Seated Body Research Articles

Initial estimation of the in vivo material properties of the seated human buttocks and thighs

On average, people are sedentary for 70% of the day, either seated or lying down. Of particular interest for this work is the seated posture; while seated the buttocks and thigh tissues experience loading for prolonged periods of time. When modeling human body interactions with seating devices and in the design of these devices, it is necessary to appropriately model the material characteristics of the soft tissue in the thigh and buttock areas. Thus, there is a pressing need for in vivo, experimentally measured data that describe the mechanical behavior of the soft tissue within the buttocks and thigh regions of humans in the seated posture. We developed an innovative experimental set-up to gather in vivo force–deflection data for both men and women in six regions located along the buttocks and thighs. Then, using anatomical measures, cadaver measures, and reported literature on regional tissue thickness, the data were converted to normal stress and strain curves. Next, these curves were described using the Mooney–Rivlin material model. Finally, best-fit mechanical parameters were reported, along with the original force–deflection data. Results indicated that, in general, the male tissue differed from female tissue, and that the mechanical properties differed between regions. Thus, when developing computer models that represent the human body, different material properties should be considered for men and women. Furthermore, appropriate properties should be used for the body region being evaluated. These data are unique in that they are quantified in vivo, represent the seated body position, and include multiple regions for both men and women. The material properties presented in this work will lead to improved numerical models, and consequently improved device design.

Effect of sitting posture and seat on biodynamic responses of internal human body simulated by finite element modeling of body-seat system

This study aimed at developing a detailed three-dimensional (3D) finite element (FE) model of the body-seat system to investigate the effect of sitting posture and seat on the biodynamic responses of internal human body under vertical white noise excitation. Three postures of human models (vertical seated body (VSB), reclined seated body (RSB), and seated body in driving posture (DSB)) and three different stiffness seats were modeled. The initial boundary condition between human body and seat for biodynamic response analysis was obtained by static analysis of the body-seat system under the action of gravity. Random response analysis was performed under white noise excitation at 0–20 Hz in the vertical direction by a modal superposition method. Principal responses of lumbar intervertebral discs (LIDs) in VSB were in the range of human body natural frequencies. The damping ratio greatly decreased the peak response values of LID. When compared with the supports of the elastic seat in VSB, those of the elastic seat in RSB increased the peak response frequencies of LID in the anterior-posterior direction and decreased them in the vertical direction, thus making them equal in both directions. Compared to RSB, DSB not only increased the peak response frequencies but also changed the peak response values. The principal response frequencies of LID in both directions increased with the increase in seat stiffness. The proposed FE model of biodynamic random response analysis is a new fundamental method for gaining insights on the responses of internal human body to vibrations and can be used to investigate the responses of the human body-seat system instead of conducting experiments to evaluate the dynamic characteristics of newly designed seats and guide the design of seats for reducing the injury risk of the lumbar spine.

시트-인체 진동 모델링을 이용한 승차감 해석

In this paper, dynamic modeling with viscoelastic properties of a human body resting on a seat is presented to quantitatively analyze ride quality of passengers exposed to vertical vibrations. In describing the motions of a seated body, a 5 degree-of-freedom multibody model from the literature is investigated. The viscoelastic characteristics of seats used in railway vehicles are mathematically formulated with nonlinear stiffness characteristics and convolution integrals representing time delay terms. Transfer functions for the floor input are investigated and it is found that these are different in accordance with the input magnitude due to nonlinear characteristics of the seat. Measured floor input at the railway vehicle is used to analyze realistic human vibration characteristics. Frequency weighted RMS acceleration values are calculated and the effects of the seat design parameters on the frequency weighted RMS acceleration values are presented.

Apparent Mass and Seat-to-head Transmissibility Responses of Seated Occupants under Single and Dual Axis Horizontal Vibration

The apparent mass and seat-to-head-transmissibility response functions of the seated human body are investigated under exposures to fore-aft (x), lateral (y), and combined fore-aft and lateral (x and y) axis whole-body vibration. The experiments were performed to study the effects of hands support, back support and vibration magnitude on the body interactions with the seat pan and the backrest, characterised in terms of fore-aft and lateral apparent masses and the vibration transmitted to the head under single and dual-axis horizontal vibration. The data were acquired with 9 subjects exposed to two different magnitudes of vibration applied along the individual x- and y- axis (0.25 and 0.4 m/s(2) rms), and along both the-axis (0.28 and 0.4 m/s(2) rms) in the 0.5 to 20 Hz frequency range, and analyzed to derive the biodynamic responses. A method was further derived to obtain total seated body apparent mass response from those measured at the backrest and the seatpan. The results revealed coupled effects of hands and back support conditions on the responses, while the vibration magnitude effect was relatively small. For a given postural condition, the biodynamic responses to dual-axis vibration could be estimated from the direct- and cross-axis responses to single-axis vibration, suggesting weakly nonlinear behaviour.

Comparisons of Seated Postures between Office Tasks

Seating is an active area of ergonomics research; however, little research has been performed that evaluates seated body postures for a variety of tasks and chairs. The goal of this study was to evaluate the effects of common office tasks on head, upper extremity, torso, hip, and leg postures. Prolonged awkward postures may contribute to musculoskeletal pain and disorders. Twenty-five participants performed four office tasks using four office chairs while postures were recorded using a 3D optical marker system. The tasks were typing on a desk fixed at 73.7cm (29“), typing with the desk adjusted to be slightly above elbow height, reclined movie watching, and a forward leaning writing task. Head angle, pelvis-to-head angle, thoracic cage angle, and pelvis angle were all significantly different between tasks ( p = <0.0001). This method for full body posture measurement will allow further research into the effects of tasks and chairs on seated posture.

Relationship between measured apparent mass and seat-to-head transmissibility responses of seated occupants exposed to vertical vibration

The “to-the-body” and “through-the-body” biodynamic response functions of the seated human body exposed to vertical vibration are measured and analyzed in an attempt to identify relationships between the apparent mass and seat-to-head transmissibility measures. The experiments involved 12 male subjects exposed to three magnitudes of whole-body vertical random vibration (0.25,0.5,1.0 m/s 2 rms acceleration) in the 0.5–15 Hz frequency range, and seated with three back support conditions (none, vertical and inclined), and two different hands positions (hands in lap and hands on the steering wheel). The vertical apparent mass and seat-to-head transmissibility responses were acquired during the experiments, where the head acceleration was measured using a light and adjustable helmet-strap mounted accelerometer. The results showed that both the measured responses show good agreements in the primary resonances, irrespective of the back support condition, while considerable differences between the normalized apparent mass and seat-to-head transmissibility could be seen in the secondary resonance range for the two back supported postures. The seat-to-head transmissibility responses are further shown to be relatively sensitive to back supported postures compared with that of apparent mass responses. Relatively stronger effects of hands position were observed on the seat-to-head transmissibility responses compared with the apparent mass responses under back supported conditions. From the results, it is further concluded that seat-to-head transmissibility emphasizes the biodynamic response in the vicinity of the secondary resonance compared to the apparent mass. The seat-to-head transmissibility measure is thus considered to be more appropriate for describing seated body response to higher frequency vibration.

Modeling of a Seated Human Body Exposed to Vertical Vibrations in Various Automotive Postures

Although much research has been devoted to constructing specific models or to measuring the response characteristics of seated subjects, investigations on a mathematical human model on a seat with a backrest to evaluate vehicular riding comfort have not yet attracted the same level of attention. For the responses of a seated body to vertical vibrations, mathematical models of the mechanisms must be at least two-dimensional in the sagittal plane. In describing the motions of a seated body, two multibody models representative of the automotive postures found in the literature were investigated, one with and the other without a backrest support. Both models were modified to suitably represent the different automotive postures with and without backrest supports, and validated by various experimental data from the published literature pertaining to the same postural conditions. On the basis of the analytical study and the experimental validation, the fourteen-degrees-of-freedom model proposed in this research was found to be best fitted to the test results; therefore, this model is recommended for studying the biodynamic responses of a seated human body exposed to vertical vibrations in various automotive postures.

The effect of macronutrients on gastric volume responses and gastric emptying in humans: a magnetic resonance imaging study

The effects of macronutrients on gastric volume changes, emptying, and gastrointestinal symptoms are incompletely understood. Three liquid meals of 500 ml (fat emulsion, 375 kcal; protein solution, 375 kcal; glucose solution, 400 kcal) were infused into the stomach of 12 healthy volunteers on three occasions. Studies were performed in seated body position using an open-configuration magnetic resonance imaging (MRI) system. MRI imaging sequences, assessing stomach and meal volumes, were performed prior to and at times t = 0, 3, 6, 9, 12, 15, 25, 35, 45, 60, 75, and 90 min after meal administration. Areas under the curve for the early emptying phase (0-15 and 0-45 min) were calculated, and characteristics of the volume curves were analyzed by a gastric emptying model. Gastrointestinal symptoms were assessed by a self-report scale. Initial (t = 0 min) and early postprandial gastric volumes were highest for glucose because of lower initial emptying. However, in the early emptying phase the characteristics of the volume curves for stomach and meal were uniform for all macronutrients. Perceptions of fullness and satiety were linearly associated with postprandial gastric volumes, but not with macronutrient composition. Isovolumic macronutrient meals modulate gastric volume response by initial meal emptying patterns. Macronutrient specific accommodation responses, as shown in barostat studies, are not reflected as gastric volume responses under noninvasive conditions.

Magnetic resonance imaging to investigate the influence of posture on gastric physiology

Magnetic resonance imaging to investigate the influence of posture on gastric physiology

Effect of elbow flexion contractures on the ability of people with C5 and C6 tetraplegia to lift.

It is commonly assumed that minor elbow flexion contractures prevent people with C5-C6 tetraplegia and paralysis of the triceps brachii muscles from bearing full body weight through their upper limbs. The aim of the present study was to determine the effect of simulated bilateral elbow flexion contractures on the ability of these individuals to bear weight through their upper limbs and to determine whether full passive elbow extension is truly critical for lifting body weight. A biomechanical study was performed. Body weight lifted was measured under conditions that simulated bilateral elbow flexion contractures. Five people with motor complete C6 tetraplegia and one person with motor complete C5 tetraplegia, all with bilateral paralysis of the triceps brachii muscles, were recruited to the study. Subjects were fitted with bilateral elbow splints that restricted elbow extension but did not restrain elbow flexion nor prevent the elbow from collapsing, and were seated on an instrumented platform that measured vertical forces under the buttocks. Subjects pushed down through their hands and lifted under five different conditions, namely: with no elbow splints; with bilateral elbow splints adjusted to restrict elbow extension by 5-10 degrees; by 15-20 degrees; by 25-30 degrees and with bilateral elbow splints adjusted to allow unrestricted movement of the elbow joint. Maximal weight lifted from under the buttocks, for each condition, was expressed in relation to weight under the buttocks during unsupported sitting (that is, 'seated body weight'). Subjects lifted progressively less weight from under their buttocks as passive elbow restriction was progressively restricted. However, one subject lifted all his seated body weight when elbow extension was restricted by 5-10 degrees and another lifted all his seated body weight when elbow extension was restricted by 5-10 degrees and 15-20 degrees. Minor elbow flexion contractures will not alone prevent people with tetraplegia and paralysis of the triceps brachii muscles from lifting. Full passive elbow extension is not critical for the performance of this task.

Weight bearing through flexed upper limbs in quadriplegics with paralyzed triceps brachii muscles.

A biomechanical analysis of lifting through flexed and extended elbows in C5 and C6 quadriplegics. To determine the mechanisms used by C5 and C6 quadriplegics to prevent elbow collapse when bearing weight through flexed upper limbs. A biomechanics laboratory. Six motor complete C5 and C6 quadriplegic subjects with paralysis of their triceps brachii muscles were recruited. A three dimensional kinematic and kinetic analysis of the upper limbs was performed whilst subjects attempted to lift their body weight through their upper limbs under four different conditions. In one condition subjects lifted with their hands placed at the same height as the seat upon which they were sitting, whilst in the other three conditions subjects lifted with their hands placed on blocks of various heights. The four different conditions required subjects to bear weight through their upper limbs with their elbows initially flexed between 15 and 40 degrees. Angular displacements and corresponding moments about the shoulder, elbow and wrist joints. In addition, EMG data were collected from the upper pectoralis, anterior deltoid and biceps brachii muscles during all lifts and expressed as a percentage of maximal isometric voluntary contractions. As block height and initial elbow flexion increased, subjects lifted progressively less weight. However, even under the high block conditions when subjects' elbows were initially flexed up to 40 degrees, subjects lifted a mean+/-SD of 43%+/-20.4 of their seated body weight with one subject lifting 76% of his seated body weight. Subjects lifted by generating shoulder and wrist flexor moments. Quadriplegics with paralyzed triceps brachii muscles can bear moderate and sometimes substantial weight through flexed elbows. This is largely achieved by the generation of shoulder flexor moments.