8.4 Lifting and Handling
8.4.1 Workspace layout
The first and most fundamental principle of safe lifting is that the load should at all times be as close as possible to the body. There are two reasons for this. First, the closer the load, the less is its leverage about the various articulations of the body;
hence less muscular effort is required and there is less mechanical stress on potentially vulnerable structures (e.g. those of the back). Second, the closer the load,
Figure 8.7 Strength of a static lifting action as a function of height above ground and foot
placement. Left: freestyle placement (F) and feet placed behind the axis of lift. Data kindly supplied by Anne-Marie Potts. l=16 men; ¡=14 women. Right: feet placed 20 mm behind the axis lift 400 mm apart, various distances to the left. The placement figures are for the mid-line of the body.
NH=knee height, KH=knuckle height, EH=elbow height, SH=shoulder height of the 21 male subjects. Data kindly supplied by Jane Dillon of the Furniture Industry Research Association.
the more easily it is counterbalanced by the weight of the body so it is less likely to get out of control. Thus the strength of the lifting action falls off rapidly as a function of the distance of the load from the body—and the weight that can be handled safely becomes correspondingly less (see Figure 8.7).
A second important principle is that symmetrical lifting actions are in general safer than asymmetrical lifting actions, particularly if the latter involve turning actions which impose a rotational twist on the spine. This is partly because the lumbar spine is anatomically vulnerable to injury under torsional loading; and partly because in turning actions we naturally tend to ‘lead with the hips’, thus exposing the lumbar spine and its musculature to particularly high levels of loading.
In practice the distance of the load from the body and the symmetry of the lifting action will be largely determined by foot placement—and this in turn is determined by the presence or absence of obstacles that prevent the person from getting his feet beneath or around the load (Figure 8.8). Lifting and turning actions likewise very often stem from deficiencies in workstation layout.
Given good foot placement, the strength of the lifting action is greatest at around knuckle height (c. 700–800 mm) and falls off rapidly above and below this level.
When exerting a vertical lifting force at knuckle height or thereabouts, the upper limbs are vertical and almost straight and the hips and knees are slightly flexed. The muscles of the lower limb thus exert a powerful extensor thrust along the line of the almost straight limb at their best possible mechanical advantage. When the force is exerted at a distance from the body, however, this peak in lifting strength disappears (see Figure 8.7).
If the lift commences at much below knuckle height, the person will either have to incline his trunk and therefore increase the loading on his spine (which will tend also to be flexed upon itself and thus anatomically vulnerable to injury), or else
Figure 8.8 Lifting at a distance: palletization task. From an original in the author’s collection.
(From S.Pheasant, Ergonomics, Work and Health, Macmillan, 1991, fig. 15.17, p. 302, reproduced with kind permission.)
strongly flex the knees, thus reducing the mechanical advantage and also rendering the knees anatomically vulnerable to injury. In either case the power of the lifting action is diminished and the weight that can be handled safely is likewise reduced.
A lift that is commenced at knuckle height or lower can (provided the load is not excessive) be continued comfortably to elbow height or a little more. If the load is a box or carton which is held by its lower edges, or a crate with handholds on the sides, the lifter will then begin to encounter difficulties as his wrist reaches the limit if its range of abduction. He will thus either have to change his grip or else make awkward compensatory movements of his upper limbs and trunk, neither of which is at all desirable (see Figure 8.9). The wrist is also anatomically vulnerable in this position.
Lifts that commence at elbow height may be continued to shoulder height or thereabouts without too much difficulty, but beyond that point the reduction in strength really begins to tell. There is a particular danger that loads that must be handled at shoulder height and above will get out of control. In this author’s experi-
Figure 8.9 Lifting outside the normal height range. Note the hyperextension of the lumbar spine.
ence, lifting tasks that entail the handling of loads outside the comfortable height range of the person in question are a very common cause of injury.
On the basis of these considerations we may divide the reach envelope of the standing person into lifting zones, as shown in Figure 8.10 (after Pheasant 1991a;
Pheasant and Stubbs 1992b). The heights given for the various landmarks are based upon anthropometric data for the ‘standard reference population’ (see section 2.4) but rounded up to convenient whole numbers. The verbal categories describing each zone may be regarded as giving a general indication of the weight of load that might be considered acceptable in each zone (see also below).
When carrying a load such as a box or carton, the person will generally hold it by the lower edges at hip height or above (800–1100 mm) so as not to impede walking.
The effort required to lift loads from conveyor belts, etc., may often be reduced therefore by setting the belt at a level that allows the person to pull the load towards him and take its weight at a suitable height for carrying. (This will depend in some measure on the nature of the load.)
Pushing and pulling actions are generally performed most easily at between shoulder height and elbow height or a little below, depending on the circumstances.
According to biomechanical studies by Ayoub and McDaniel (1973), the optimum
Figure 8.10 Height ranges for lighting actions. (From S.Pheasant, Ergonomics, Work and Health, Macmillan, 1991, fig. 15.20, p. 305, reproduced with kind permission.)
level is 70–80% of shoulder height—which works out at a little below elbow height or about 1000 mm for men and 900 mm for women. Fixed horizontal handles on trolleys, carts, etc., should be at this level; but vertical handles will often be a better solution in that they allow the user to find his or her own level.
Pushing actions are strongest when the feet are placed as far back as possible;
pulling actions when the feet are as far forward as possible. High-friction shoes and flooring materials are important. An unobstructed floor space of 1000 mm is required;
1800 mm is preferable for pulling actions.
Tasks involving the storage of items on shelving and racks constitute an important class of handling problems. In general the heaviest and/or most commonly used items should be stored in the most accessible positions (see Section 5.1). Table 8.3 provides some guidance in these matters (based on the above anthropometric considerations and also user trials reported by Thompson and Booth 1982).