Effect of seat cushion resilience and hardness on lower-limb loading during sit-to-stand

Seat cushion materials affect the mechanical demands of sit-to-stand (STS) movements; however, the effects of specific material properties, such as resilience and hardness, remain unclear. Understanding how these factors influence lower-limb joint moments and movement strategies during STS may contribute to the development of seat designs that assist individuals with reduced lower-limb strength. Therefore, this study aimed to clarify the fundamental mechanical effects of seat cushion resilience and hardness on STS.

Fifteen healthy young adults performed STS from five polyurethane foam cushions that differed in resilience (14-55%) and 40% compression hardness (66-336 N). The material ranges were determined with reference to technical documents and a published patent specification to ensure they were within the range commonly used in everyday seating products. Kinematic and kinetic data were collected using a motion capture system and two force plates. Net joint moments were calculated via inverse dynamics, and differences among seat conditions were analyzed using repeated-measures ANOVA or the Friedman test, with Bonferroni-adjusted pairwise comparisons (α = 0.05).

Seat resilience significantly affected the peak hip and knee extensor moments (p < 0.01, η² = 0.35-0.48). High-resilience cushions delayed seat-off timing and maintained greater seat reaction force at the timings of peak hip and knee extensor moments. In contrast, seat hardness mainly influenced horizontal center-of-mass (COM) velocity and hip joint moment (p < 0.01, η² = 0.31-0.37), with softer seats producing larger values.

High-resilience cushions delayed seat-off and maintained buttock support for a longer duration, thereby reducing the peak hip and knee extensor moments. In contrast, softer seats promoted a strategy involving greater horizontal momentum generation by the upper body, which consequently required an increased hip extensor moment to decelerate this momentum. Cushions with a resilience of ≥ 53% and 40% compression hardness of ≥ 180 N effectively reduced lower-limb joint loading. The results of this study provide fundamental insights that may contribute to future research on chair design and cushion selection in clinical and caregiving environments.

Not applicable.