立ち上がり動作は，日常生活で頻繁に行われる動作でありながら，十分な下肢筋力やバランス感覚が求められるため，高齢者を含む身体機能が低下した方にとって難易度の高い動作である．したがって，立ち上がり動作を補助する環境の整備が必要であるが，現行の家具設計において立ち上がり動作を考慮したものは少ない。また，既往研究では，座面の高さや角度と立ち上がり動作の関係性を調査しているものの，座面素材の性質に着目した研究は少ない．以上より，本研究では座面素材の力学的特性の違いが立ち上がり動作に及ぼす影響の解明を目指した．
座面材として3種類の軟質ポリウレタンフォーム（柔軟・低反発、柔軟・高反発、硬質・高反発）を用い、異なる素材を上下層で組み合わせた2種類の多層構造条件を加え、計5条件において比較を行った。実験は健常な学生12名を被験者とし、3次元動作解析装置および床反力計を用いて、各条件下における立ち上がり動作の計測を実施した。
膝関節および股関節にかかるモーメントの最大値をそれぞれ算出した結果、いずれにおいても柔軟・低反発の単層クッション条件が最も高い値を示した。いずれも低い値となったのは硬質・高反発の単層クッション条件で，両条件間には有意な差が認められた。
膝関節モーメントの最大値を比較した結果、低反発の座面条件において膝関節への負荷が増大することが分かった。また、上層が低反発、下層が硬質・高反発で構成された多層クッション条件においても、膝関節モーメントが高い値を示した。これらの結果から、座面上層の反発性の低さが膝関節への負荷増大に寄与する可能性が示唆された。一方、股関節モーメントの最大値に着目すると、柔軟・高反発の単層クッション条件においても高い値が観察された。よって、柔らかい座面材は立ち上がり動作中の身体重心の移動量を過剰に促進し、その結果として股関節への負荷が増加したと考えられる。
Sit-to-Stand (STS) is a movement frequently performed in daily life. Yet, it requires sufficient lower limb muscle strength and a sense of balance, making it a highly challenging movement for people with reduced physical function, including the elderly. Therefore, it is necessary to provide an environment that assists in STS. However, few current furniture designs consider STS. In addition, although previous studies have investigated the relationship between the height and angle of the seat surface and STS, few studies have focused on the properties of the seat surface material. Therefore, this study aimed to elucidate the effects of differences in the mechanical properties of seat materials on STS.
Three types of soft polyurethane foam (flexible/low resilience, flexible/high resilience, and rigid/high resilience) were used as the seat material, and two multi-layer construction conditions were added, combining different materials in the upper and lower layers, for a total of five conditions for comparison. Twelve healthy students participated in the experiment, and their STS were measured under each condition using a 3D motion capture system and two force plates.
The maximum moment applied to the knee joint and hip joint was calculated for each condition, with the flexible, low-resilience single-layer cushion condition having the highest value in each case. The lowest values were found in the rigid, high-resilience single-layer cushion condition, and there was a significant difference between the two conditions.
A comparison of the maximum knee joint moment values showed that the load on the knee joint increased in the low-resilience seating condition. The knee joint moment was also higher in the multi-layer cushion condition, where the upper layer consisted of low resilience and the lower layer consisted of hard and high resilience. These results suggest that the low resilience of the upper layer of the seat surface may contribute to the increased load on the knee joint. On the other hand, focusing on the maximum value of the hip joint moment, high values were observed even in the flexible and highly resilient single-layer cushion condition. Therefore, it is considered that the soft seat material promoted excessive body center of gravity shift during STS, resulting in an increased load on the hip joint.
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