The Efficacy of a Spring Support, Split-Seat Chair to Enhance Mobility When Sitting
Richard Robertshaw DC
Published on: 2021-05-28
Ergonomic studies usually involve physical support and positioning but also includes ease of utilization at a work station. The effects of long term sitting has been addressed from many different biomechanical philosophies. However, movement and mobility while sitting have not been studied or addressed sufficiently from an anatomical perspective.
It is important to study the effects that immobility has on an individual when seated for long periods of time, as is typical with the hours spent in front of computers. Therefore, a study on the efficacy of a spring supported split-seat was performed to evaluate the benefits of a chair that allows and even enhances mobility compared to a single seat pan. Results of this study reveal a clear difference in the structural movement and the importance of mobility of the body with a spring supported split-seat chair compared to the single seat chair.
KeywordsErgonomic; Split-Seat Chair
Introduction and Aims
Workstations are constantly being revised to reduce wear and tear on the worker. Long term sitting has become so injurious that people prefer to stand for many hours at work or at least alternate between sitting and standing. We sit at home, work and in our cars. Rather than eliminate sitting altogether, an alternative can be offered to allow more mobility in the chair itself and not with just a moveable back and rocker. There are two pelvic bones and they can be independently supported to allow the pelvic bone to move reciprocally from side to side thereby reducing compression on the lower spine, hips and shoulders.
London’s Brunel University, School of Engineering and Design under the direction of Jinsheng Kang, PhD, Stephan Orphanides and Jie Hong performed a study to evaluate the structural mobility of sitting in a single seat chair compared to a spring supported, split-seat chair.
The spring supported split seats that were used in the study were custom prototypes where the seat pan is split from front to back with a gap between the two sides being 1 inch wide. Each side of the split seats in supported by modified bicycle seat springs
Arranged in a diamond configuration (Figures 1,2). The front and the rear spring are set at 8.5 inches apart and from side to side by 4.5 inches. This is to allow a reciprocation from side to side (Figures 3,4).
Figure 1: Illustration of spring configuration.
Figure 2: Front view of seat.
Figure 3: Front of prototype with split-seat and spring support.
Figure 4: one of the prototypes used in the Brunel study.
The study included 11 cameras, (Figure 5) taking 120 frames per second, set up in a circle around people sitting in different chair while performing desk related tasks.
Figure5: Camera positioning.
The hardware used to capture the body motion of the participants was the Motion Analysis Eagle Digital System. Reference markers were placed on the underside of each chair used in the study as well as on the sides of the desk to record accurate heights.
A custom marker set was used to analyze in greater detail the back motion of each participant. The custom marker sets were composed of 39 custom signal markers which were placed on the back (Figure 6), front (Figure 7), sides (Figure 8), shoulders, neck, head, hips, knees and feet of each participant. They were then recorded doing desk tasks while seated in a regular single seat chair and the spring supported split-seat chair.
Figure 6: Custom markers, posterior.
Figure 7: Anterior Custom Markers.
Figure 8: Lateral Custom markers
Eight Tasks Were Selected
- Sit, stand up and sit down in one cyclic motion.
- Lean far to the left and then to the right without support.
- Reach to a file cabinet located behind and to the right of the participant.
- Reach for paper on the floor (front left).
- Reach for a glass of water on the desk, take a drink and then place the glass back in the original position on the desk.
- Reach for a phone on the desk (right in front of the participant), move it towards the participant’s ear, hold 5 seconds, then return it to the original position on the desk.
- Move a mouse in a square motion around the perimeter of a piece of white A4 paper taped to the top of the desk. Two pieces of A4 paper were placed to accommodate for left and right handed participants.
- Type at a keyboard, participants were asked to type one paragraph of the text displayed in front of them.
17 Subjects Participated In the Workstation Study
Eight common workstation motion tasks were designed in order to test the main function and design of the spring supported, split seat chair in comparison with a standard office chair. After analyzing all the results of this motion study, a clear difference between the two types of chairs became apparent. After examination of all joint angles across eight workstation motion tasks, a summary of the split-seat, spring supports chair show to be effective in allowing more mobility of the body while sitting.
The main distinct difference between the split-seat chair and the standard seat was evident in the hips, mid-back and upper back. Each of these was effected by the designs presented in both chairs. Overall the subject’s hips bent less while completing the motion tasks in the spring supported, split-seat than in the standard chair. Subjects were inclined to bend their mid-backs and upper backs instead of their hips. The difference was more obvious in some actions which leads subjects to change their center of gravity while reaching for items on the desk (left/right or right/left), as well as reaching behind. The difference was more apparent when the subject were seated at the front of the seat rather than sitting at the back of the split-seat.
In some other motions, subjects showed a wider varied scale about the hip angles including hip abduction and hip adduction. In addition subjects had almost the same hip flexion but very different hip abduction and adduction results when asked to pick an item up from the floor. Subjects had much more freedom about how they positioned their hips and legs in the split-seat, spring supported chair when completing these tasks.
Subject also had a higher shoulder flexion value in most actions. When twisting their body, left/right shoulder flexion were both higher. Upon reaching for a glass of water in the left front desk, right shoulder flexion was higher in the spring supported, split seat than in the standard chair. For the opposite type of motion, reaching to answer the phone on the right front desk, the left shoulder flexion was higher in the split-seat.
According to the results the study showed that the split-seat chair had a clear advantage over the standard chair for allowing the mobility of the body when sitting and performing specific tasks associated with a normal movement at a workstation.
When a change in the center of gravity occurs, the split-seat offers more support which protects the hips. The spring supported, split-seat also provides more flexibility about the hips and legs when completing general actions such as standing, typing, etc., the split seat increases balance as subjects gained a higher level of shoulder flexion on the opposite arm when completed reaching tasks.
The results also show the spring supported, split-seat was more effective when the subjects were seated toward the front of the split-seat.
The eight tasks that were performed in the Brunel study showed greater mobility amongst the participants in the spring supported split-seat than in the single seat chair in various ways. Depending on the task performed there was greater movement in the upper back, pelvis and mid back. The results showed that in the spring supported, split seat chair that when the lumbar spine was not impeded by a single seat and the pelvis could reciprocate, there was greater range of motion in the rest of the body while sitting. The results also showed that even when sitting on the front of the split-seat there was more mobility then when seated in the back of the split-seat.
The reason this chair was studied was to find a seat that did not impede mobility.
The advantages to that mobility were not in the study. However, it has been proven in many scientific experiments that immobility is a detriment to the body when seated for long hours at a workstation, flying, while seated on the current airline seat and even the long hours of commuting. We sit on our way to work, seat all day at work, sit on our commute home from work and at home in the evening.
The spring supported, split seat was designed to allow, and even, enhance mobility and the inherent rhythms of the body, external and internal rotation as well as flexion/extension of the skeletal system. Consider the prototypes used in this study an evolution of the current chairs in a field devoted to reducing stress, structural damage and illness do to extended hours of sitting.
Richard Robertshaw, D. C.
Practice of Chiropractic in San Francisco, CA. for 42 years.
Technique instructor at Life West Chiroprratic College for 22 years.
Relicensing Provider for California Chiroprcators for 12 years.