With the growing number of people in offices, good office furniture is always in demand. The most important piece of office furniture is the chair. It is very important for an office chair to be comfortable and supportive because it maybe used for 8 hours a day or more. When considering a chair's comfort, the tilt of the chair plays a key role. There is only a basic understanding of what factors help to qualify a chair's tilt as being good, and this understanding is only qualitative. The purpose of this research was to quantify the kinematics of an office chair's tilt.

To study the tilt of the eight chairs that were tested, an Optotrak was used. The Optotrak is a set of three cameras that track markers. These markers are infrared emitting diodes which can be placed on the chair and subject and then tracked through 3-dimensional space to 0.1 mm accuracy. With this motion analysis equipment, each chair was put through ten trials. Each trial started with the subject sitting in the chair. The subject would then recline and return to an upright position. This constituted one trial. For each trial, data was collected by the Optotrak for 6 seconds @ 100 Hz.

In order for the data to mean something, it is pertinent that there be a set of factors to look for in the analysis. Six tilt factors were analyzed. These factors were range of motion, the synchronization of the chair back and seat, smoothness, heel lift, shear movement, and the location of the pivot points for the chair. The range of motion for a chair is the difference in angles of chair back and seat between the fully upright and reclined position.

The synchronization of a chair is the measure of back rotation to seat rotation. For obvious reasons, chairs always have a synchronization greater than 1. There is a large variation in chairs today, without a complete understanding as to what a good value for this is.

The smoothness of the chair is related to how secure a chair feels. To measure the smoothness, we plot the angular velocity vs. the angle. By looking at these phase plots, we can see accelerations that occur during the motion of the chair. With the phase plots, a judgement can be made as to how smooth a chair is.

Heel lift is measured by placing a marker on the subject's heel. The displacement of this marker is the amount of heel lift. Heel lift is an important factor when considering that if the heel leaves the ground, only the toe is in contact with the ground. This in turn causes the calf muscle to tighten and the purpose of reclining in a chair is to relax.

Another factor dealing with the long-term effects of sitting in a chair is the shear movement between the user's back and the chair back. For our analysis, both the tangential shear and the perpendicular motion between the user and chair back were measured. If a chair has a large amount of perpendicular motion, then it is not always in contact with the user's back. This can lead to back strain and after long periods of time, problems can develop. The tangential shear is less important to the long-term effects of sitting. It can cause a user's shirt to become untucked, or pull the shirt down in the back causing the user to experience a strangling sensation.

The last and possibly the most important factor to consider in chair tilt is the location of the pivot points of a chair. The location of these affect both the heel lift and the shear movement between the user and the chair. For an office chair, there are three pivot points. There is a pivot point for the seat, the back, and a pivot point between the two. In the past, it was believed that a good pivot point location for the seat was one that was close to the knee. Today, manufacturers try to create a pivot point near the ankle to minimize or prevent heel lift. It is not feasible to actually have a mechanical pivot near the user's ankle, so a system of multiple pivots is used. In today's office chairs, one of these systems is called 4-bar linkage. With a system like this, it is possible to create pivot points that are not located on the chair itself.

To date, 8 chairs have been tested. By looking at the data from these chairs, we have quantitatively distinguished tilt factor differences. In the future, these quantitative measures may be related to user comfort, and thus be used in the development and design of new chairs.