - to remove weight from the feet and maintain a stable posture so muscles not directly involved with the work can relax.
What is the advantage of sitting correctly? ›
Good posture helps prevent excessive strain on the joints and muscles of your body, which helps to reduce pain and minimize your risk of injury. Good posture can even boost your self- esteem and mood.
What are the physical benefits of sitting? ›
By sitting and standing correctly, you'll strengthen your core, buttock muscles, and back extensions while relieving unnecessary tension. The reality is that good posture distributes the force of gravity evenly throughout your muscles, so no one structure is over-stressed.
Sit up with your back straight and your shoulders back. Your buttocks should touch the back of your chair. All 3 normal back curves should be present while sitting. You can use a small, rolled-up towel or a lumbar roll to help maintain the normal curves in your back.
What does your sitting posture say about you? ›
Sitting upright with good posture indicates that you're confident and ambitious. If this is your typical posture when sitting down, you're a strong and reliable person that people often turn to when in need of help. Despite sitting perfectly erect, you still know how to have fun and relax.
What can sitting for too long do? ›
Research has linked sitting for long periods of time with a number of health concerns. They include obesity and a cluster of conditions — increased blood pressure, high blood sugar, excess body fat around the waist and unhealthy cholesterol levels — that make up metabolic syndrome.
What should you avoid when sitting? ›
What to avoid doing while sitting?
- Tilting the head forward to prevent neck injury.
- Sitting without lumbar support to prevent back pain.
- Working with arms raised to prevent neck and shoulder pain.
- Bending wrists to prevent wrist pain and muscle cramps.
- Working with unsupported forearms to prevent shoulder and back pain.
Under the OSH Act's General Duty Clause, employers must keep their workplaces free from recognized serious hazards, including ergonomic hazards. This requirement exists whether or not there are voluntary guidelines.
What are 3 things you should do to setup your work area ergonomically? ›
Making the desk tall enough for your arms to rest comfortably at your sides. Raising your monitor to eye level. Positioning your keyboard and mouse as close as you can while maintaining a neutral position in your hands and wrists. Moving around at least once an hour.
How should I be sitting at my desk? ›
Adjust the chair height so your feet are flat on the floor and your knees are in line (or slightly lower) with your hips. Sit up straight and keep your hips far back in the chair. The back of the chair should be somewhat reclined at a 100- to 110-degree angle. Ensure the keyboard is close and directly in front of you.
What is ergonomic checklist? ›
A good ergonomic environment allows for maximum adjustability of the office equipment including desk height, chair height, backrest, seat pan, armrests, footrests, computer screens, keyboard position, document holders, and lighting.
What are positive ergonomics? ›
Positive ergonomics: Positive ergonomics refers to a new type of ergonomics that stresses the positive aspects of the man-machine system. Its major interest is to make “human-machine system” enjoyable where the human feels pleasant.
What is basic ergonomics? ›
Ergonomics is the field of study concerned with finding ways to keep people safe, comfortable and productive by accommodating human characteristics, capabilities and limitations in the product or process both at work and home.
The 10 Basic Principles of Ergonomics
- Work in a neutral position. ...
- Decrease the need for excessive force. ...
- Keep materials within easy reach. ...
- Work at the proper height. ...
- Reduce unnecessary motions. ...
- Minimize fatigue caused by static load. ...
- Minimize contact stress. ...
- Leave adequate clearance.
Ergonomic risk factors are workplace situations that cause wear and tear on the body and can cause injury. These include repetition, awkward posture, forceful motion, stationary position, direct pressure, vibration, extreme temperature, noise, and work stress.
What are the 2 key components of ergonomics? ›
Ergonomics Domains of Specialization. According to the International Ergonomics Association, there are three broad domains of ergonomics: physical, cognitive, and organizational.
What is the healthiest way to sit? ›
Adjust your chair
Move your seat up or down until your legs are parallel with the ground and your knees are even with your hips. Your arms should be parallel to the ground, too. Your feet should be resting on the floor. If they aren't, use a stool or foot rest to elevate your feet until you are in this position.
What are the two types of sitting? ›
There are two major styles of sitting on a raised surface. The first has one or two of the legs in front of the sitting person; in the second, sitting astride something, the legs incline outwards on either side of the body.
What happens if you don't sit properly? ›
Slouching, slumping, and other types of poor posture can cause muscle tension, as well as back pain, joint pain, and reduced circulation. Poor posture can even lead to breathing issues and fatigue. The benefits of good posture include: Improved balance.
Why is sitting better than standing? ›
The science of standing
Standing requires more effort and energy than sitting, which means that it burns more calories. However, the research shows that standing at work instead of sitting will not help you lose weight because the difference in calories burned is minimal.
How many hours of sitting is healthy? ›
LOW risk indicates sitting less than 4 hours per day. MEDIUM risk indicates sitting 4 to 8 hours per day. HIGH risk indicates sitting 8 to 11 hours per day. VERY HIGH risk indicates sitting more than 11 hours per day.
How many hours of sitting is unhealthy? ›
But when experts analyze the handfuls of studies examining the effects of prolonged sitting, the data shows that sitting for more than eight hours a day can have a serious impact on a person's health.
What muscles get weak from sitting? ›
Legs and gluteals (bum muscles)
Sitting for long periods can lead to weakening and wasting away of the large leg and gluteal muscles. These large muscles are important for walking and for stabilising you. If these muscles are weak you are more likely to injure yourself from falls, and from strains when you do exercise.
The ideal seating position for your posture is with your feet flat on the floor and your back fully supported by the sofa or some cushions. In this position, the body is in perfect alignment with your head in line with your shoulders and spine and your hips level to your knees.
What does good sitting posture look like? ›
Sit up with your back straight and your shoulders back. Your buttocks should touch the back of your chair. All 3 normal back curves should be present while sitting. You can use a small, rolled-up towel or a lumbar roll to help maintain the normal curves in your back.
How does ergonomics affect seating? ›
Correct sitting posture at work is essential in helping you to avoid neck and back pain. Your chair should be comfortable and the controls easy to reach. Adjust the seat and back frequently to suit the task and maintain full back support.
How can I make sitting less harmful? ›
How can you save your health from the dangers of sitting?
- Walk or cycle, and leave the car at home.
- For longer trips, walk or cycle part of the way.
- Use the stairs instead of the lift or escalator, or at least walk up the escalator.
- Get off the bus one stop early and walk the rest of the way.
When seated, keep your back straight and your feet flat on the ground. Try to keep your knees and your hips level. You may need a footrest to keep your hips and knees level. If you sit for long periods, you should support your lower back against the back of your chair.
What is the best ergonomic position? ›
Keeping your back, neck, and head straight and upright, with your shoulders up and back. Your feet should stay flat on the ground or propped up and flat against a footstool, with your knees ideally positioned at the same level as your hips or a little higher.
What is the most ergonomic seat angle? ›
Holding this slightly forward pelvic posture on your chair puts pressure on the muscles around the pelvis and can be hard to hold, so forward tilted chair seats (FTS) are recommended. The angle that has been shown to be optimal is 20-30 degrees forward and down.
Is it better to sit on the floor or in a chair or sofa for back pain? ›
Despite this, health professionals are increasingly advising that sitting on the floor helps to maintain the natural curvature of the spine and so helps people sit more upright and improve posture.
