The design of seating can be traced at least to antiquity. The stool, for example, had already been developed into a valued article of furniture by the Egyptians as far back as 2050 B.C. and the chair as far back as 1600 B.C.1 Despite its ubiquity and long history, however, seating is still one of the most poorly designed elements of interior space. Industrial designer Neils Diffrient has said that “Chair design is the acid test for designers.”2 One of the major difficulties in the design of seating is that sitting is, quite frequently, viewed as a static activity while, in actuality, it is a rather dynamic one. Accordingly, the application of static two-dimensional data, alone, to solve a dynamic three-dimensional problem, involving biomechanical considerations, is not a valid design approach. Paradoxically, a chair that is anthropometrically correct may not necessarily be comfortable as well. If the design, however, is simply not responsive at all to human dimensions and body size, there is no question that the seating will, in fact, be uncomfortable.
Another difficulty is that very little data are available with respect to the biomechanics of chair design and practically no research has been published with respect to “comfort.” All that can be provided in this section, as well as in Part C, are some broad guidelines, basic concepts, and recommendations.
The dynamics of sitting can be more clearly illustrated by studying the mechanics of the support system and the general bone structure involved. According to Tichauer, “The axis of support of the seated torso is a line in a coronal plane passing through the projection of the lowest point of the ischial tuberosities on the surface of the seat.”3 Figures 4-1 and 4-2 show the tuberosities. Branton makes two observations in this regard. The first is that, when sitting, about 75 percent of the total body weight is supported on only 4 square inches (sq in), or 26 sq cm, of these tuberosities.4 This constitutes an exceptionally heavy load, distributed over a relatively small area, and as a result, very high compressive stresses are exerted on the areas of the buttocks beneath. Tichauer indicated that these stresses have been estimated at 85 to 100 pounds per square inch (psi).5 Other data have shown the compression pressures on the areas of the skin between the buttocks and a hard seat pan as high as 40 to 60 psi and the pressure a few inches away as only about 4 psi.6 These pressures cause fatigue and discomfort and result in a change in the sitter’s posture in an attempt to alleviate the condition. Prolonged sitting, without change in posture, under the compressive pressures cited may cause ischemia, or an interference in the blood circulation, resulting in aches, pains, and possible numbness.
Figure 4-1. A sectional view of the seated figure showing the ischial tuberosities.
Figure 4-2. An enlarged posterior sectional view of the ischial tuberosities.
It becomes obvious that the design of seating should provide for the distribution of the body weight supported by the ischial tuberosities over a larger area. Proper padding on the seat pan can accomplish this. It is apparent, too, that the design of the seating should also permit the sitter to change posture when necessary to alleviate discomfort. In this regard, proper anthropometric data are essential in determining the proper measurements and clearances required.
Branton’s second observation is that, structurally, the tuberosities form a two-point support system which is inherently unstable.7 The seat pan alone, therefore, is not sufficient for stabilization. Theoretically, the legs, feet, and back, in contact with surfaces other than the seat pan, should produce the necessary equilibrium. This would presuppose that the center of gravity was directly over the tuberosities. The center of gravity of the upright seated body, however, is actually located outside the body, about 1 in, or 2.5 cm, in front of the navel, as indicated in Figure 4-3. The combination of the two-point support system, in addition to the position of the center of gravity, has led Branton to suggest a scheme “in which a system of masses is inherently unstable on the seat.”8 He further suggests that if the system is to remain as stable as it normally appears to be, some internally active (muscular) forces must be assumed to be at work.
Figure 4-3. Center of gravity of the upright seated figure.
Given the many body postures assumed during any sitting period, in addition to the muscular activity involved, even when the body seems to be at rest, sitting is not the static type of activity it frequently is conceived to be. According to Branton, “the sitting body, therefore, is not merely an inert bag of bones dumped for a time in the seat, but a live organism in a dynamic state of continuous activity.”9
It has also been contended that the many postures assumed while sitting are attempts to use the body as a lever system in an effort to counterbalance the weights of the head and trunk. Stretching the legs forward and locking the knee joints, for example, enlarges the base of the body’s mass and reduces the effort of other muscles to stabilize the trunk. Other postures, such as holding up the chin with the hand while the elbow rests on the armrest or the lap, or supporting the head by leaning it against the headrest, are still other examples of the body’s attempt at stabillization, providing relief to the muscle system and, in turn, alleviating discomfort. More significantly, these changes in posture occur without deliberation. Branton attempts to explain this phenomenon by suggesting the existence of an “internal ‘posture program,’ which enables the body to strike a running compromise between its twin needs for stability and variety.”10
Of particular significance to the designer is the importance of the location of back-, head-, and armrests as well as their size and configuration, since it is these elements of the chair or seat that function as stabilizers. If the seat does not provide for proper body stabilization, the user must stabilize himself by assuming the various postures mentioned earlier. This requires the expenditure of additional energy due to the muscular effort involved and increases discomfort.
In view of the elusive nature of sitter comfort and the fact that sitting is more of a dynamic activity than a static one, the relative importance of an anthropometrically oriented approach to seating design has occasionally been challenged. Although, as mentioned earlier, there is no guarantee that an anthropometrically correct chair will be comfortable, there seems to be general agreement that the design must, nevertheless, be based on properly selected anthropometric data. If it is not, there is little doubt that the seating design will cause the user discomfort. The essential anthropometric dimensions for seating design are shown in Figure 4-4 and Chart 4.1.
It should also be noted, however, that the data cannot be applied in a vacuum. In establishing chair dimensions, the anthropometric aspects must be related to the biomechanical demands involved. It was demonstrated previously, for example, that body stabilization involved not only the seat pan, but the legs, feet, and back in contact with other surfaces. In addition, some muscular force was also required. If, through improper anthropometric design, the chair did not allow the majority of users to, in fact, have foot or back contact with other surfaces, body instability would be increased and additional muscular force would have to be introduced in order to maintain proper equilibrium. The greater the degree of muscular force or control required, the greater the fatigue and discomfort.
It is necessary, therefore, that the designer become familiar with the anthropometric considerations involved in the design of seating and their relationship to the biomechanical and ergonomic imperatives implied. To deal with one without knowledge of the others is to solve only a part of the design problem. In this regard the generally accepted basic dimensions required in the design of seating include seat height, seat depth, seat width, backrest height, and armrest height and spacing.
One of the basic considerations in the design of seating is the height of the top of the seat surface above the floor. If the seating surface is too high, the underside of the thigh becomes compressed, as illustrated in Figure 4-5. This can cause considerable discomfort as well as a restriction in blood circulation. If the height of the seat does not permit the soles of the feet proper contact with the floor surface, body stability is weakened. If the height of the seat is too low (Figure 4-6), the legs may become extended and positioned forward. The feet then are deprived of any stability. By and large, however, a tall person would be far more comfortable using a chair with a low seat height than a short person using a chair with a seat height that is too high.
Figure 4-4. Key anthropometric dimensions required for chair design.
Note: No published anthropometric studies concerning lumbar height can be located. A British study [H-D. Darcus and A.G.M. Weddel, British Medical Bulletin 5 (1947), pp. 31–37], however, gives a 90 percent range of 8 to 12 in, or 20.3 to 30.5 cm, for British men. Diffrient (Humanscale 1/2/3) indicates that the center of forward curvature of the lumbar region for adults is located about 9 to 10 in, or 22.9 to 25.4 cm, above the compressed seat cushion.
Chart 4-1. Selected body dimensions, taken from Tables 2 and 3 of Part B, useful in the design of seating. Little detailed published data are available with regard to lumbar heights. Estimates, however, vary from a range of 8 to 12 in, or 20.3 to 30.5 cm, and 9 to 10 in, or 22.9 to 25.4 cm.
Figure 4-5. A seat surface placed too high causes the thigh to become compressed and blood circulation to be constricted. In addition, the soles of the feet are not permitted proper contact with the floor surface, thus weakening body stability.
Figure 4-6. A seat surface located too low may cause the legs to become extended and positioned forward, depriving them of any stability. In addition the movement of the body forward will also cause the back to slide away from the backrest and deprive the sitter of proper lumbar support.
Anthropometrically, the popliteal height (the distance taken vertically from the floor to the underside of the portion of the thigh just behind the knee) should be the measurement in the tables used as reference in establishing the proper seat height. The lower range of the table, such as the 5th percentile data, would be appropriate since these will serve the segment of the population with the smallest body dimension. The rationale, as discussed earlier, is that a seat height that will accommodate a person with a smaller popliteal height measurement will also accommodate one with a larger measurement. Chart 4-1 indicates a 5th percentile popliteal height of 15.5 in, or 39.4 cm, for men and 14.0, or 35.6 cm, for women. The measurements, however, were recorded with the examinee stripped to the waist, pockets emptied, without shoes, and wearing a knee-length examining gown—hardly the kind of attire most people normally wear while sitting. It is necessary, therefore, to compensate for these conditions by increasing the measurements accordingly.
Since the items of clothing as well as the shoes are a function of climate, time of day, location, socioeconomic class, age, culture, and fashion, it is obvious that the factor to be added is, at best, an educated guess or reasonable approximation. Given the dangers involved in making the seat height too high, it would make sense to be conservative in estimating this factor and to err on the smaller side. It is suggested, therefore, that 1.5 in, or 3.8 cm, be added to both measurements; the figures then become 17 and 15.5, or 43.2 and 39.4 cm, respectively. These figures, however, could just as easily be increased if boots or very high heels were assumed to be the footwear. Similarly, the figures would be smaller if the user were lounging at home in slippers and a bathrobe. Given the great variation possible in popliteal height due exclusively to attire, not to mention body size, a very strong argument can be presented for adjustability in all chair types. It should be noted that in determining seat height the type, resiliency, and sag of padding or upholstery should be considered. Moreover, when the chair is used in conjunction with a table, desk, or other work surface, or footrest, seat height dimensions can vary. These conditions, as well as others involving the anthropometrics of seating, will be examined graphically in Part C.
Another basic consideration in chair design is the depth of the seat. If the depth is too great, the front surface or edge of the seat will press into the area just behind the knees, cutting off circulation to the legs and feet as shown in Figure 4-7. The compression of the tissues will also cause irritation and discomfort. A greater danger, still, is the possibility of blood clotting, or thrombophlebitis, if the user does not change body position. To alleviate the discomfort in the legs, the user may move his buttocks forward, in which case his back becomes unsupported, body stability is weaker, and greater muscular force is required to maintain equilibrium. The result is fatigue, discomfort, and back pain. Too shallow a seat depth (Figure 4-8) may result in an awkward situation where the user has the sensation of falling off the front of the chair. In addition, a shallow seat depth will also result in a lack of support of the lower thighs.
Figure 4-7. If the depth of the seat is too great, the seat front will press into the area just behind the knee, causing discomfort and problems with blood circulation.
Figure 4-8. A shallow seat depth will deprive the sitter of proper support under the thighs. It may also give the sitter the sensation of tipping off the chair.
Anthropometrically, the buttock-popliteal length (the horizontal distance from the rearmost surface of the buttock to the back of the lower leg) is the measurement in the tables to be used to establish the proper seat depth.
Chart 4-1 indicates a 5th percentile buttock-popliteal length of 17.3 in, or 43.9 cm, for men and 17.0 in, or 43.2 cm, for women, while the smallest measurement indicated in Table 2K in Part B is the 1st percentile female data, with a measurement of 16.1 in, or 40.9 cm. Accordingly, a depth of seat measurement that exceeds about 16 in, or 40.6 cm simply would not accommodate the very small user, while a seat depth of 17 in, or 43.2 cm, for an easy chair, however, would accommodate about 95 percent of all users.
Although the size, configuration, and location of the backrest is one of the most important considerations necessary to ensure a proper fit between user and chair, it is also the most difficult component to dimension in reference to published anthropometric data. Despite the availability of those body measurements required in dimensioning basic chair parts, such as seat height, seat depth, seat width, and armrest heights, there is a paucity of data relating specifically to the lumbar region and spinal curvature. Accordingly, it will be necessary to limit discussion of the backrest to guidelines and some generalizations.
Figure 4-9. The primary function of the backrest is to provide support for the lumbar region or small of the back. Provisions should also be made for the protrusion of the buttock area.
There appears to be general agreement that the primary function of the backrest is to provide support for the lumbar region, or small of the back (Figure 4-9). This is the concave lower portion which extends approximately from the waist to about the middle of the back. The configuration of the backrest, therefore, should to some extent accommodate the spinal profile, particularly in the lumbar area, as shown in Figure 4-10. Caution should be exercised, however, not to provide so close a fit as to prevent the user from shifting body position.
Figure 4-10. The lumbar region.
The overall height of the backrest may vary depending on the type and intended use of the chair involved. It may be just sufficient to provide lumbar support and little more, as in the case of the typical secretarial chair; or it may extend all the way to the back of the head or nape of the neck, as in easy chairs or reclining chairs, or possibly somewhere in between, as in general purpose seating. Provisions should also be made for necessary clearance to allow space for the protrusion of the buttock area. This clearance may take the form of an open area or recess between the seat surface and the lumbar support. Soft padding in this area will also accommodate the protrusion in the buttock region.
Armrests serve several functions. They support the weight of the arms and assist the user in lowering himself into the seat or in pushing or raising himself out of the seat. If the chair is used in conjunction with some work task, for instance, one involving the manipulation of sensitive console dials or controls the armrest can also function to steady the arm during the performance of the particular activities. Anthropometrically, several factors must be taken into consideration in sizing and locating the armrests. For the height of the armrest, the elbow rest height would appear to be the proper anthropometric reference measurement to apply. This measurement is the dimension from the tip of the elbow to the seat surface. The decision to be made is the particular percentile data to be selected.
But consider the problem of one user with a large body breadth dimension as opposed to another extremely slender person with a relatively small body breadth dimension, who both have identical elbow rest height measurements. It has been observed in such cases that the user with the narrow body breadth would require higher armrests, because as the arms swing outward to connect with the armrest, the vertical distance from the elbow to the seat increases. Since no basic relationship exists between transverse measurements and those in the vertical plane, it has been suggested that the armrest should accommodate the higher range elbow rest height. Those users with shorter elbow rest heights can use the armrests by abducting the arms or raising the shoulders. However, if the armrest is too high, the user may have to force or lever the trunk out of the chair and round the shoulders, resulting in fatigue and discomfort due to the muscular activity generated. Chart 4-1 shows the highest measurement for elbow rest height to be the 95th percentile male data, or 11.6 in, or 29.5 cm. Such an armrest height would, in fact, be uncomfortable for most people. The 70th percentile data would appear to be an optimal high range limitation and the 5th percentile the low limit. Most sources, therefore, recommend an armrest height between 7 and 10 in, or 17.8 and 25.4 cm.
The purpose of cushioning is essentially to distribute the pressure, due to the weight of the body at the point of interface, over a larger surface area. The danger, however, is for the designer to assume that the more opulent, deeper, and softer the cushioning, the greater the degree of comfort. This simply is not the case. All too often it is the very seating that appears overstuffed that, in fact, can provide the most discomfort, fatigue, and pain. Where the bone structures are closest to the skin are the areas of greatest potential discomfort due to the compressive stresses imposed on the body tissue. The ischial tuberosities in the buttock area mentioned previously are an excellent example of a sensitive area, in which the importance and need for proper cushioning is shown.
If cushioning is not properly designed, it is possible that relief from compressive stress may be obtained at the expense of body stability. Branton suggests that a state could be reached whereby the cushioning could deprive the body structure of support altogether. The body would “flounder about” in the soft mass of cushioning with only the feet resting on the floor, thereby increasing the burden of body stabilization on internal muscular activity.11
Still another source of discomfort may develop if the body weight causes the front end of the seat cushion to elevate, placing pressure on the bottom of the thigh and the nerves in that area. Similarly, if the body sinks too deeply into the cushioning, the sides and possibly the rear portions of the seat cushion may also elevate, producing additional pressures on the various parts of the body involved. In addition, the deeper the body sinks into the chair, the more effort is required to get out of the chair.
It is obvious that hard, flat seats are uncomfortable for extended use. It has also been suggested that excessively deep, soft cushioning can result in extreme discomfort. Although more research is required to objectively study the entire notion of sitter comfort, certain guidelines for proper cushioning have been suggested. Diffrient recommends that, for comfort, an average padded seat would have about 1.5 in, or 3.8 cm, of medium foam padding over .5 in, or 1.3 cm, of firm closed-cell padding, or a total of about 2 in, or 5.1 cm, with a maximum allowable seat compression of about 1.5 in. The seat compression allowance is based on a 172-lb, or 78-kg, male. For every 30 lb, or 13.6 kg, less, .25 in, or 6.4 mm, should be deducted. For every additional 30 lb, .25 in should be added.12 Croney recommends a depression of about ½ in, or 13 mm.13 Damon et al. suggest that 1 to 2 in, or 2.5 to 5.1 cm, of compression would suffice.14
