The functions described in (2) and (3) involve looking, listening, and speaking while at the same time working with objects and tools. The function may either be fulltime (such as a chair for typing work) or occasional (such as a visitor’s chair or a conference chair). For the full-time function, it is important for the operator to be able to adjust every part of the chair so that it is suitable for just that one person. The chair does not need to be adjustable for the occasional function. There may even be advantages for nonadjustable chairs. For example, in some environments (such as a
waiting room) difficulties may occur if every user were to make adjustments to the chairs. Instead, the chair should be designed so that it is suited to as many of the user population as possible. Regarding chair design, there are various seemingly conflicting philosophies. By studying all the arguments for and against these philosophies, it becomes clear that they are not in complete contradiction but to a large extent complement each other as they represent different types of sitting.
The four different philosophies are:
1. The ‘classical sitting’ style, first described systematically by Akerblom (1948) and later developed by Floyd and coworkers (1958, 1969), among others.
2. ‘Active sitting’, ‘discovered’ by Mandal (1982), and more recently marketed by the Scandinavian furniture company Hag.
3. ‘Armchair sitting’, proposed primarily by Grandjean (1988).
4. Culturally-specific sitting.
In this section we describe in detail the principles for these four types of sitting, together with some of their advantages and disadvantages.
1. Classical sitting is best suited to ordinary office work, where the work consists of many different tasks and where it has been possible to design the work surfaces to suit the work done on them. The sitting position starts from a relatively low (minimum height 35 cm) seat surface, in order to reduce pressure on the underside of the thigh, which leans slightly backwards to prevent the worker from sliding off. There is a small backrest that should provide support for the lower back and to help straighten it up. This position gives a limited range of movements and reach. When sitting correctly, the body is relatively relaxed and balance is achieved by voluntary action. The weight of the body is taken mainly on the ischial tuberosities (see Figure 6.9).
2. Active sitting is based on a higher seat tilted forwards, which straightens up the back ‘naturally’ (see Figure 6.10). The sitting position gives greater
FIGURE 6.9 Classical sitting posture.
FIGURE 6.10 Active sitting posture.
freedom of movement and reach, achieves a better balance, and less room is needed for the legs. At the same time, the legs are subjected to a relatively high loading, as active sitting gives less support for the body. Recent studies show an increase in foot volume and foot complaints for this type of sitting. A correct posture for this type of sitting is adopted more or less automatically. The weight of the body is taken on the feet and the ischial tuberosities. It is well suited to those workplaces where long reach is required (due to a large work area) and/or where there is limited leg room. It is also suited to situations where there is a frequent change between sitting and standing. Combined with a sloping desktop it is also good for handwriting and ordinary reading tasks.
3. In armchair sitting, the body weight loading is distributed between a large part of the back, the ischial tuberosities, and the thighs. There is a large backrest and a large seat, both of which lean backwards. The sitting position is relatively low, but at the same time allows the best relaxation for the body and the best balance. It is best suited to looking/listening functions, for example, in conference rooms, control rooms, and for certain types of VDU work where the majority of the work is on the VDU with few other tasks (see Figure 6.11).
4. In culturally-specific sitting, there are many different types of traditions. Most of these can be related to cross-legged sitting directly on a floor or on any type of flat surface. For a person with sufficient flexibility in the legs to be able to sit cross-legged, this sitting position has many advantages. Automatically, the spine remains erect and in balance without tension in the
FIGURE 6.11 Armchair sitting posture.
back muscles. This is the obvious reason why this sitting position is used for mediation and in yoga. Preferably the sitting surface should not be too hard or too soft. Some people prefer to sit on a small cushion (a few centimetres high).