BODY PRESSURE DISTRIBUTION OF AUTOMOBILE DRIVING HUMAN MACHINE CONTACT INTERFACE.doc

body pressure distribution of automobile driving human machine contact interface*abstract: aiming at the fatigue and comfort issues of human-machine contact interface in automobile driving and based on physiological and anatomical principle, the physiological and biochemical process of muscles and nerves in the formation and development of fatigue is analyzed systematically. the fatigue-causing physiological characteristic indexes are mapped to biomechanical indexes like muscle stress-strain, the compression deformation of blood vessels and nerves etc. from the perspective of formation mechanism. the geometrical model of skeleton and parenchyma is established by applying ct-scanned body data and mri images. the general rule of cor/fort body pressure distribution is acquired through the analysis of anatomical structure of t-jitocfci and femoral region. the comprehensive test platform for sitting comfort of 3d adjuitabie intact interface is constructed. the test of body pressure distribution of hunian-/jachme contact interftcc sd its comparison with subjective evaluation indicates thoi ue biomechaiiical indexes r,f zufumobile driving human-machine contact interface anil body pressure distribution rule studied can effectively evaluate the fatigue and comfort issura of human-n;a6!ii!vs contact interface and provide theoretical basis for the optimal design of huii.n-i.tichbiv- contact interface. key words: biomechanics contact interface fatigue body pressure distribution0 introductionautomobile driving fatigue has always been an important subject in ergonomic studies, and human-automobile contact interface is one of the important factors that affect automobile driving fatigue. based on biomechanical principles and combined with the experiment on body pressure distribution of human-machine contact interface, this paper investigates the body pressure distribution of automobile driving human-machine contact interface.akerbloms m research (1948) shows that anthropometry-related ergonomic standard has been taken into consideration as a key factor of sitting comfort design. in 1965, drury, et al, published trunk drivers preference of driving and controlling positions, which showed a wide distribution. in 1978, jacob-son, et al(3, investigated mathematical models to predict mans comfort response in different automobiles and environments. however, this model couldnt be applied to the nonexistent ones. in 1988, matsuoka, et a!w, the existing automobiles and was more likely to be used carried out the research on passengers seat comfort by investigating pressure distribution and put forward the position of passengers comfort posture. in early 1990s, some scholars such as reed, et al5, evaluated the sitting comfort through measurement of the testees contact pressure or electro-myography. zhanq et al16, proposed the hypothetic model for sitting comfort and discomfort. in 1999, denis, et alm, started with friction and pressure to discuss the influence of seat surface rim on the support and comfort in seat design and to stress the consideration of the adjustability, intent, design constraints and variation of human body measurements in the design of seat surface rim support. in 2000, mehta, et al1*1, put forward a basic model for comfort seat design based on biomechanics.researches above show that all the current seat designs meet the ergonomic guidelines that mostly are traditional anthropom-etrical standards and physiological standards obtained through traditional electromyography. however, researches on body pressure distribution rule for direct evaluation of human body comfortselected from proceedings of the 7th international conference on frontiers of design and manufacturing (1cfdm 2006). this project is supported by national natural science foundation of china (no. 50475090) and program for new century excellent talents in university, china (040927). received july 14, 2006; received in revised form march 21, 2007; accepted march 30, 2007 under sitting postures are not very mature. this paper analyzes the mechanical features of comfort and.fatigue issues under driving postures from biomechanical point of view and puts forward the body pressure distribution rule for comfort evaluation to provide theoretical basis for ergonomic design of automobile cab.1 physiological mechanism of driving comfortfatigue is a very complicated physiological state, mostly displaying the exhaustion of body and mind, fatigue and decline of work capacity and concentration. if the automobile driving contact interface is not properly designed, drivers will easily get uncomfortable and tired, which may physiologically cause loading of lumbar vertebrae, limb soreness and uncomfortableness, or even affects the drivers driving controllability.1.1 mechanical mechanism of m usele fatiguemuscle fatigue is the decrease of motor ability caused by human muscle maintaining a certain state fof long or contracting and stretching continually. excitement of cell membrane leads to a series of biochemical reactions, which can release energy to change the positions of thick myofilaments and thin myofilaments in muscle fibers, and further cause muscle constraint. the direct energy source for muscle constraint is atr in chemical reactions, it is converted from high-energy state to low-energy state with energy released. meanwhile, the decomposed low-energy phosphate compound is continually being converted into atp through synthesis. the energy storage components of human body are sugar, fat and protein whose degradation metabolization depends on oxygen supply. when there is enough oxygen supply, the energy storage components can be decomposed thoroughly and produce water and carbon dioxide, releasing large amount of energy to recompose atr otherwise, adp is mainly composed by the energy released in anaerobic glycolysis of sugar and plentiful lactic acid will be produced at the same time. too much accumulation of lactic acid will cause the decline of muscle cell ph value, thus the contracting characteristics of muscle fiber will be affected and muscle will exhibit ponderosity, soreness and inability, resulting in fatigue.when human body is under driving postures, the buttocks and legs have a long-term contact with seat surface. when muscle in local area bears long term pressure or is under intense static contraction, blood flow reduces because of the contraction of internal musculature pressure within blood vessels, the amount of oxygen and blood sugar carried by blood reduces correspondingly. as a result, normal aerobic degradation cannot be carried out and anaerobic degradation becomes the major reaction instead. as the wastes produced in metastasis (especially lactic acid) cannot be discharged through blood in time, the deposit will lead to muscle soreness and cause fatigue. as fig. 1 shown, to ease or delay muscle fatigue, certain condition should be created to maintain or even increase the effective flow of blood fed to muscle. therefore, stress deformation of muscle and blood vessel is defined as the biomechanical index for muscle fatigue.1.2 mechanical mechanism of nerve fatiguenerve fatigue refers to headache, distraction and obtuse response. it can result from the change of central nervous neuro-transmitters, and it can also be a normal physiological reaction towards peripheral fatigue.neurotransmitters, neuromodulators like ammonia and enzymatic reactions are the biochemical basis for nerve center fatigue. besides, there are direct or indirect relations among them, which complicate the mechanism of central nervous fatigue. amino acid neurotransmitters in cerebral tissues are the majority of neurotransmitters in central nerve endings, which are categorized into two types: the excitatory ones and the inhibitory ones. with the catalysis of enzyme, excitatory glutamate (glu) can be converted into inhibitory gamma-aminobutyric acid (gaba). in normal physiological states, their metabolisms are in the balance. once the balance is broken, the content of gaba increases and the cerebral cortex inhibitory process becomes dominant, resulting in fatigue in the central nervous system. 5-hydroxytryptamine (5-ht) is the inhibitory central neurotransmitter in cerebral tissues and its precursor is tryptophan which exists in blood in combinative or dissociative forms. when skeletal muscle contracts, dissociative tryptophan in blood plasma increases correspondingly, which facilitates tryptophans entry to the cerebrum, accelerates s-ht composition, increases its density, strengthens its inhibition towards cerebral cortex and leads to the central nervous fatigue. ammonia, which is poisonous to cerebrum, is released into blood .by muscle in movements. it can directly and indirectly affect the cerebral nervous system to change the function of central nervous system and result in nerve fatigue.when human body keeps the driving postures continuously, especially when local pressure is formed because of the uncomfortable seat, the repetitive, rapid pessimal stimulus is constantly transmitted as signal into the cerebral cortex and thus causes the excitement of cortex cells. when local or most cerebral cortex cells work for excessive long period or the consumption of synapse ending neurotransmitters reaches a certain limit, the inhibitory neurotransmitters will be activated in cerebrum and thus causes nerve fatigue. motor nerve endings within the muscles can transmit the impulse from nerve central system to the skeletal muscles, causeing the muscle contraction and adjusting the muscle tensity to maintain different body postures. when skeletal muscle contracts, there will be changes of internal environment, such as the increase of carbon dioxide, the accumulation of metabolic product like phosphonic acid, lactic acid etc., and the lack of oxygen, which can work on the synapse to change its excitability and affect synapse transmission. meanwhile, the density of chemicals that can activate the inhibitory neurotransmitters rises, which causes the cerebral dysfunction. consequently, protective inhibition happens in the cerebral cortex, resulting in the occurrence of fatigue. as fig. 2 shows, to release or delay nerve fatigue, the pressure distribution of local nerves is defined as the biomechanical index for the research on nerve fatigue.2 body pressure distribution rule based on biomechanics2.1human body 3d modelinga complete understanding of stress distribution of parenchyma in buttock and posterior femoral region under sitting postures can provide us with very useful information. therefore, whether the analysis of geometrical model of different tissues in buttock and thigh can be carried out from anatomical point of view becomes crucial for the biomechanical research of buttock and femoral region.the general processing procedures for human body 3d modeling based on ct and mri images are: first, put the collected cross-sectional images in sequence number order and pre-process them. through contour tracking and coordinate transform the vector cross-sectional contour sequence is obtained. after topology reconstruction and cross-sectional contour interpolation, set up the tri patch connection among the corresponding contours of adjacent layers based on the interpolative points and accomplish the triangulated surface reconstruction of the cross-sectional contour sequence. complete the modeling of different biological tissues (skeleton, muscle, skin and blood vessel) and create different types of visualized displays through data reduction of angle-chord deviation criterion and coordinates unification of 3 points location, as shown in fig. 3 and fig. 4.2.2body pressure distribution rulebody pressure distribution is a key factor that affects sitting comfort. proper body pressure distribution can help to relax muscles over the whole body and ensure the normal blood circulation and nerve transmission. at the early stage of the research, the focus is on the geometrical model of tissues in buttock and femoral region. the general rule is obtained through the description of anatomical structure of tissues in skeleton, muscle and blood vessel.when a man is seated, most (about 80%) of the body weight is pressed upon the seat surface through buttock, hunch part of the back and the attached muscle. the supporting framework of buttock and femoral region is formed through the connection of pelvis structure (including sacrum, coccyx, left and right hipbones) and thighbones of both sides. hipbone lies at both undersides of the trunk. the bone substance at its back, which is called ischium node, is rough and fleshy (fig. 4). the ischium of human body is strong and can endure greater pressure than its surrounding muscles. therefore, the weight of upper body carried by vertebral column is distributed on the surface of pelvis through buttock muscles and transferred outwards through the contact surface of buttock-seat. ischium node bears most of the upper body weight. when the seat surface under the ischium node cannot keep approximately horizontal, the thighbone beside the two ischium nodes turns upwards, and further affects the stress distribution of the whole pelvis, makes the stress distribution in local area of pelvis unbalanced and causes the abnormal pressure on sciatic muscles, which may easily leads to discomfort.muscles in buttock and posterior femoral region may be pressed because of the hardness and shape of the seat surface. after muscles get pressed and deformed, nutrition of the circulatory system cannot be imported and the metabolic product cannot be discharged in time, which may cause muscle fatigue and pain. buttock muscle region, which is right under the ischium, is divided into three layers. the one that receives the greatest influence of contact surface shape is gluteus maximus muscle on the shallow layer, which almost takes up the whole buttock subcutis and forms the gibbous shape of buttock together with the buttock subcutaneous fat tissue. the part through which thigh contacts with seat surface is the posterior femoral muscle group, including biceps femoris muscle, semitendinosus muscle and semimem-branosus muscle. they all start with the ischium node and cross down to the back of coax and knee joint. gluteus maximus muscle is a flat muscle in the shape of an irregular square. being relatively thick, it is less sensitive to pressure and can bear more pressure than the posterior femoral muscle group. under standing posture, gluteus maximus muscle covers the ischium node, but when one is sitting, the ischium node is uncovered and the gluteus maximus muscle skims over the side faces, which indicates that muscle fiber cannot stand long term pressure.when muscle is under pressure, so are the blood vessels and nerves in the muscle. as there are large numbers of blood vessels and nerves in buttock and femoral region, if pressure is not prop-erly distributed, blood circulation and nerve conduction will be affected and man will feel uncomfortable. the blood supply of gluteus maximus muscle mainly comes from cutaneous branch of superior gluteal artery, inferior gluteal artery in the second place. the branches of superior gluteal artery cutaneous branch are distributed in gluteus maximus muscle, some of them even cross the gluteus maximus muscle and reach the surface skin. the ending branches of inferior gluteal artery lie in the backside of gluteus maximus muscle and the foreside of biceps femoris muscle. regarding the geometrical structure of sitting posture, the backside of the gluteus maximus muscle and the foreside of biceps femoris muscle are close to the ischium node, therefore, they can bear relatively heavy pressure. however, as the branch inosculation of superior gluteal artery, inferior gluteal artery and lateral sacral artery lies in the medial gluteus maximus muscle, it cannot bear too much pressure. although there is no main artery in the posterior femoral muscle group, the branch inosculation of internal lilac artery, femoral artery and popliteal arteiy is formed in the posterior femcral region as fig 5 she1 vs. besides, instead of crossing along the thiibone, the femcral main anery slopes towards the iowtt surtax of tbe thigh until reaches the popliteal space and than crosses the close body surface, the bearing capacity displays the descending tendency from ischium node to the popliteal space. nerve dominating the gluteus maximus muscle is inferior gluteal nerve, the concomitant of inferior gluteal artery, whereas the dominating nerve of the posterior femoral muscle group comes from the branch of sciatic nerve. as the trend of dominating nerves in buttock and femoral regions is consistent with that of blood vessels, its influence upon the body pressure distribution is approximately the same with that of blood vessels.therefore, pressure on the seat cushion should be designed in line with the principle that pressure varies with different parts of the buttock. most of the human body weight should be properly distributed on the seat cushion with relatively large bearing area and small unit pressure. besides, the transition of pressure distribution from low to high should be smooth and abrupt change should be avoided. therefore, it is generally considered that comfortable body pressure distribution should follow the rules below: cd pressure reaches the climax around the ischium andgenerally decreases outwards. (d pressure in the posterior femoral region descends gradually and reaches its nadir at the lower surface of the thigh which contacts with the front edge of the seat cushion. popliteal space cannot bear the pressure. medial gluteus maximus muscle should not bear too much pressure. no obvious abnormal peak value, namely no peak value pressure which causes foreign body sensat