The Different Mechanism Options
Spring Tilt Mechanism
Exactly what the name says. These mechanisms are not ergonomic, but they are mostly used on value oriented task chairs or leather conferencing chairs. They go up and down, swivel, and have one locking position. A variation on these is the multi-lock mechanism, which goes up and down and swivels, but allows for multiple locking positions. The pivot point for these is found at the center of the chair, which is why they are not ergonomic. When you recline on a chair that has this mechanism, the front of the seat reclines at the same rate as the backrest, thus it puts pressure on your thighs and limits the circulation of blood.
The classic ergonomic mechanism. It is also known as a mid-pivot tilt mechanism, as the pivot point is in the center of the chair. It doesn't have the ergonomic features of the more advanced, expensive mechanisms, but it has just enough to cross that threshold. To be considered multi-function, one of these mechanisms will have at least 3 paddles that do the following:
1. Control the height of the chair.
2. Lock the back rest at multiple angles.
3. Knee Tilt control - allows the user to lock and adjust the pitch of the seat pan, i.e. to control at what angle the seat is.
Other features with these mechanisms usually include manual back ratchet control for back height and a seat slider. The seat slider, however, is an add on for most mechanisms, and not built in.
The Knee-Tilt Mechanism
The knee tilt promotes keeping your feet on the ground and thighs parallel to the ground when you recline, thus reducing the pressure on your thighs and achieving more bloodflow - more bloodflow means less yawning, so on and so forth. A knee tilt mechanism will have height adjustment, tilt tension and be able to lock the back rest in multiple positions. The 2:1 Syncro tilt option keeps your knees and legs in an ergonomically correct position while allowing you to comfortably recline. The ratio means that for every 2 degrees of back recline, the seat reclines 1 degree, thus ensuring ergonomic comfort. Most of these mechanisms are sloped down and back, putting the attachment point of the mechanism and chair towards the front of the seat.
It is an important part of proper office work ergonomics to keep one's feet grounded, instead of hanging in the air. The knee-tilt feature allows for such a position. Hanging puts feet to sleep, which is bad for circulation, which we know is bad for your blood flow and health.
Final Word: The type of mechanism you choose greatly depends on the application you need the chair for. For quick tasking or conferencing purposes, where sitting is limited to around 3 hours at a time, simple spring tilt mechanisms suffice. Conversely, any seating application where sitting durations range from 6 to 8 hours a day, leather office chairs, ergonomic office chairs, or what have you, a knee tilt mechanism is highly recommended.