A dynamic backrest may help accommodate trunk movement, and multi-device work elicits more of it: An exploratory comparison of high-end office chairs under single- and multi-device tasks

Dynamic-sitting office chairs are usually compared with static sitting rather than with each other, and the contribution of backrest deformability (lateral backrest movement) is unknown. Office work also increasingly spans multiple devices. Twenty healthy adults (10 men, 10 women) completed a 15-minute single-device and a 15-minute multi-device proofreading task on each of three chairs: two commercial dynamic-sitting chairs (T90, Aeron) and a T90 whose backrest lateral movement was mechanically fixed (T90-fixed). We recorded subjective ratings, head and trunk posture (inertial measurement units), upper trapezius and lateral gastrocnemius activity (electromyography), and task performance. Chair effects were largely null: T90 and Aeron did not differ in ratings or performance, posture and muscle activity showed only isolated, direction-inconsistent differences, and preferences were polarized. The standard T90 showed greater trunk movement (mean absolute angular velocity in yaw) than T90-fixed during the multi-device task (p = 0.042), which may indicate that its movable backrest helps accommodate trunk movement under demanding work; no other whole-sample posture, muscle, rating or performance measure differed between these two configurations, and replication is needed. Task effects were large and consistent: the multi-device task elicited greater and faster head, trunk and shoulder movement and higher upper trapezius activity than the single-device task, with lower reading throughput. Multi-device work thus imposes substantially greater movement demands than single-device work, which supports designing chairs that accommodate movement in modern work environments.