Biomechanical effects of sitting with adjustable ischial and lumbar support on occupational low back pain: evaluation of sitting load and back muscle activity

Mohsen Makhsous, Fang Lin, James Bankard, Ronald W Hendrix, Matthew Hepler, Joel Press, Mohsen Makhsous, Fang Lin, James Bankard, Ronald W Hendrix, Matthew Hepler, Joel Press

Abstract

Background: Compared to standing posture, sitting decreases lumbar lordosis, increases low back muscle activity, disc pressure, and pressure on the ischium, which are associated with occupational LBP. A sitting device that reduces spinal load and low back muscle activities may help increase sitting comfort and reduce LBP risk. The objective of this study is to investigate the biomechanical effect of sitting with a reduced ischial support and an enhanced lumbar support (Off-Loading) on load, interface pressure and muscle activities.

Methods: A laboratory test in low back pain (LBP) and asymptomatic subjects was designed to test the biomechanical effect of using the Off-Loading sitting posture. The load and interface pressure on seat and the backrest, and back muscle activities associated with usual and this Off-Loading posture were recorded and compared between the two postures.

Results: Compared with Normal (sitting upright with full support of the seat and flat backrest) posture, sitting in Off-Loading posture significantly shifted the center of the force and the peak pressure on the seat anteriorly towards the thighs. It also significantly decreased the contact area on the seat and increased that on the backrest. It decreased the lumbar muscle activities significantly. These effects are similar in individuals with and without LBP.

Conclusion: Sitting with reduced ischial support and enhanced lumbar support resulted in reduced sitting load on the lumbar spine and reduced the lumbar muscular activity, which may potentially reduce sitting-related LBP.

Figures

Figure 1
Figure 1
Experimental setup for laboratory test. The picture shows the actual experimental chair with Xsensor pressure mats on. It is shown in the Off-Loading configuration. The inset shows how the subject fit in the chair in an Off-Loading configuration, and the adjusting angle of the BPS.
Figure 2
Figure 2
Representative data of interface pressure from a subject. Upper row: interface pressure between the backrest and the subject. Lower row: interface pressure between the seat cushion and the subject. Left column: recording in Normal posture. Right column: Recording in the Off-Loading posture. Recordings were done in the same trial.
Figure 3
Figure 3
Average changes (in percentage, Mean ± SE) of parameters of seat interface pressure induced by sitting posture change from Normal to Off-Loading for both Asymptomatic and LBP subjects. A. Total contact area (TCA); B. Average pressure (AP); C. Peak pressure (PP).
Figure 4
Figure 4
Average changes (in percentage Mean ± SE) of parameters of backrest interface pressure (total contact area "TCA", average pressure "AP", peak pressure "PP", vertical location of PP "PPZ" and "horizontal location of PP "PPY") induced by sitting posture change from Normal to Off-Loading posture for both Asymptomatic and LBP subjects.
Figure 5
Figure 5
Average changes (in percentage, Mean ± SE) of surface EMG from the back muscles induced by sitting posture change from Normal to Off-Loading posture for both Asymptomatic and LBP subjects. A. Left side; B. Right side.

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