What are the basic requirements for sitting comfort?
The basic requirements are to have a workstation and job tasks designed to allow a person to remain in a neutral posture, regardless of the work being done. The CSA Standard "CSA Z412-17 Office ergonomics – An application standard for workplace ergonomics" recommends that alternating between sitting and standing, or variations of sitting and standing postures (e.g., reclining, forward tilting, and semi-standing) are encouraged in order to avoid the negative effects of prolonged sitting or standing in one posture. CSA also states that there is no uniquely correct working posture that fits a user for an extended period of time.
This document covers working in a sitting position. Please see other OSH Answers, including Sit/Stand Desk, Working in a Sitting Position, and Working in a Standing Position, for more information.
Workstation
A workstation should allow the worker to sit in a balanced body position. Use a workstation that:
- context Allows for the alignment of the spine whether the worker sits or stands.
- context Allows the worker to move the spine freely.
- context That can be adjusted to the needs of the worker.
Job design
A job's design should allow the worker to work in a variety of balanced positions.
- context Design tasks so they require movement of the spine and encourage the worker to alternate positions frequently.
- context Providing all materials at the working level.
- context Positioning tasks within easy reach.
- context Avoiding lifting and transferring loads while sitting.
- context Avoid an excessive range of movement by:
- context Avoid static positions.
- context Position visual tasks that maintain the vertical alignment of the head and spine.
- context Provide good general lighting.
- context Provide task lighting where necessary.
- context Avoid glare.
- context Design the lighting system to:
What is meant by a neutral posture, reference posture, or balanced position?
Gravity constantly exerts a downward force on our bodies. The parts of our body in contact with a supporting structure (the seat of chairs, armrests, floors, etc.) will feel pressure from the structure to balance this force. Parts of our bodies that are not in contact with a supporting structure will use muscles, tendons, and ligaments to balance the force of gravity with an upward force of their own.
The body position that has the highest strength to handle the pressure felt by the supporting structure, while causing the least amount of strain on the muscles, tendons, and ligaments, is called the neutral or balanced body position.
The following recommendations outline how to maintain a neutral body position while sitting:
Note: No one seating position can remain comfortable over a long period of time. For this reason, multiple positions in which the body can be in the neutral position are recommended. As mentioned above, the workstation should allow the worker to adjust and move between these positions.
- context Soles of the feet make an angle of 90° with the lower legs.
- context Thighs approximately in a horizontal position and lower legs vertical with feet resting on the floor or on a footrest.
- context Erect or upright spine.
- context No twisting of the upper torso.
- context Shoulders are relaxed and upper arms hang normally at the side of the body.
- context Elbows stay close to the body and are bent between 90° and 120°.
- context Hands, wrists, and forearms are straight, in-line, and roughly parallel to the floor.
- context Head and neck are upright (not turned, tilted, flexed, or extended). Generally, they are in-line with the torso.
- context Feet are fully supported by the floor or a footrest may be used if the chair height is high and desk height is not adjustable.
- context Back is fully supported with appropriate lumbar support when sitting vertically or leaning back slightly.
- context Thighs and hips are supported by a well-padded seat and generally parallel to the floor.
- context Knees are about the same height as the hips with the feet slightly forward.
- context The in-line sight is between the horizontal (that is, straight ahead when the head is in the neutral position) and 60° below the horizontal (that is, the eyes looking down). The side to side optimum viewing is directly in front of you, with 15° to either side. 35° side to side is the maximum. Please see the OSH Answers Office Ergonomics - Positioning the Monitor for examples.
How should manual tasks be designed?
Seated manual tasks should be designed to maintain vertical alignment of the spine, and avoid constant stress on the shoulder region. This alignment can be done by:
- context Providing materials at or slightly below elbow height.
- context Providing armrests where appropriate.
- context Providing tool balancers and avoiding heavy hand tools.
- context Avoiding tasks that require the shoulder or arms to be excessively high.
- context Primary zone – immediate reach, for items needed most frequently or of highest importance.
- context Secondary zone – within arm’s reach, for items of lower importance or used occasionally.
- context Tertiary zone – area farthest away which requires you to bend forward to reach, for rarely used items.
- context Avoiding tasks that require excessive reach. Place items according to importance and how often you will use them