Located underneath the seat of an office chair is the chair mechanism. This is a mechanical component that enables you to make specific adjustments to the seat and backrest of the chair. For example, changing the seat height and the angle of the backrest. This allows you to personalise your seat position for optimal comfort and support. There are many types of mechanisms available, so what is the difference between chair mechanisms?
In an office chair, the mechanism is regarded as the “heart” of the chair. Selecting the right one can make all the difference for your long-term comfort. Because there are so many types, it can become confusing when you have to make a choice. Most mechanisms have multiple adjustment levers and knobs which control the various movements, viz. seat height, backrest tilt, seat tilt, tension adjustment and seat depth (slider) adjustment.
Let’s take a look at all the major mechanism types and discuss their differences. However, it is important to first understand the concept of static and dynamic seating, and how this is influenced by the chair mechanism.
Static and Dynamic Seating
a. Static Seating
Ergonomic office chairs should offer adjustments that allow both "static” and “dynamic” seating. The correct static seated position is shown below:
The checklist for a correct static seating posture is where your:
Sitting statically in the above posture minimises the strain on your body. However, maintaining this posture continuously for a long period is not possible. As you work, the continual inactivity (or static loading), of your core muscles causes them to get tired, and so you begin to slouch in your chair.
To improve your overall comfort, it is critical that you constantly change from a static to a dynamic seated posture.
b. Dynamic Seating
Dynamic seating occurs in an office chair that is fitted with a mechanism that allows the backrest and seat to move relative to each other. This movement automatically stimulates, or activates various muscles in your lower back, core and legs. By doing so, the blood-flow through these muscles increases, which prevents them getting tired. Dynamic Sitting has long been acknowledged as the global benchmark for ergonomic comfort.
If you spend long hours at your workstation, a chair fitted with a mechanism that facilitates dynamic sitting will provide far greater comfort, than one that does not.
Categories of Office Chair Mechanisms
There are 7 major categories of mechanisms available for office chairs, all of which will allow you to adjust the height of the seat. With the exception of the swivel-only mechanism, they all provide additional functionality. What is the difference between chair mechanisms? How do they compare when it comes to providing dynamic seating?
Below we describe each category and give the mechanism a Dynamic Rating Score based on it's ability to facilitate dynamic sitting.
1. Swivel-Only Mechanism
The Swivel-Only Mechanism is typically found on a sit/stand stool. These mechanisms are normally used for stools in areas where you continuously alternate between sitting and standing. Good examples are laboratories, factories and salons.
2. Swivel & Tilt Mechanism
This mechanism is found on chairs where the seat and backrest are one continuous piece. In these chairs, the seat and backrest cannot be separately adjusted and will always move together in the same direction, at the same rate.
The pivot point for these mechanisms is usually in the centre of the seat. This means that when you tilt backwards in the chair, the front edge of the seat will have a tendency to lift your feet upwards, and so increase the pressure on the underside of your thighs.
3. Frontal Pivot Mechanism
A Frontal Pivot or Knee-Tilt mechanism, is exactly like a normal Swivel & Tilt (#2 above), except that it's pivot point in not in the centre of the seat, but closer to the front of the chair. This results in a wide-angle tilt that keeps the front of the seat relatively level. Unlike a normal swivel & tilt mechanism, the frontal pivot does not result in an increase in pressure on the underside of your thighs when you tilt backwards in the chair.
4. Permanent Contact Mechanism
A Permanent Contact or Back-Rake Mechanism, allows you to adjust the angle of the backrest. The angle of the seat cannot be changed and remains fixed in a horizontal position. Office chairs that are fitted with a Permanent Contact mechanism are ideally suited for occasional use and are not recommended for intensive use.
A Permanent Contact mechanism allows you to lock the backrest into any desired position, often referred to as infinite lock. Typically, the height of the backrest can also be adjusted so that you can align it with the natural curvature of your spine.
5. Synchronous (Synchro) Mechanism
Synchronous or “synchro” Mechanisms have a mechanical tilt that moves both the chair's backrest and seat together in a fixed ratio. All synchro mechanisms ensure that the movement of the backrest is greater than the movement of the seat. Typically synchro mechanisms have a 2:1 ratio i.e. the backrest moves 2 degrees for every 1 degree of movement in the seat.
The seat and backrest can be locked into a number of static positions. If the mechanism is unlocked, it will be in a free-float position. In this position, the chair is dynamic and the backrest provides continual back support by moving with you. A synchro mechanism enables dynamic seating and is therefore recommended for professional intensive use.
6. Synchronous Frontal Pivot Mechanism
A Synchronous Frontal Pivot mechanism combines the features of a Frontal Pivot and Synchronous mechanism (#3 and 5 above), to create an overall superior product. This chair mechanism is ideal for professional intensive use.
7. Free-Float Mechanism
A Free-Float or Multi-function mechanism allows you to independently adjust the backrest and seat of the chair. This enables you customise the exact angle of the seat and backrest to create your optimal sitting posture.
The seat angle can also be set at a negative tilt (forward sloping), to create an ‘open’ angle at the hips of approximately 100 degrees. This ‘open’ angle is regarded as the ideal ergonomic seated angle – see The Perfect Ergonomic Office Setup. According to Occupational Safety and Health Administration (OSHA), a forward sloping seat helps increase blood flow to the lower extremities and at the same time reduces lower back pressure and pain in the forward leaning position.
In a static seating position you can lock the movement of both the seat and the backrest independently and in any position. In the free-float position, the chair is dynamic and the backrest provides continual back support by remaining in contact with your back and moving with you. Due to its highly advanced functionality, the free-float mechanism is suitable for professional and intensive use.
A Free-Float mechanism offers you the best ergonomic comfort and has the highest Dynamic Rating of all mechanisms.
Choosing the right office chair for your needs is critical for your long-term comfort and wellbeing at work. The mechanism plays a huge role in the overall comfort of any chair. An office chair with a mechanism that has a "good" or "excellent" Dynamic Rating, is essential if you spend many hours sitting at your workstation.