Reducing chair seat pan ischial tuberosity support, combined with increasing chair backrest lumbar support (called an Off-Loading sitting posture), has been found to lessen lumbar paraspinal muscle activity while concurrently decreasing spinal compression when compared to a Normal sitting posture (sitting upright, flat back rest, full seat pan support). This recent Northwestern University study involved 35 subjects, some of which were asymptomatic, while others had low back pain.
When changing from the Normal sitting posture to the Off-Loading posture, there was a significant:
• decrease in average pressure, total contact area and peak pressure on the posterior seat pan
• increase in average pressure and total load along the chair backrest
• increase in peak pressure on the anterior seat pan toward the thighs
• superior shift in peak pressure along the chair backrest within the low back pain group
• decrease in lumbar EMG activity
The Bottom Line – How This Applies To Ergonomists
Although the laboratory chair used in this study was somewhat unique in design, the concepts it is based on and subsequent research findings are consistent with the field practice of many ergonomists. Using chairs that allow the seat pan to be tilted forward along with increasing back rest lumbar support has been a basic protocol applied to clients with low back pain. This research provides a scientific rationale to justify this approach and can be cited when making recommendations during these times of tight business budgets.
It is hoped that this research continues to investigate the impact of the Off-Loading sitting posture on the degree/frequency/duration of symptoms among low back pain subjects.
Other Key Points
In the Normal sitting posture for both the symptomatic and asymptomatic groups:
• The posterior region of the seat pan had the highest average pressure and highest peak pressure
• The posterior and the middle regions of the seat pan had nearly the same total contact area
Subjects were excluded who had structural abnormalities of the spine (i.e., scoliosis, stenosis or significant disc space collapse).
The study chair was designed in the laboratory and permitted the posterior third of the seat pan to tilt inferiorly 20° relative to the anterior two-thirds of the seat pan. The seat pan was adjustable in height, depth and width. The seat pan was parallel to the floor while the backrest-seat pan angle was placed at 100°.
A lumbar air bladder was within the backrest and could be adjusted in height and protrusion.
Normal sitting was defined as sitting upright using a flat seat pan and flat back rest. Off-Loading sitting was defined as sitting upright with the posterior third of the seat pan tilted downward 20° relative to the anterior two-thirds of the seat pan. Also, the lumbar air bladder was activated.
After adjusting the laboratory chair to the subject, a Normal sitting posture was held for 10 minutes, followed by 10 minutes in an Off-Loading posture. These posture positions were repeated for a total of three times while data was collected by two pressure mapping mats.
This position paper can be acquired at: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2654542
Article Title: Biomechanical effects of sitting with adjustable ischial and lumbar support on occupational low back pain: evaluation of sitting load and back muscle activity
Publication: BMC Musculoskeletal Disorders, 10:17, 2009
Authors: M Makhsous, F Lin, J Bankard, R W Hendrix, M Hepler, and J Press
This article originally appeared in The Ergonomics Report™ on 2009-07-01.