Due to human physiology, maintaining sitting position for a couple of hours is not beneficial for the nervous and musculoskeletal systems. Despite the fact that maintaining such a position is physically less tiring compared to the standing position, then with an incorrect position of the body it can cause a much greater (by approx. 40 %) load of the lumbar part of the spine. In addition, the wrong distribution of weight on a seat can cause point loads on the cardiovascular system. As a consequence, long-term positioning of the body in the wrong position, on an
improperly fitted base, often causes pains, changes in degenerative arthritis, blood clots, as well as superficial inflammation of the venous system of lower limbs (Kaminska 2001).
Each day, one can see that the sitting position has dominated the human being’s contemporary lifestyle. Even passively, he generally relaxes on furniture for sitting and resting, which most frequently cause only mental relaxation for the user, not physiological. This is caused by a poor distribution of stresses on the entire contact surface of the user’s body with the flexible layer of the furniture piece. The first step to solve this problem can be the analysis of the human-seat system (Smardzewski et al. 2008), which will enable to assess the accuracy of the human body’s position on a selected furniture piece for sitting.
The following parameters for assessing the body’s position can be assumed as the most important (B^dzinski 1997):
• personal information—gender, age, somatic type, body weight and height,
• concerning the sections of the spine and defining their mobility, flexibility and
shape of physiological curvatures,
• anthropometric—determining the dimensions and proportions between the basic
sections of the body, and
• relating to the musculo-nervous system and musculoskeletal system.
Knowledge associated with being familiar with the mechanical and structural properties of the human body is a fundamental preliminary prerequisite for any theoretical, numerical or experimental approximations in the analysis of adjusting the technical means, which the seat is, to the physiological functions of the body.
Understanding the values of reaction forces at play when the human body is in contact with a technical object requires reducing the human body to the scheme of a multi-joint beam, where each element of the beam between the joints reflects a specific part of the body. (Figure 8.49). Therefore, the centres of gravity of certain parts of the body are defined, like the head, the torso—analysed together with the upper limbs, thighs, shanks and feet. Between these parts, joints are located which reflect the possibility of movement.
The places where reactions occur have been identified based on the measurement of the distribution of stresses using a sensory mat (Fig. 8.50). These studies were carried out on the model of an armchair with geometry that ensures minimal impact of mass forces on the user’s body.
By replacing the actual object with a calculation model, simplifications have been introduced not only in the system of the beam, but also in the system of external forces, where the concept of concentrated loads has been applied. External forces were represented by the reaction-supporting system and the mass forces of body parts applied to a construction element (Zielnica 1996).
The presented calculation scheme of elements’ reaction forces of the furniture piece on the human body-free load (Fig. 8.51) does not take into account the friction coefficient of each body part on the surface of the furniture piece and changes resulting from the tension of the human muscular system.
Fig. 8.49 The human-seat system as a model of a joint beam: a the centre of gravity for individual parts of the body, b beam joint
Fig. 8.50 Distribution map of surface stresses
On the basis of the calculations conducted (Smardzewski et al. 2008), it was found that the reactions of supports affecting the beam diminish along with a reduction in the values of mass forces and represent a constant value of share percentage of the sum of all reactions of supports working on the model of the multi-joint beam.
Fig. 8.51 The calculation scheme of the base’s reaction
In the analysed structure, a certain regularity was found in the percentage distribution of forces on individual parts of the body and they are as follows:
• the head—9.60 % of the sum of mass forces and 8.17 % of the sum of reactions of supports,
• the torso—57.40 % of the sum of mass forces, as well as 35.27 % for RT1 and 12.94 % for Rt2 of the sum of reactions of supports,
• the thigh—21.14 % of the sum of mass forces and 33.17 % of the sum of reactions of supports,
• the shank—8.96 % of the sum of mass forces and 6.91 % of the sum of reactions of supports and
• the feet—2.90 % of the sum of mass forces and 3.56 % of the sum of reactions of supports.
Furthermore, it is clear that the multi-point support of the body is beneficial for the reduction of bending moments. If in the human-seat system, the human body is supported by a greater number of points, therefore, the contact surface with the piece of furniture for sitting is greater, then the forces that occur inside the body are smaller. Limiting the forces affecting the user’s body while sitting has a positive impact on his musculoskeletal system, which is associated with comfort of use.