Posture is an active process and is the result of a great number of reflexes, many of which have a tonic character. The attitudinal, as well as the righting reactions, are involuntary.
(H. D. Denniston, 1935)
It is easy to overlook the fact that the human body is a mechanical system that obeys physical laws. Many of our postural and balance control mechanisms, essential for even the most basic activities, operate outside of conscious awareness. Only when these mechanisms break down - as in slipping or losing balance - are we reminded of our physical limitations. An understanding of these limitations is fundamental to practically all applications of ergonomics.
The skeleton plays the major supportive role in the body. It can be likened to the scaffolding to which all other parts are attached. The functions of the skeletal and muscular systems are summarised in Table 2.1.
Like any mechanical system, the body may be stable or unstable and is able to withstand a limited range of physical stresses. Stresses may be imposed both internally or externally and may be acute or chronic. A useful starting point in the discussion of mechanical loading of the body is to distinguish between postural stress and task-induced stress. According to Grieve and Pheasant (1982), postural stress is the term used to denote the mechanical load on the body by virtue of its posture. Posture is defined as the average orientation of the average orientation of the body parts over time. Task-stress depends on the mechanical effort needed to perform the
Table 2.1 Functions of the skeletal and muscular systems
To produce movement of the body or
Protection (the skull protects the body parts brain and the rib cage protects the heart and lungs
To maintain posture
Heat production (muscle cells produce heat as a by-product and are an important mechanism for maintaining body temperature)
Movement (muscles are attached to bone; when they contract, movement is produced by lever action of bones and joints)
Homopoiesis (certain bones produce red blood cells in their marrow)