Effect of fluctuation in the parallel force exerted on the buttocks on discomfort during sitting in experimental ergonomic chair with tilt-in-space and reclining functions.

Background: Tilt-in-space and reclining functions are commonly used to improve comfort in ergonomic seating. However, fluctuations in shear (parallel) forces on the buttocks during postural changes may cause discomfort, yet this relationship is not well understood.

Objectives: To examine how combined use of tilt-in-space and reclining functions affects fluctuations in parallel forces on the buttocks, and how these fluctuations relate to subjective discomfort.

Study design: Cross-sectional experimental study.

Methods: Nineteen healthy adult males (mean age: 27.7 ± 10.3 years) participated. An experimental ergonomic chair was used under 2 conditions: 1 with a seat-cover assembly that conforms to the back shape, and a control condition with a flat back-support. Participants experienced a 10 degrees tilt-in-space and reclining angles from 100 degrees to 130 degrees. Fluctuations in parallel forces on the buttocks were measured using a force plate, and subjective discomfort was assessed with a visual analog scale. Stepwise multiple regression was used to analyze associations between force fluctuation and discomfort.

Results: The seat-cover assembly condition resulted in significantly lower visual analog scale scores (median: 4, interquartile range: 0-13) compared to the control (median: 46, interquartile range: 24-72; p < 0.01). Regression analysis revealed that only the fluctuation in parallel forces significantly predicted discomfort (β = 0.722, R² = 0.507, p < 0.01).

Conclusions: Fluctuations in parallel forces during tilt and recline use are significantly associated with discomfort. Ergonomic designs that reduce shear force variability may enhance seating comfort during postural adjustments.