Factors affecting seat contour characteristics

Effects of elastic seats on seated body apparent mass responses to vertical whole body vibration

Apparent mass (AM) responses of the body seated with and without a back support on three different elastic seats (flat and contoured polyurethane foam (PUF) and air cushion) and a rigid seat were measured under three levels of vertical vibration (overall rms acceleration: 0.25, 0.50 and 0.75 m/s(2)) in the 0.5 to 20 Hz range. A pressure-sensing system was used to capture biodynamic force at the occupant-seat interface. The results revealed strong effects of visco-elastic and vibration transmissibility characteristics of seats on AM. The response magnitudes with the relatively stiff air seat were generally higher than those with the PUF seats except at low frequencies. The peak magnitude decreased when sitting condition was changed from no back support to a vertical support; the reduction however was more pronounced with the air seat. Further, a relatively higher frequency shift was evident with soft seat compared with stiff elastic seat with increasing excitation.

PRACTITIONER SUMMARY

The effects of visco-elastic properties of the body-seat interface on the apparent mass responses of the seated body are measured under vertical vibration. The results show considerable effects of the coupling stiffness on the seated body apparent mass, apart from those of excitation magnitude and back support.

A Comparison of the Average Sitting Pressures and Symmetry Indexes between Air-adjustable and Foam Cushions

[Purpose] The purpose of this study was to determine the best adjustable cushions, for the maintenance of normal sitting balance by healthy persons in wheelchairs, from a foam cushion, and a newly-developed air-adjustable cushion. [Subjects] Eighteen healthy participants (9 men, 9 women) were recruited. [Methods] Participants were evaluated using the Force Sensing Array System to assess average sitting pressure and the symmetry index of chair sitting under the following conditions: no seat cushion, a foam cushion, and a newly-developed air-adjustable cushion. [Results] The results show that there were no significant differences among the average sitting pressures which were measured on the hard surface of a chair, a foam cushion, and the newly-developed cushion. The air-adjustable cushion's symmetry index turned out to be closer to 0 than those of the foam cushion and the hard surface of the chair. [Conclusion] We suggest that the newly-developed air-adjustable cushion contributes to a more symmetrical sitting posture than the basic foam cushion or no seat cushion.

THE INTEGRATION OF 3-D HIP-SHAPE ANTHROPOMETRIC DATA BANK AND HUMAN-ENGINEERED SEAT-CUSHION: THE CLASSIFICATION OF HIP SHAPES AND CATEGORY OF SEAT CUSHIONS

Custom fitted seat cushions work to improve posture, alleviate fatigue and prevent bedsores by evenly distributing body mass and relieving single or multiple points of pressure. Existing commercially-available bedding and seating is unsatisfactory to patients and professionals alike. Three-dimensional anthropometric measurement technology is employed to establish a database of hip-form profiles. This database includes 400 individuals grouped into five categories based on the shapes and measurements of 3-D hip-forms analogous to women's brassiere cup sizes. Each class is matched within a range of a anthropomtric values such as height, weight, hip-circumference and thigh-diameter. With such human factor engineering considerations, the preliminary 3-D hip-form database has created practical distinctions between the five hip-form classes that could help to design patient specific seating that is both comfortable and healthy.

Seated buttock contours: a pilot study of Australian senior high-school students

Both posture and comfort of a chair are influenced by the contour and characteristics of the seat. Knowledge of seat contours of a student population could thus be useful in the design of school chairs. This study investigated seated buttock contours of senior high-school students in order to determine: (a) their general characteristics, (b) the effect of gender and sitting posture and (c) the relationship between the contours and selected anthropometric variables (stature and mass). A contour measurement device was developed and used to measure buttock contours in five sitting postures (typing, sitting up, sitting back, slumping and writing). Buttock contours were quantified by constructing anterior-posterior (AP) and lateral profiles from which six discrete profile dimension measurements were made. AP and lateral profiles were found to have a consistent shape across all participants. Five out of six profile dimensions were significantly different between genders, with just one significantly different between sitting postures (typing and sitting back). Correlations between anthropometric measures and profile dimensions were relatively low (r < 0.34) with no clear patterns evident. Overall results of this study suggest that buttock contours are influenced by gender to a greater extent than sitting posture.

Reliability of the ISO Wheelchair Cushion Test for Loaded Contour Depth

Standardized test methods that report cushion characteristics and performance can simplify cushion selection by helping clinicians and users identify types of cushions that may meet a user's needs. The loaded contour depth (LCD) test is one of the test methods included in the International Organization for Standardization (ISO) standards for wheelchair cushions. LCD measures the depth of immersion into a cushion. The objective was to determine the reliability of LCD and overload deflection measurements. LCD and overload deflection tests were repeated on 17 cushions. Intraclass correlation coefficient (ICC) and repeatability coefficient (RC) were calculated. The reliability of LCD (ICC = 0.98; RC = 0.21) and overload depth (RC = 0.15) were very high. LCD measures can reliably be used to differentiate cushions that vary by at least 1 cm. The overload test can be used as a criterion-referenced measure, but it is not sensitive to variation.

Buttock and back pressure distribution tests on seats of mobile agricultural machinery

Multidisciplinary effort is usually required in the evaluation of comfort problems in the working environment of mobile agricultural machinery workers. Comfort can be partly assessed from the study of the pressure distribution of the human-seat interface. Four combine foam seats and a new air-based seat were compared with regards to static buttocks and back support pressures. Within and between subject variability and the issue of measurement repeatability were addressed. Significant differences were found in the maximum pressure profiles of the four seating systems with seats 2 and 3 performing better than seat I (seat 4 showed no significant difference when compared to the other seats). There is an almost linear relationship between mean pressure and body mass index. In all cases the air seating system performed better with regards to the static pressure gradients.

Determination of generic body-seat interface shapes by cluster analysis

The purpose of this study was to determine typical or generic shape patterns of the buttock-seat interface for elderly wheelchair users. The group of subjects was composed of 30 elderly people (aged 65 or older) and the shapes of the body-seat interface were measured by the electronic shape sensor (ESS). By analyzing the dissimilarity in geometrical shape descriptors or parameters, four distinct generic shapes were identified by means of the cluster analysis method. The results suggest that the generic shapes were mainly characterized by the lateral symmetry of the shapes. The determination of elderly people's seat interface shapes into distinct clusters may lead to a more comprehensive understanding of the seat support interface and more effective seat cushion designs.

Seat cushion design for elderly wheelchair users based on minimization of soft tissue deformation using stiffness and pressure measurements

A method for designing seat support surfaces using interface pressure and soft tissue stiffness criteria was evaluated. An algorithm designed to drive a rigid support surface on a programmable seating system to a shape for which the externally applied pressure is inversely related to the measured stiffness of adjacent soft tissue was evaluated on 30 elderly subjects (age 65 years or older). The resulting support surface shapes were transferred to compliant foam cushions and evaluated using interface pressure measurements. Pressure and stiffness measurements on the seating system indicated the surface shape control algorithm met the desired programmed criteria by achieving an inverse relationship between pressure and stiffness, as it converged to an "optimal" support surface shape. Evaluation of interface pressures on the compliant foam cushions showed that the pressure distributions on the cushions contoured to the optimal surface shapes were more uniform and had lower values than distributions on flat foam cushions and foam cushions contoured to shapes measured using state-of-the-art load-deflection devices. The results suggest that support surfaces designed using tissue stiffness as a criteria can provide loading conditions intended to minimize relative deformation and, thus, stress in load-bearing soft tissue.

A system for the analysis of seat support surfaces using surface shape control and simultaneous measurement of applied pressures

A system for the design and analysis of seat support and buttock tissue interfaces has been developed. It has the ability to control the seating surface shape while measuring the pressure applied to the buttocks by the surface. Pressures are measured over an 11 x 12 rectangular array of support elements using silicon pressure sensors mounted in a swiveling head atop each support element. Control of surface shape is mediated by selective linear translation of the support elements along their respective vertical axes. Closed-loop control of the system allows for the dynamic formulation of a support surface on the basis of programmable criteria. The system is intended to function as a research tool to facilitate the study of the relationships between support surface shape and interface pressure, and support surface shape and soft tissue distortion. The purpose of this paper is to present the system instrumentation and the rationale behind its design and development. The paper also presents the results of several tests to evaluate the accuracy and performance of the system. This evaluation included a pilot study on 10 able-bodied subjects. The results of these system evaluations indicate that the system is capable of making repeatable and precise measurements of pressure and surface element position and can formulate support surface shapes that satisfy specified optimization criteria.

Using seat contour measurements during seating evaluations of individuals with SCI

Measuring the shape of the buttock-cushion interface has been used successfully in research to study tissue loading and as a means to fabricate custom contoured cushions. Seat contours are also able to provide useful clinical information on the weight-bearing surface of the cushion, which can be used to address posture. This article offers specific case studies that demonstrate how the analysis of seat contours can be used to identify pelvic tilt, pelvic obliquity, and areas of high loading. Seat contour measurements complement other clinical measures, such as seat interface pressures and general postural assessments, to form a more complete picture of the buttock-cushion interface. They have become useful in the clinical management of various pressure and posture problems experienced by individuals with spinal cord injury and other wheelchair users.