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Risk Factors and Sources of Injury > Incorrect Seating Posture and Chair Adjustment          
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Poor Seating Posture / Incorrectly Configuring (or Failing to Adjust) the Chair

Graphic of Ergonomic Considerations When Setting
            Up a Sitting Workstation

What is the Correct Seating Posture and Chair Configuration to Minimize Risk Factors when Seated?

Seat Height: The chair should be adjusted to the height where the thighs are parallel to the floor, or slightly inclined towards the knee, with the ball and heel of the feet in full contact with the ground.

Back Rest:
The backrest should be flush against the back to provide support for the lower back (including lumbar support).

Lumbar Support:
The lumbar support should be adjusted to the optimal position in the curve of the lower back to ensure an orthopaedically supported S-Curve in the spine.  

Position of Calves and Feet:
The angle at the hips and knees should be slightly greater than 90° to allow the feet to be slightly forward of the knee of the user, under the desk. 

Seat Pan Depth:
There should be 2" of space behind the back of the leg to prevent the front edge of the seat from putting pressure on the back of the calf.

Arm Rests (Optional):
These can either be on the chair or attached to the work surface.  In no case should the rests impinge upon or limit access to the workstation. 

Arm Placement: The arm should be positioned at a position close to the line of the torso (as shown in the picture at right) so the joint where the forearm meets the upper arm is slightly above 90°. 
Arm Rest Height: The height of the rest should be set such that the shoulders are relaxed, and in the same position as when the user is standing with arms resting at their sides.
Arm Rest Width: The supports should be close enough to the body that there is no strain on the shoulder joint (typically within 1"-3" of the shoulders). 

Why is Improper Seating Posture and Chair Adjustment a Risk Factor?

If the configuration or usage of the chair is incorrect, circulation can become impeded and muscle fatigue and cramping can occur.

If Seat Height is Too Short...
There will be increased pressure on the underside of the thigh, which has no support.

If Seat Height is Too High...
The knees may impinge on access to the workstation, and circulation to the legs will be greatly reduced.

If Back Rest is Not in Contact with the Back...
The user will have to rely upon their own resources to maintain an upright posture, will fatigue at a much faster rate, and will be prone to adopting poor postures without realizing it.

If Lumbar Support is Not in Contact with the Small of the Back...
The user will be unlikely to maintain the proper S-Curve posture, and will likely start to move to a hunched over C-Curve posture which will lead to back pain and discomfort.

If Seat Pan Depth is Too Short...
There will be increased pressure across the underside of the unsupported thighs.

If Seat Pan Depth
is Too Deep...
There will be pressure on the back of the calf..

If Arm Rests are Incorrectly Adjusted:
Arm rests can be helpful as they will provide relief to the shoulders and upper back by supporting the weight of the the arms.  Arm rests at improper heights, depths, or widths will only create additional postural problems and risk factors for users.

How Close to Ideal does the Seating Posture and Chair Adjustment have to be to avoid it being a Risk Factor?

There is a fair amount of variance in tolerance to poor posture throughout the population. However, individuals know when they are experiencing discomfort, although they may not always realize that the source is an improperly configured chair. While many individuals can tolerate a degree of discomfort before permanent damage is done it needs to be remembered that discomfort is a warning sign. As such, steps should be taken to address discomfort as soon as it is identified. Being proactive in addressing the potential causes of the discomfort will greatly decrease the risk of developing pain, which is the way our body tells us that damage has been done. The line between discomfort and pain is a fine one which is why it should be avoided.

What are the Symptoms of a Chair which is in the Wrong Position or Improper Seating Posture?

Symptoms to look for include forced shifting of weight distribution on the seat pan of the chair to address lower back pain/discomfort or pressure points on the seat or underside of the thighs, or a desire to regularly move legs to relief stiffness, numbness, aches or pains.

How Can the Choice of Accessories or Workstation Help to Reduce Risk Factors from a Chair which is Incorrectly Configured or Poor Seating Posture?

Foot Rests can be added to support the foot and prevent plantarflexion of the ankle if the feet are too high (i.e. dangling in mid air, or even if both the ball and heel are not in full contact with the ground). Foot Rests will also address the potential for restriction of circulation that can occur if feet are not properly supported.

Forearm Supports which attach to the worksurface can serve a similar role to arm rests on a chair.

Arm Pads which attach to the arm rests on a chair can provide extra cushioning and raise an arm rest higher than the normal range of adjustment allows.

Seat Support Inserts which can be for the seat pan only or both seat pan and back can enhance lumbar support, improve seat pan cushioning, remove pressure points, and address specific issues such as tailbone sensitivity.

Leg Supports can elevate the leg for specific situations where a user needs to elevate their leg to address thrombosis or other conditions.

Last edited July 20th, 2018

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