ABSTRACT
The body framework consists of the skeletal system, comprising a subsystem of bones linked by joints and strengthened by ligaments, and also the muscular system, comprising muscles attached across the joints and capable of moving the joints. The whole is surrounded by various body organs. The soft tissues are surrounded by a varying thickness of fat and ultimately covered by skin. The various dimensions measured are a function of the size of bones and muscles, which in turn is determined largely genetically and partly by nutrition and training. They also depend on the thickness of various layers.As noted, the bones are linked by joints. There are various kind of joints, some of which, such as the knee, permit motion in only one plane of space; others, such as the shoulder, permit free movement in all planes; still others, such as the small joints of the back, permit limited movement in all planes. Joint movement is restricted by tough fibrous bindings, called ligaments, the variety, number, and attachment of which varies from joint to joint.The bones are made of a fibrous material heavily infiltrated with calcium and other minerals. Their structure is not static like that of the beam of a bridge, but varies continuously as calcium is deposited and re-absorbed and as the fibrous material is destroyed and restored. 80
The epidemiological approach is based on the science of epidemiology, which is concerned with identification of the incidence, distribution, and potential control of disease and injury, including the identification of factors which serve to modify risks of overexertion injury, with particular emphasis on the incidence and severity of low-back pain in industry.Factors pertaining both to the job and to the person can be identified. Job risk factors include the weight and size of the object and the force required to lift it, as well as the location of the object, and its centre of gravity with respect to the worker; the further away the load centre of gravity is from the body, either from bulk of the object or layout of the workplace, the greater is the frequency and severity of problems. The frequency of the lift is also significant, as are the spatial aspects of the task in terms of movement distance, direction, the presence of obstacles and constraints derived from postural demands. Coupling between the worker and the object, for example the type and location of handholds, if any, can greatly modify the difficulty of the task; properly designed handholds placed in the line of the centre of gravity and, if possible, no wider than the natural hanging position of the arms can greatly improve the manoeuvrability of a load.Personal factors include that of gender, not so much because of sex
differences p e r se, but because females statistically speaking are less strong than males. It is also suggested that because of the structure of the female pelvis the centre of the hip socket in females is located in front of a line through the centre of mass of the body, while that of males is in the same plane; hence a greater bending moment exists on the spine of women to the extent that the lifting stress on the back muscles of women for the same object is some 15 per cent higher than for men (Tichaeur, 1978).Age is another personal factor that requires consideration. Many studies have shown that the greatest incidence of low-back pain and back strain occur among workers in the 30 to 50 year old group. This, however, does not necessarily mean that the group is more susceptible; it is more likely that either workers over age 50 have selected themselves out of tasks that require heavy manual lifting, or that with experience they have learned techniques of avoidance of injury.Some studies quoted in the NIOSH Manual suggest that increased body weight is favourably correlated with the ability to lift, probably because the heavier person is usually stronger than his lighter peer, and also that he has the mass required to counter-balance the handling of large objects. There is, however, controversy over whether stature in itself plays any part in the ability of a person to lift. Consensus suggests that there is little or no significance.The style, or technique, of lifting can be significant. Despite protestations by various safety oriented organizations, there is no single ideal technique that will meet all circumstances. It is not always appropriate to adopt the commonly taught squatting method with the back straight while the knees are used to provide the lift. This might be appropriate in some circumstances, but in others, for example, it might be better to bend at the back and use the arms in a swinging ballistic movement, say, to complete several sequential actions at a low level. In general it is desirable where feasible, to separate the feet in order to maintain a balanced distribution of weight, then bend at the knees and hips with the back reasonably straight, keeping the arms as near to the body as possible with the load as close to the body as possible, and then lift smoothly with the legs.The question of strength has been mentioned in connection with discussion of the significance of gender. It is generally considered that strength, and particularly back strength, is significant in protecting the back. Studies by Chaffin and his colleagues (1973, 1977) showed that the incidence of back pain in workers engaged in frequent lifting tasks was three times greater among persons whose strength as measured by isometric strength tests was not equal to or exceeding that demanded by their jobs. It was concluded that overstressing a person beyond his/her demonstrated strength cannot be tolerated by a person’s musculoskeletal system, especially when such exertions are performed more often than about 100 times per week.
