Document NGak3kzg9zj45VLngMRbe4Y4g

80 ROSS A. MC FARLAND this would not be feasible unless there were no disadvantages entailed in sliding the buttocks forward in the seat. It should be noted that Garner,27'28 in two studies of industrial seating, believes that a depth of 11 inches will be ample. Seat height. The height of the seat above the floor is most important to the comfort and efficiency of the worker. This is particularly true with excessive heights, which will cause pressures of a more or less injurious nature on the muscles and nerves of the under part of the thigh. Consequently, the seat should be sufficiently low to insure that the thighs can be kept free or almost free of the seat. This does not mean that a person may not find it comfortable to rest his thighs. on the seat at times, but in general this is not a suitable position for extended periods. The pertinent body measurement in this case is taken from the floor to the tendons of the thigh in back of the knee (popliteal area) when the subject is in the sitting position.29 In the general population, this body dimension (including shoe thickness) measures 16.9 inches for the 5th percentile or shortest group (men 17.6, women 16.7) ,12 It is obvious that the maximum seat height should not exceed these figures; 1 inch less would be more satisfactory. If the legs must be extended for ward for the operation of foot pedals, lower leg. height is reduced, and it may be necessary to reduce further the height of the seat. Under most ciroumstances, provisions for the vertical adjustment of industrial seats will be both desirable and feasible. For many groups a 6-inch adjustability will be adequate, but since the requirements might vary considerably depending both on the body size range of ' tfieworking population ahTTbirt'Ke~type of worlTana movements requiredT^ wout be best to determine this range experimentally for individual situations. Shape of seat. The shaped saddle-type seat with an elevation between the thighs generally has been found unsatisfactory since it requires the buttocks to be. ( correctly fitted to the corresponding concavities of the seat. In theory, this would ; result in pressure being evenly distributed over the buttocks, a disadvantageous feature since both anatomical and physiological considerations make it desirable that the ischial tuberosities (the parts of the hip bones in contact with the""seat) bear the larger , part of the body weight. Edwards and Duntley have shown that the blood supply to the skin over the tuberosities is peculiarly adapted to weight bearing.30 The curvature of the tuberpsities and their surface irregularity make , I't^:i5fficul:t^pp^Mi3efinfte:rmeasufeIitTaltlI6'figlfrifi^n^irtK^TwFsTigfi61y|T ovfS'fei&^fel^SlfetiBdsiaibout I*/*-to 2.inehieanhsle^gtM?/9;6fi^i|i^^Ke;'di8to^ between'the*.tu'6,eirtisities is approximately 4V inches in men and 5 inches in women, there'is'-'eriough variation in the size and shape of the buttocks so that a given-'shap'ed=ffeBt,iwldu!ld-beiiuncom'fortable-fora-large'-percentagfriof-!any-popul'a'! tion. A generally flat surface would be superior. Since the tuberosities are so small,;( there is little use in having a very soft and resilient seat, for a large amount of. * B. Akerblom, Standing and Sitting Posture. A. B. Nordiska Bokhandeln, Stockholm, ,1948 "A. E. Edwards and S. Q. Duntley, The pigments and color of living human skin, Am. J. Anat.t 65, 1-34 (July, 1939). HUMAN ENGINEERING AND INDUSTRIAL SAFETY 81 ** would be transferred to tissues that are not adapted for bearing it. The ^Swlif'S^^flf CODsistency, therefore, is one that gives only slightly under pressure.29 \cjf<prest. Two types of back rest may be used in industrial seating. One j|||cjnsists of a large flat or curved surface that can support the entire area Awl>k The back rest found in motor vehicles would be one such example, only that they be about 18 to 20 inches high and about 20 inches wide, of this type will be able to accommodate almost everyone in regard to ^^^^^gwnent body dimensions.18 ^V^The'second, and probably more common, variety consists only of a support J^^j^Jc^bar region of the back, the area subject to the greatest strain. The t'S^rym.^'s. or-stenographer's chair offers an example of this type of backrest. In such 18 desirable to leave a space below the lumbar support so that the butl^Pks^can^be shifted to the rear of the seat, thus giving a slight arch to the small ^^Mkck. Increasing or decreasing the arch in this manner creates further Jp^^Snrties for the change of position important in maintaining comfort and in . onset of fatigue. ^v^Ph^ertinent body dimensions in the construction of chairs with a lumbar umK^^^j'fhe vertical distance from the seat to the top of the sacrum, at the ri?o=ition|bf the first lumbar vertebra. Fortunately, individual variation in this ^Hjdvi||feen8ion is slight, and the position of the lower edge of the lumbar support ^j^hQflvdequate if it is located no more than 7 to 7V2 inches above the level of nfic,'jppttt'3,J~It--should"not'be^so~high"as'to come into contact with the shoulder hhid^Sn addition, the back rest should be curved to correspond as accurately as the natural curvature of the lumbar region in the horizontal plane. On ^^|||Bopulation, this curvature was found to approximate the arc of a circle r-inrh radius.82 If the back support is vertically adjustable, adequate iffpr'muiiqgjcan be made for virtually the entire population. seat an* rest- the slopes of threat and back rest are ^jA^ily related to anthropometric measurements^brief mention of them &^i;rmade in any discussion of seat design. If the back rest is vertical it ^n^y^i|fe;|upport to the small of the back, but it will be unable to support directly of the body. When this is desirable, it;,is.necessary.;itpi,slope the I^orT"wgigh`t..is"~8I^Te'dTh^'"tKS,^CKTr^`^it8 slope j. I'1 this way the supporting organs and muscles of;'.the body will be their burden. Although the degree of inclination, if any, must "tvliiTfKSwjjersonal preference and job requirements, a slope lir'the ..vicinity of ^th'e=horizontaHs"Often-found satisfactory;'Under'SUch'-c6nditions;-if the brjfcbntal, pressure on the back rest will give rise to an opposing force that ggush.the buttocks forward on the seat. For this reason,i.the seat must mm* . JSAkt erblom, "Chairs and Sitting." in W. F. Floyd and A. T. Welford, eds., Human Design. H. K. Lewis, London, 1954, Ch. 5. - T.M. Weddell and H. D. Darcus, Some anatomical problems in naval warfare, Brit. 77-83 (April, 1947). :