According to McCormick (1957), a person is more comfortable in a seated position if two basic conditions are met:
• The weight of the body is adequately distributed between the seat and the backrest and
• muscle tension necessary to maintain a seated position is as small as possible.
Fig. 3.39 The dimensions of space for the free movement of the lower limbs straight at the knee joint
Identifying technical requirements for furniture for sitting and relaxation requires
collecting data on:
• dimensional characteristics of the human body, including the dimensions and weight of the body and its individual parts (anthropometry),
• distributing the centres of gravity and the range of movements of parts of the human body (biomechanics),
• the construction of the body, that is tissue types, their distribution and mechanical properties (anatomy),
• reactions of the user’s body to the impact of external factors, including the behaviour of the circulatory, nervous and bone system (physiology), and
• the construction and technology of making furniture for sitting, that is geometry and dimensional proportions of the seat construction and characteristics of upholstery systems and materials used (stiffness, strength, thermal insulation, air circulation, etc).
Fig. 3.40 Ergonomic dimensions of a table, taking into account the elbow width
The factors that have the most important impact on the comfort of sitting are as follows:
• the level of contact pressures occurring at the junction of the seat and the user’s body, and the value of stresses inside the soft tissues,
• body build, including weight, sizes and gender,
• time of sitting, and
• way of sitting.
Usually, the value of contact stresses between the human body and an upholstered system increases in proportion to the body weight of the user and strictly depends on the stiffness of the materials used (Smardzewski et al. 2008). A mattress obtains optimum stiffness when the spring layer is characterised by Young’s modulus of a value similar to the elasticity of the human body’s soft tissue (Smardzewski et al. 2006).
Depending on the geometry and constructional solution of the seat, contact stresses can develop within the muscle tissue stresses limiting the normal functioning of the circulatory system. By averaging the pressure in the arterial system,
we obtain a value of around 100 mmHg, in the capillaries around 25 mmHg, and in the final part of the venous system, this pressure amounts on average to 10 mmHg (Guzik 2001). The border value of surface pressure equal to 32 mmHg refers to the pressure reducing the light of capillaries (Krutul 2004b; Stinson 2003). Each pressure greater than this value may result in limiting or closing the lumen of veins, and arteries, which slows down the flow of blood or stops its circulation, causing local ischaemia. Time for which the stress is exerted on the human body plays an extremely important role. Large stresses within a short period of time can lead to deep damage to the muscle tissue. And stresses of small values, but lasting for a longer period of time, cause damage to soft tissue (Stinson 2003). Such a situation can take place in the event of long-term maintenance of a uniform seated position.
Fig. 3.42 Dimensions of a chair-table set including space for the free movement of the lower limbs bent in the knee joint
The effect of this can be the compression of the spine muscle and gluteal muscle by the sacral vertebra and sciatica, which results in an increase of stiffness of the muscles where the bone meets the muscle. This is revealed by pressure pains at the height of the ischiatic bone or so-called pins and needles of the lower limbs caused by ischaemia. The studies of Gefen et al. (2005) showed a stiffening of muscle tissue during 35 min under external pressure 35 kPa (262.5 mmHg) and during 15 min under pressure of 70 kPa (525 mmHg).
By simulating the immobility of a person sitting on a hard ground, using the finite element method, these authors generated in the cross section of the stress compressing at a level of 35 kPa. In this way, they demonstrated that the intensity of the injury damaging cells grows within the first 30 min of sitting in stillness and is the cause of the formation of pressure pain.
Fig. 3.43 Dimensions of a chair-table set including frontal hold reach on the worktop surface and space for the free movement of the lower limbs bent in the knee joint
Defloor and Grypdonck (1999) identified the influence of the seated position on the distribution and value of contact stresses between the seat and the body of the user. The dimensions were carried out on 56 volunteers aged 19-46 years (average age 23.8 years, average weight 64.8 kg, BMI (body mass index 22.4 kg/m2). The lowest stresses, at the level of 39.5 mmHg, have been assigned for the position adopted on a seat inclined backwards with a tilted backrest and support of legs on a footrest. The position with feet resting on the floor, without the possibility of resting the arms, forced the creation of surface stresses with an average value of 48.7 mmHg. However, the highest average value of surface stresses, equal to
55.4 mmHg, was obtained in a position when the body slides down on the seat by 200 mm from the upright position, and the line of the trunk forms an angle of 45° with the line of the seat surface.
By examining the impact of changes in the angle of inclination and the height of the backrest on pressure in the intervertebral disc, Nachemson (1976) demonstrated that the smallest stress and pressure in the disc occur when the angle of inclination is 120° and the support of the lumbar is located at a height of about 20 cm. The highest pressure occurred when the angle of inclination amounted to 90° (Fig. 3.49).
Fig. 3.44 Dimensions of a chair-table set including frontal hold reach on the worktop surface and space for the free movement of the lower limbs straight at the knee joint