HISTORICAL RESEARCH REPORTResearch Report TM/85/01

1985

Manual handling: limits to lifting Graveling RA

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Manual handling: limits to lifting

Graveling RA This document is a facsimile of an original copy of the report, which has been scanned as an image, with searchable text. Because the quality of this scanned image is determined by the clarity of the original text pages, there may be variations in the overall appearance of pages within the report. The scanning of this and the other historical reports in the Research Reports series was funded by a grant from the Wellcome Trust. The IOM’s research reports are freely available for download as PDF files from our web site: http://www.iom-world.org/research/libraryentry.php

HISTORICAL RESEARCH REPORTResearch Report TM/85/01

1985

ii Research Report TM/85/01

TM/85/1UDC: 612.76

MANUAL HANDLING : LIMITSTO LIFTING

R A Graveling

January 1985

Price: £10.00

MANUAL HANDLING : LIMITS TO LIFTING

Contents Page No

1. INTRODUCTION 1

2. ASSESSMENT OF CRITERIA 3

2.1 Biomechanical Criteria 3

2.1.1 Intradiscal Pressure 32.1.2 Intra-Abdominal Pressure 72.1.3 Electromyography and Muscle Strength 14

2.2 Psychophysical Criteria 182.3 Metabolic Criteria 22

3. A 'HYBRID1 CRITERION 25

4. HEALTH AND SAFETY COMMISSION: REGULATIONS AND GUIDANCE 28

5. CONCLUSIONS 28

6. REFERENCES 30

ERG2/3

MANUAL HANDLING ; LIMITS TO LIFTING

R A Graveling

1. INTRODUCTION

Published limits can be divided into three broad categories*

Acceptable limits, derived from studies of what a worker is willing to lift

or carry. Safe limits, based on some physiological or biomechanical

criterion level which it is considered potentially harmful to exceed and

Official or Legal limits. These latter limits might be expected to reflect

those set according to the previous two categories. However, they may

sometimes be based on political expediency. For example, Carter (1969)

commented on the frequency with which 50 Kg or 1 cwt, commonly occurring

package weights, have been cited as limits in the national standards of

various countries. It could however be argued that the use of these weights

has evolved over years of experience and has some intrinsic validity as an

acceptable load.

The establishment of limits is complicated by the diversity of factors

which have been reported as influencing the amount which an individual can

lift or carry. Drury and Pfeil (1975) listed over twenty variables which

could affect lifting or carrying performance, classified under three general

headings of 'operator variables', 'task variables' and 'environmental

variables'. This list, adapted to Table 1, is by no means exhaustive, but

does give an indication of the complexity of the problem.

Table 1 Variables Potentially Affecting Lifting Performance(based on Drury and Pfeil, 1975)

Type Variables

OperatorVariables

SexAgeSizeStrengthSkillMotivationMethod of lifting

Task Variables Size of object liftedShape of object liftedPosition of handlesHeight at start of liftHeight at end of liftTime for which object is supportedDistance over which object is carriedFrequency of lift

EnvironmentalVariables

Temperature/humidity/air movementLighting, noise, chemical environmentSize and layout of workplaceDiscrete hazards (stairs, corners,slippery floors etc)

Various criteria have been adopted in determining what constitutes a

safe or acceptable lifting limit. Garg and Ayoub (1980) classified these

under three categories: (1) biomechanical: used to determine safe lifting

limits, (2) psychophysical: the determination of what is acceptable to a

person required to lift, and (3) metabolic/heart rate. This third category

reflects the physiological parameters usually adopted in establishing

criterion levels and is used to assess repeated lifting demands over a

period of time. Such parameters are not appropriate for the assessment of

single lifts.

2. ASSESSMENT OF CRITERIA

2.1 Biomechanical Criteria

2.1.1 Intradiscal pressure

One of the most widely adopted biomechanical criteria is that of the

compressive force acting on the vertebral discs. Such forces can be

directly related to those necessary to damage discs. Studies on'discs

obtained from cadavers showed a mean 'yield pressure1 of 710 Ibs (321 kg)

with a range of 350-1400 Ibs (158-634 kg) (Bartelink, 1957). These

pressures should probably be regarded as lower limits as the discs examined

were mainly from people of 60-80 years of age. A value for a younger person

has been quoted as 600 kg (Nachemson, 1966). Measures taken in vivo with a

needle inserted into the disc showed that, in a subject holding a 10 kg

weight in each hand and bending forwards at an angle of 20°, total

intradlscal pressures of between 250 and 340 kg could be recorded

(Nachemson, 1965). Resting values with the subjects standing in a relaxed

posture were 80 - 140 kg total load (588 - 883 kPa). More recently, resting

values of 331 ± 34 kPa have been quoted (Andersson et al, 1977). Although

the actual values are considerably different, possibly a function of

improved measurement techniques, again they found an increase in pressure

proportional to the back angle, with the pressure linearly related to the

sine of the angle. Similarly, for a fixed back angle, a linear relationship

was demonstrated between load and spinal pressure.

Fish (1978) described the calculation of the compressive forces

associated with lifting a 20 kg load by two different methods (straight

back/bent knee and flexed back/straight knee) and repeating the flexed back

lift with a 0.2 kg load. Forces were calculated for the L4/5 disc. The

mean values obtained (351 kg and 383 kg for 25° and 40° back angles) are

remarkably similar to those measured directly by Andersson et al (1978)

again for a 20 kg load (326 kg at 30°) despite apparently not allowing for

the support provided by the abdominal musculature (see below).

The degree of agreement between predicted and measured spinal forces

were reported by Schultz et al (1982). Studies were carried out with a

variety of arm positions and weights in sitting and standing postures. The

regression analysis of predicted compression forces against intradiscal

'pressure yielded a straight line with a correlation coefficient of 0.98.

Although the actual regression equation was not cited it can be seen from

the graphical presentation of the data that the origin is quite close to

zero on both axes but that the predicted force is consistently greater than

the measured pressure (slope > 1). The authors also cited earlier work by

Nachemson (1960) reporting in vitro studies which showed a direct

relationship between the compressive force actually applied, and disc

pressure.

The studies by Andersson et al and Fish showed a similar pattern of

results with an increase in disc compressive forces with increased load.

Neither study however showed any significant differences between the forces

associated with the two lifting methods. Andersson et al (1978) reported

that all means of peak force values were higher in back lifting whereas the

figures published by Fish (1978) showed the early stages of leg lifting to

result in;higher calculated forces than back lifting.

Ayoub and El-Bassoussi (1978) also published calculated spinal forces

(L4/5) for back and leg lifting of different weights, again showing a linear

increase in calculated force with increased load* They reported that

maximum compressive forces for back lifting were clearly higher than those

for leg lifting although no level of statistical significance was reported

and one figure, showing the time course of changes during a lift, showed

remarkably similar initial curves for both lifts with an unexplained

secondary maximum for the leg lift.

Unlike Fish (1978), Ayoub and El-Bassoussi do reportedly allow for the

effect of abdominal pressure in their calculations. However, despite this

fundamental difference, the compressive forces at the start of the lifting

cycle are very similar when the differences in load lifted are allowed for.

Thus, interpolating between values of 433 kg for a. 20 kg load and 246 kg for

a 0.2 kg load (Fish, 1978) gives a prediction of 287 kg for a 4.53 kg load

compared with the value of 280 kg (from graphical display) calculated by

Ayoub and El-Bassoussi.

The calculations of Fish differ in one major aspect from those of Ayoub

and El-Bassoussi or the direct measurements of Andersson et al. The former

paper does not indicate any real increase in compressive force after the

onset of a lift (two graphs showing an increase of the order of 5%, one a

decrease of a similar magnitude). However, the latter papers both show a

considerable increase. Ayoub and El-Bassoussi calculate an initial increase

of 37-47% Whereas the graphs reported by Andersson et al suggest changes of

the order of 100%.

Apart from a rather rudimentary comparison of two simple lifts, neither

of the two papers dealing with calculated spinal forces has any real

discussion of the application of their work. Neither paper commented on the

fact that, even with comparatively low loads, the calculated forces were

greater than those which, as detailed above, can destroy a disc. Similarly,

no attempt was made to apply this or any other criterion to determine

lifting limits. The calculations were, in both cases, limited to examining

the effects of increased load and to comparing the time course of force

changes during two 'idealised' lifts.

A recent NIOSH report on manual lifting (NIOSH, 1981) utilised data

published by a number of authors based on a variety of techniques including

disc pressure to develop criteria and subsequent lifting limits. The report

concluded that:- ' jobs which place more than 650 kg compressive

force on the low-back are hazardous to all but the healthiest of workers.

In terms of a specification for design a much lower level of 350 kg or lower

should be viewed as an upper limit'. Working on a mixed population, the

report uses data for an average female to produce the lower (350 kg) limit

and an average male for the upper (650 kg) limit.

Calculations based on a static loading model resulted in lower limiting

values of 4.5-83 kg and upper limits of 22-100 kg depending on the

horizontal and vertical location of the load, and assuming the 'best*

posture in each case. However, calculations for an average male, based on a

dynamic lifting model, indicated that 400 kg compressive force would be

generated by loads as light as 2-8 kg for leg lifts and 2-4.5 kg for back

lifts, varying according to horizontal location. Interpolating between the

600 and 700 kg lines gives 650 kg values of 16-55 kg and 13-45 kg for leg

arid back lifts respectively.

Despite these conflicts, the report nevertheless concluded that back

compressive forces could be used to produce practical recommendations for

acceptable lifting tasks and, subsequently, used spinal force criteria in

conjunction with others to produce composite lifting limits (see-below).

However, a more recent publication (Hutton and Adams, 1982) reported

substantially higher compressive strengths for spinal discs. The authors

hypothesised, that whereas in previous studies compressive forces had been

applied perpendicularly, with the two faces of the intervertebral disc

parallel, discs were flexed in vivo, and that this may alter their

compressive strength. Consequently, they examined compressive strengths of

discs over the probable range of flexion angle and consequent shear

component encountered in life. A sample of 16 disc specimens from 8 male

cadavers aged 22-46 years yielded a mean compressive strength of 10, 219 N

(sd ±";1711N), considerably higher than the figure of 6376N (650 kg) cited by

NIOSH (1981) as 'hazardous to all but the healthiest of workers'.

In an illustration of the significance of this finding, the authors

calculated that an average young male would need to increase his maximum

lifting strength by 44% before he could lift enough to crush his average

strength lumbar disc, assuming that no relief was provided by the intra

abdominal pressure (see below). No publication has been found to date which

modifies the biomechanical criteria to accommodate these revised data.

Direct measurement of disc pressure during load holding or lifting has

been carried out, but only in a clinical environment. Andersson et al

(1978), carried out direct measurements of the disc pressure during

laboratory lifting simulations, but concluded that such measurements were

not possible in field studies. It therefore appears from the literature

that, despite its intrinsic validity as a measure of lifting strain,

intradiscal pressure is not a feasible measure outside the clinical

laboratory.

Several groups of workers have calculated disc forces through

biomechanical models and, as cited above, Schultz et al (1982) have

demonstrated a close agreement between predicted forces and measured

pressures in the disc. However, apart from the NIOSH document, these

predictive models have not been used to produce any limiting lifting

criteria. In addition to the conflicts between the models, such as these

described above, most predictive programs published to date are based on

symmetrical, two-handed lifts in the sagittal plane (ie, the 'simplest1

possible lift). Frequently, the model is essentially static, generating

'movement* through a series of static postures rather than a genuinely

dynamic model incorporating inertial forces etc. They are therefore of

limited utility for industrial application.

2.1.2 Intra-Abdominal (intragastric) pressure

Another parameter examined by Andersson et al (1978) was the pressure

in the abdominal cavity, the intra-abdominal pressure or, more correctly,

as it is actually measured within the stomach, intragastric pressure.

Bartelink (1957) examined the intra-abdominal pressure using a stomach

balloon attached to a mercury manometer, in order to attempt to find a

possible explanation for the discrepancy between the measured forces for

destroying spinal discs of 158-634 kg, referred to earlier, and theoretical

predictions of the load placed on the base of the spine on lifting a 100 Ib

(45.4 kg) weight of 1600 Ibs (726 kg). It was found that when the back was

bent so that the hands were placed on the floor (or as near to the floor as

possible) the intra-abdominal pressure was comparatively low. As the trunk

was raised, the pressure rose to a maximum, the exact position of which

appeared to depend on the load being lifted but which occurred at about the

'half-way' stage. The pressure then dropped to a minimum with the subject

standing erect. As with the intradiscal pressure, the intra-abdominal

pressure was greater with heavier loads. Bartelink hypothesised that the

'abdominal balloon' as he described the abdominal cavity, and the muscles of

the trunk, in particular the transverse abdominal muscles, acted to provide

a 'muscle skeleton' which transmitted some of the load to the pelvic girdle

thus reducing the forces acting on the spine.

Eie et al (1962) reached the same conclusion as a result of their

"'studies on weightlifters. With more sophisticated equipment they were able

, .to study the pattern of changes far more closely than the earlier work with

_ a mercury manometer. Intra-abdominal pressure, again measured with a

stomach balloon was found to rise with forward bending of the trunk,

reaching a maximum when the trunk was at an angle of approximately 45°.

Higher pressures were reached during lifting than during lowering. As with

the previous study, pressures were found to be proportional to the weight

lifted. One item of particular interest was the markedly higher pressures

reached when a load (10 kg) was lifted incorrectly. Correct lifting

produced a pressure rise of 8-10 mm Hg whereas the abdominal pressure rose

by 50-60 mm Hg if the load was raised or lowered incorrectly. It was

concluded that the raised intra-abdominal pressure tended to counteract the

compression of the lower vertebrae and discs and made it possible for loads

to be lifted which would otherwise cause spinal damage.

In a recent review, Andersson (1980), referred to this hypothesis

stating that, although the positive relationship which had been demonstrated

between intra-abdominal pressure and biomechanical force analyses supported

the idea, further studies were necessary to test the hypothesis. Troup

(1979), however, described the mechanism for this supportive role,

suggesting that the abdominal musculature could take as much as 25% of the

overall compressive force induced by a load, particularly when the trunk was

flexed.

Andersson et al (1978) reported a series of experiments which examined

the effects of posture, and static (pulling) and dynamic (lifting) loads, on

the intradiscal and intra-abdominal pressures and the electromyographic

signals from back muscles. It was found that all three variables responded

systematically to changes in posture and load.

Davis et al (1964) examined intra-abdominal pressure (intra-gastric

pressures) and intrathoracic pressures during pushing, pulling and lifting.

It was found that the pressure changes displayed two clear phases. These

were an initial sharp increase, the 'snatch* pressure followed by a

sustained but lower pressure while maintaining the load. This pressure was

sometimes no greater than the resting pressure. Pressures in the thoracic

cavity (measured by a balloon in the oesophagus) were generally lower than

those in the abdomen. As would be expected, higher snatch pressures were

observed in those subjects who tended to jerk in the initial stages of the

movement whereas those who adopted a slower, smoother, approach generally

produced lower pressure changes. Intra-abdominal pressure changes, either

snatch or sustained, most commonly occurred and were of a greater magnitude

in pushing, with lifting second and pulling last. They were again found to

be related to the weight moved. It was concluded that pulling was the least

likely to overstress the trunk and that it was preferable to carry out the

manoeuvre in a slow, smooth fashion without jerking.

The measurement of intra-abdominal pressure was applied to a simulated

'real' task by Davis et al (1966). Three different procedures were studied

for raising one end of pit props in restricted headroom. Intra-abdominal

pressure measurements was carried out with radio pills rather than gastric

balloons, a useful development which allowed the subjects more freedom of

movement and presumably produced less discomfort. The pressure changes were

again studied in two ways, the peak pressure and the pressure-time product

(FTP), a development of the sustained pressure studied earlier by the same

authors. One lifting procedure produced significantly lower values for both

parameters than the other two. Again, speed of lift was shown to be an

important factor in determining peak pressure. However, when PTP was

plotted against speed of lift (lift time) it was found that the lowest PTP

values occurred at a time approximately one third as long again as the

fastest lift. It was suggested that this might be a valuable measure for

determining optimum rates for a particular manoeuvre. One matter for

caution referred to by the authors was that, although one particular lift

produced the least strain as shown by the pressure measurements, the body

movements resulted in an unstable working position towards the end of the

maneouvre. Clearly therefore, in assessing work routines, pressure

measurements should not be' considered in isolation, but in conjunction with

other parameters.

Davis and Stubbs (1977a) developed loading criteria based on intra-

abdominal pressure (IAP). They reported previous work in the construction

industry (Stubbs, 1973) which had shown an increased liability to back

injury in those members of the workforce carrying out tasks known to produce

frequent peak IAF values above 100 mm Hg. Adopting a 10% safety margin

(Davis and Stubbs, 1978b) they subsequently carried out an extensive series

of laboratory studies to determine the external forces exerted by lifting,

pushing and pulling in various postures which would induce IAP values of

90 mm Hg. These values were first published for small (5th percentile)

males, under 35 years of age (Davis and Stubbs, 1977a, 1977b and 1978a).

They were subsequently republished with additional data for different age

groups (Materials Handling Research Unit, 1980).

One slight anomaly in the figures presented is that the force data are

those for workers with 5th percentile height and weight whereas the arm

length dimensions are based on the 50th percentile functional arm length

(British industrial male population).

' The data are presented in the form of isobars of equal external force,

and are applied by determining the pathway of a lift through these isobars,11 adopting the lowest force indicated as the safest force for that particular

manoeuvre. Due to the wide variety of postures and lifting which man can

adopt, those covered by the published data are necessarily limited. They

cover a number of one-handed and symmetrical two-handed manoeuvres, all

executed with a straight back. Stooping is deliberately excluded to avoid

any suggestion of acceptability for this lifting posture. However, postures

such as may be adopted during kinetic lifting, which may involve a non-erect

10

back, are also omitted. The data set assumes lifting in unrestricted space,

and a lifting rate of up to once per minute (Materials Handling Research

Unit, 1980). Bates up to six per minute are covered by a reduced loading

limit of 70% of those for less frequent lifts. This limit is not derived

from fatigue effects on the IAP, but on heart rate and 'other physiological

measurements' (Davis and Stubbs, 1978b). Legg (1981) reported a study of

the effects of fatigue and training on 1AF levels during lifting tasks.

Neither parameter was found to influence 1AP levels and it was concluded

that any increased risk of back injury due to abdominal muscle fatigue would

not be mediated via IAP changes.

A subsequent paper from the Materials Handling Research Unit (Davis,

1981) indicates that the data set on which these contours are based has a

typical coefficient of variance of over 32%. For a 5th percentile IAP of

90 mm Hg, the 50th percentile pressure would therefore be of the order of

58 mm Hg, indicating the conservative nature of this limit.

The requirements for symmetrical lifting of evenly balanced objects,

and the erect postures covered, reduce the general applicability of these

lifting limits to industrial tasks. Ridd (1983) presented data on the

effects of spatial restraints on intra-abdominal pressure. The author

examined the effects of a number of factors including reduced headroom and

asymmetrical lifting. None of the tables of data has standard deviations

associated with the means and no statistical significance is attributed to

any of the differences. The asymmetrical lifting data were equivocal. In

free space, most mean pressures quoted were higher than those for

symmetrical lifts with a typical pressure increase of 11%. However, with

reductions in available headroom down to 80% of stature, the asymmetrical

lifts showed a slight tendency to produce lower pressures (typically 2%

down). Further bending to a fully stooped posture again produced increases

in the IAP relative to the symmetric lift, with an average increase of 16%.

Symmetrical lifting in reduced headroom produced a reduction in lifting

capacity as evidenced by an increase in 1AP for a given lift. Under a

ceiling height equivalent to 90% of the individuals stature, lifting

11

capacity was reduced to an average of 42% of the restricted load with little

further change with any additional reduction in headroom.

Graveling (1984) in a literature review on forces in awkward postures

concluded that there was a considerable degree of similarity in the data for

reduced headroom situations derived in different ways using differing

criteria and went on to produce composite guidelines largely using the more

complete data set based on 1AP criteria but backed, where possible, by

corroborative values from other sources. However, the limited data which

are available for restricted headroom/awkward postures made this a

comparatively limited exercise.

Such a compromise approach forms the essential basis of the N10SH

guidelines discussed below. An alternative approach which could be adopted

would be to carry out direct measurements of the 1AP changes associated with

different mining manual handling tasks. However, the documentation of the

experimentation which resulted in the selection of 100 mm Hg, as the

critical level is not very clear. Stubbs (1973) observed the postures

associated with various occupations in the construction industry, and

subsequently simulated the most frequently occurring lifting postures in a

laboratory. No information was given in this document relating to the

actual IAP values recorded during these studies. Details of quantification

were limited to a comparison of lifting manoeuvres such as:- lift A resulted

in 20 - 25% higher truncal stress than lift B. In addition, the nature of

the subjects who participated in these laboratory studies was not stated.

Davis and Stubbs (1978b) gave more information on these studies, concluding

that those at risk were those 'sustaining repeated, frequent high-trunk

stresses inducing peak intra-abdominal pressures above 100 mm Hg'. This

paper was actually presented in 1976, and in 1977 this conclusion was

modified to an increased liability in 'occupations in which peak intra-

truncal pressures of 100 mm Hg or more are induced'. (Davies and Stubbs,

1977a). This shift of emphasis from frequent peaks above 100 mm Hg to what

is subsequently interpreted as any peaks above this level is not

substantiated either by reworking of the original data or by the

presentation of additional data.

12

A subsequent publication (Stubbs, 1981) quoted some actual 1AP values

from the laboratory simulations carried out as part of the construction

industry study (Stubbs, 1973). Data are only published for full stoop

lifting of 3, 15 or 25 kg weights through distances of 0.42, 1.11 and

1.53 metres. Twelve readings, from an unspecified number of subjects were

reported for each lift, with a maximum of 50% of peaks exceeding 100 mm Hg

(stoop lifting 25 kg through 1.53 m) and a minimum standard deviation of 42%

of the mean. It is apparently this series of readings which was used to

formulate the 100 mm Hg limit. In this paper, the author repeated the

earlier statement relating back injury to repeated, frequent peak IAP values

in excess of 100 mm Hg. However, a further publication from the MHRU

(Nicholson, Davis and Sheppard, 1981) identified tasks on the basis of any

peaks over 90 mm Hg, including several where well under 10% of all peaks

exceeded this level.

Given the questions concerning the validity or precise interpretation

of the 100 mm Hg criterion, absolute assessment of lifting tasks using

direct measurement may not be appropriate. However, comparative evaluations

of alternative lifting conditions using IAP as an objective index of truncal

strain may provide useful information and should be considered further.

In conclusion, a limiting criterion based on intra-abdominal pressure

has been proposed for the assessment of manual handling tasks. The

derivation of this criterion level of 100 mm Hg is not at all well

documented for such a fundamental statement, and appears to be based on a

rather small, highly variable data set. In addition, it is not particularly

clear whether it should be interpreted as an absolute limit, ie no loads

producing peak IAF values in excess of 90 or 100 mm Hg or whether the limit

should be on the frequent occurrence of such peaks. Furthermore, leaving

aside the validity of the criterion on which they are based, the force

limits derived from IAP values have only been produced for a series of

highly stylised postures, and may only be of limited utility in assessing

mining manual handling tasks. However, there may be some advantage in

investigating the use of IAP as an objective measure of truncal load.

13

2.1.3 Electromyography and muscle strength

Andersson et al (1977) studied intradiscal and intra-abdominal

pressures in conjunction with a third measure, electromyography (emg), in

an examination of spinal loading and posture. It was found that all three

variables responded systematically to changes in postures and load, although

the variability of emg and intra-abdominal pressure was high. Intradiscal

and intra-abdominal pressures each showed a significant linear relationship

with the sine of the angle of flexion. The emg recordings were more

complex, and the relationship varied according to the spinal level from

which the measurements were made. Full wave rectified and averaged

electromyographic signals were used for quantitative analysis. Recordings

were made from the trapezius and from the levels of the fourth and eight

thoracic vertebrae (T4 and T8) and the first, third and fifth lumbar

vertebrae (Ll, L3 and L5). Recordings from the trapezius were found to

reach a maximum of 10° of flexion and subsequently to decrease. 14, T8, Ll

and L3 values reached a maximum at 40° and decreased at 50°. L5 values

increased with increasing angle up to the maximum studied of 50°. The three

parameters were also studied when the angle of flexion was held constant at

30° and the external load increased. Both pressure values showed a

statistically significant linear relationship with the external load. Emg

values also increased with greater external load but no statistical

relationship was reported. A given load was found to produce its greatest

effect at higher levels on the back, with the greatest increase in

electromyographic activity from the trapezius.

Two other aspects of posture were also studied. These were lateral

flexion and a combination of spinal rotation and flexion. These were

examined in conjunction with asymmetric loading. Consistent differences

were found between different levels although no statistical analysis was

reported. The changes were not studied in such detail as those reported

above, with only one angle of flexion (20°) being adopted. Lateral flexion

was found to Increase both intradiscal and intra-abdominal pressure and to

vary according to whether the trunk was bent towards or away from the load.

A similar laterality was observed with the emg recordings, with the

trapezius and thoracic area showing greater values on the ipsilateral side

14

whilst, with the electrodes placed in the lumbar region, higher values were

found on the side contralateral to the load.

As with the disc pressure, and to a certain extent with intra-abdominal

pressure, studies have been reported on the relationship between predicted

muscle contraction forces and the myoelectric activity of the muscles

involved. Schultz et al (1982) reported a 'slightly non-linear1 regression

equation with a correlation coefficient of r = 0.992. The curvilinear

nature of the relationship was apparently related to alterations in trunk

angle. The authors reported that less myoelectric activity was exhibited

per unit calculated muscle force when the trunk was flexed than when it was

upright. Thus, when the 12 pairs of data were divided into two groups of 6

(trunk flexed and trunk upright) the best fit lines were linear in each

case, with correlation coefficients of r = 0.976 and r = 0.987 respectively.

There has also been considerable interest in the quantification of

muscle fatigue through the use of the frequency shift phenomenon (Graveling

et al, 1980) although it does not appear to have been used in the

formulation of lifting criteria.

Criteria based on muscle function have however been derived from

studies of muscle strength. Poulsen and Jorgenson (1971) examined the

relationship between the isometric strength of the back muscles and the

maximum weight which an individual could lift. No information was given

regarding how the maximum lifts were determined although it appeared that

bent leg lifting was used. Linear regressions were obtained for men and

women with correlation coefficients of 0.72 and 0.78 respectively.

Simplified forms of the regression lines were then used to calculate

recommended lifting limits for a normal population, using standard muscle

strengths published by Asmussen and Heeboll-Nielson (1961). Three sets of

values were calculated for males and females of various body heights and

weights. These were: maximum lift, calculated from the simplified

regression line, permissible single lift (70% of maximum), and permissible

repeated lift (50% of maximum). The 70% level was selected on the basis of

a visual inspection of the graphs which showed that the distribution of data

round the line went down to about 70% of the predicted values. A more

15

recent publication (Poulsen, 1978) repeated the experimental data, and re-

presented the lifting limits. In this paper, the maximum lifts were derived

from the regression values minus two standard deviations, which conveniently

also came to 30%. The 50% 'repeated lifts' value was based on a previous

publication (Molbech, 1963) which indicated that a frequency of 6 to 10

lifts per minute could be endured for at least 10 minutes if the load did

not exeed 80% of the maximum. Poulsen and Jorgensen interpreted this as 80%

of their maximum permissible (ie 70% of the predicted maximum) and rounded

the values (56% of maximum) down to 50%, to be applied for rates up to 6 per

minute for a short period of the working, day.

Poulsen (1978) did not repeat the frequent lifting category. This

latter-paper did however give more detail on the determination of the

maximum lift values. Each individual was presented with a box of

unspecified.size, loaded with the estimated maximum load for that

individual, minus 5 kg. No information was given on the manner of

estimation. Subjects adjusted the load to determine that which could just

be lifted from the floor to an upright position using 'the knee-action

technique*. This and the back strength measurements were made on a total of

50 people (25 men and 25 women).

Carlsoo (1978) included the measurement of back muscle strength in a

study of the relationships between lifting capacity and various muscle

function tests including electromyography on selected muscle groups. As

with the previous study by Poulsen and Jorgensen (1971) a good correlation

was reported between back strength and lifting capacity, in this case using

any lifting technique. Lifting capacity was determined by progressively

.increasing the weight of 'a box* until the subject's lifting limit was

/reached. No data were presented as only approximately 50 of an intended 150

''subjects had been assessed. One of the interesting features was the wide

variety of muscle activity and coordination (as indicated by the emg

recordings) during both the relatively controlled back and abdominal

strength tests and in the 'free-style* lifting tests. Although no evidence

was presented in this paper, it is conceivable that muscle coordination

rather than muscle strength per se may be the determining factor, with

16

relatively uncoordinated lifts predisposing to back pain and other lifting

problems.

Pytel and Kamon (1981) also studied the utility of strength as a

predictor of lifting capacity. In addition to static back extensor strength

which was measured in the same manner as used by Poulsen and Jorgensen

(1971) static elbow flexor strength, and dynamic lift, back extension and

elbow flexion strengths were determined. A tote box (45 x 30 x 12 cm) was

used for the determination of maximum dynamic lift and maximum acceptable

lift. In both cases, the box weight was adjusted by the subject, in the

first case to the maximum which they could lift and in the latter case to

the load which they considered to be comfortable to lift at a rate of 6 per

minute for a regular work day. The equipment for the determination of

dynamic strengths (Mini-Gym) allowed control of the speed of movement. Two

speeds were used, 0.73 and 0.97 ms . No rationale was given for the

selection of these speeds.

Static back or elbow strengths were not used as correlates on maximum

dynamic lift (MDL). However, linear regression equations and coefficients

were given for MDL on each of the dynamic strength tests. For men, the best

correlation was achieved between MDL and dynamic lift strength (DLS) at a

speed of 0.73 ms (r = 0.87) with all other coefficients being 0.65 or

less. The tests on women gave much higher correlation coefficients on all

comparisons, varying from a maximum of 0.96 (dynamic back extension strength

at 0.73 ms ) to 0.68 (dynamic elbow flexion strength at 0.97 ms ). No

comment was made on this sex difference. In both sexes, tests at 0.73 ms

produced consistently higher correlation coefficients than the equivalent

test at 0.97 ms . All the test results at this speed were therefore

combined to produce a multiple regression equation predicting MDL,

incorporating a factor for sex with a value of one for men and two for

women. However, this equation produced only a marginally higher correlation

coefficient than a much simpler equation only involving dynamic lift2 2

strength and sex. (R = 0.948 cf R = 0.941). The authors concluded that

although this latter equation (MDL = 295 + 0.66 (DLS) - 148 (SEX)) was only

derived from a small group of subjects (10 male, 10 female), the values

obtained compared favourably with a number of other published studies and

17

were therefore considered to be reliable. The equation was intended for use

in individual assessment, and no attempt was made to derive population

norms•

Garg and Ayoub (1980) compared the limits published by Poulsen and

Jorgensen (1971) with others from Martin and Chaffin (1972) and Asmussen et

al (1965). The limits recommended in the former paper were substantially

higher than those from the other two. For example, the 95th percentile

lifting capabilities from Martin and Chaffin for a similar lifting distance

(which should be comparable with the maximum minus two standard deviations

of Poulsen and Jorgensen) are 2 kg lower than the lightest limiting value

specified for any age group by Poulsen and Jorgensen. Similarly, the

repeated lifting values calculated by these authors are consistently higher

than values calculated by Asmussen et al (1965). This difference appears

superficially to be accounted for by a difference in the manner of

calculation, for the former limits are based on 50% of isometric back

strength whereas Asmussen et al used 40% values. However, despite the fact

that the Poulsen and Jorgensen values are derived from figures published by

Asmussen (Asmussen and Heeboll-Nielson, 1961), recalculation of maximum

strengths from the percentage values give maxima from Asmussen et.al

of 2.5 - 4 kg less than the lower end of the ranges cited by Poulsen and

Jorgensen. It appears therefore that several sets of lifting limits have

been derived, based on lifting strength, but that there are considerable

differences between them. There appears to be no justification for

selecting any one in preference to any of the others.

2.2 Psychophysical Criteria

Two main groups of research workers have published lifting limits in

recent years based on the psychophysical criterion of an acceptable lift.

These are the group led by Snook (eg Snook and Irvine, 1967; and

Snook et al, 1970) whose findings have been collectively published as Snook,

1978, and a group from Texas Tech University led by Ayoub (eg McDaniel,

1972; Dryden, 1973; and Knipfer, 1974). The data obtained by Snook and his

co-workers have been used to provide selected percentile ranges of

acceptable lifting weights for a number of different variables whereas those

18

data obtained by Ayoub and his colleagues have been used to prepare

predictive equations. These two approaches will be described in turn.

Both groups used the same fundamental approach to the experimental

protocol in that the subjects were instructed to adjust the workload to the

maximum amount that they could perform without strain or discomfort and

without becoming tired, weakened, overheated or out of breath.

Snook (1978) published maximum acceptable weights of lift, lower, push,

pull (initial and sustained) and carry, for 90th, 75th, 50th, 25th and 10th

percentiles of industrial male and female populations. Lifting and lowering

variables reported were: range of lift (floor to knuckle, knuckle to

shoulder, shoulder to arm reach), frequency of lift (one every five seconds

to one per eight hour period), vertical distance of lift (25, 51 and 76 cm)

and width of object away from the body (36, 49 and 75 cm). All values were

derived from lifting and lowering a regularly shaped evenly loaded box with

good handles. It was further reported that heat stress (30.3°C dry bulb and

65% relative humidity, 26.7°C ETA) reduced the acceptable lifting workload

by 20%. Object length was not reported as a variable as it was not found to

have a significant effect on weight lifted within the range 57 to 89 cm.

Similarly, age was not included as a variable but incorporated into the

percentile values, as results for three ages (20 -30, 30-40 and 40-55) did

not yield any significant differences. Although, as with all studies of

lifting, this series of studies was limited by the use of regularly shaped

objects in good.lifting conditions, the results have several advantages over

those from a number of other studies. In particular, these include the

diversity of lifting ranges and frequencies covered by the data, and the

fact that the participating subjects were allowed a free style of lift.

Ciriello and Snook (1983) reported a further series of psychophysical

studies designed to confirm or reject some of the assumptions made in

interpolating between data points during the earlier studies. The authors

drew attention to an apparent over estimation by 10-15% of acceptable lifts

for female subjects in the earlier work. However, although previous

estimates for females had been too high, a comparison of the results for

males from the two series of studies indicates that the values in the

19

earlier studies were consistently lower, sometimes by as much as 20%, than

those reported by Ciriello and Snook. Although part of the difference may

be accounted for by comparing mean values against 50th percentiles, the

differences are too large for this factor to have had much influence.

The authors also reported oxygen consumptions for a selection of the

faster lifting tasks. These generally exceeded the NIOSH criterion of

1.0 litres min~ for an 8 hour day (NIOSH, 1981) indicating that the

psychophysical technique produced overestimates when assessed against

physiological criteria.

Mital (1983) also concluded that the psychophysical technique produced

overstimates of acceptable lift from a study specifically designed to verify

the psychophysical approach. In the first part of the experiment, subjects

were randomly allotted a task from a selection of frequencies, box sizes and

heights of lift (36 different combinations, 10 subjects!). No indication

was given of the actual lifting tasks used. The subjects then estimated

their acceptable load for an 8 hour shift in the usual way. In the second

part of the experiment, subjects actually worked an 8 hour shift, starting

with the previously selected load but adjusting it if they so wished. It

was found that, by the end of the 8 hour shift, subjects were only lifting

an average of 65% of their starting load.

Although derived in essentially the same manner, the data obtained by

Ayoub and his colleagues (collectively published as Ayoub et al, 1978) were

primarily used in a totally different fashion; the acceptable lifting loads

obtained, together with selected individual characteristics, being used to.' •'•'.*"•obtain predictive equations for acceptable lifting loads. However, even the

simplest predictive equations, with average error ratios ranging from .0966

to .4435, require the determination of back strength and leg strength, and

are not therefore immediately applicable to an industrial task.

Ayoub et al, (op cit) did however use their data to produce a limited set of

percentile values of lifting capacity based on 'repeated, continuous lifting

tasks'. No detail was given on the rate of repetition. Because of this,

comparisons between these data and those published by Snook is difficult.

Ayoub (1977) carried out some comparisons of the data by his own group and

20

that published by Snook and his colleagues. These comparisons were based on

the original papers (eg Snook et al, 1970) rather than the collective tables

which may account for the discrepancy in lifting capacity reported. Ayoub

cited a value of 24 kg from Snook et al (1976) as the lifting capacity for

an average male, lifting at a rate of 1 lift min from floor level to a

height of 50-75 cm above the floor, compared with a value of 27.6 kg from

his own data. However, an examination of the data in Snook (1978) gives a

range of loads varying from 22-30 kg depending on box size and actual

distance lifted (the extremes of the lifting range produced a 1 kg

difference in acceptable load). Comparisons of the effect of lifting

frequency showed a slower decrement of acceptable load with increasing

frequency from the data by Snook than from those by Ayoub for floor to

knuckle and knuckle to shoulder lifting ranges, but that the ranking was

reversed for lifting from shoulder height to reach. Given the increased

risk to health of lifting above shoulder height then the more conservative

data of Snook may appear to be more acceptable.

In obtaining their data, Snook and his colleagues allowed individuals

to select their own preferred lifting technique whereas no reference has

been found in the publications of Ayoub and co-workers to the lifting style

adopted in their studies, although Garg and Saxena (1979) suggested that the

initial (unpublished) reports (McDaniel, 1972; Dryden, 1973 and Knipfer,

1974) were based on a single (unspecified) lifting technique. Garg and

Saxena reported a study in which a psychophysical methodology was adopted to

determine maximum work loads at four different lifting frequencies (3-12

lifts min ) using two specified lifting techniques (bent knee/straight back

and stooped) and free style lifting. The mean acceptable work load was

consistently higher for free style lifting for all four lifting frequencies,

ranging from 4.8 to 15.6 kg m min higher (stooped back was also

consistently higher than bent knee). No Indication was given regarding the

style of lift being adopted during the free style lifts. Differences of

this magnitude (8-30%) could account for the differences in lifting capacity

as determined by Snook and Ayoub. It is not possible to determine whether

the differences were genuine, or an artefact of the relative inexperience of

the subjects in one or other of the imposed methods of lifting. The six

subjects used were college students who were reported as having had 'some

21

previous experience in manual materials handling jobs'. This finding

illustrates a conflict between different assessment criteria. Lifting with

a stooped back is regarded as unsafe on biomechanical calculations because

of the forces generated in the lower spine. However, this and other studies

have shown workers to be willing to lift more in this fashion than with a

straight back/bent knee technique, presumably finding it easier.

These psychophysical criteria, particularly those of Snook and his

colleagues have an intrinsic appeal as loads which individuals find to be

acceptable. They are largely derived from industrial populations (unlike

many studies) and did not entail the imposition of unaccustomed lifting

practices. Within.the limitations of the handling situations studied they

provide comprehensive, comparatively easily applied assessment criteria.

However, the more recent papers by Ciriello and Snook (1983) and Mital

(1983) must cast some doubt on their validity. Arguably, the exceeding of

physiological criteria may be as much a reflection on the validity of these

criteria which, as indicated in the next section, are far from unequivocal.

However, the finding by Mital of an overestimation when tested against an

actual 8 hour shift causes more concern. Furthermore, as stated above, the

criteria are restricted to smooth, two-handed, symmetric lifting of

regularly shaped objects in good lifting conditions and therefore are

unlikely to be particularly appropriate in many industrial contexts.

2.3 Metabolic Criteria

In repetitive lifting the critical factor generally ceases to be

primarily muscular strength or the tolerance of other body components to

the biomechanical forces involved. Lifting tolerance becomes a function of

the ability of a particular individual to maintain an adequate oxygen and

thus energy supply to the working muscles, generally referred to as the

aerobic capacity or maximum oxygen uptake (Vo. max). Jorgensen and Poulsen

(1974) examined the relationship between oxygen uptake and lifting at

different frequencies. Using four male and four female subjects, the

authors demonstrated a curvilinear relationship between maximum lifting

frequency and relative load, based on an upper physiological limit of 50%

Vo_ max. These data were used, in conjunction with 5th percentile maximum

22

oxygen uptake population norms (mean - 2 sd) derived from Astrand and

Christensen (1964) and equivalent maximum permissible lift values calculated

according to Poulsen (1970), to derive a table of maximum lifting

frequencies for males and females of two ages (20 and 55 years) and two

heights (150 and 170 cm). The data were calculated as permissible maximum

lifting frequencies for different percentages of maximum lift (10, 25, 50

and 75%).

The values obtained are, in one respect, conservative in that both

oxygen uptakes and maximum lifts are based on the lower end of the normal

distribution. However, they are calculated on the assumption of 50% V6_ as

an acceptable work load for a normal shift whilst several authors, eg

Rodgers (1978) have suggested 33% to be a more realistic figure. The

situation is further complicated because values for Vo. max depend on the

manner in which the maximum is determined. Thus Petrofsky and Lind (1978)

drew attention to their observation that Vo. max measured by pedalling a

cycle ergometer (as were those from Astrand and Christensen) was higher than

that determined on lifting tasks. With lighter loads (7 kg) this

differential could be as much as 30%.

Garg and Saxena (1979) adopted a 'physiological fatigue criterion' of

5 Kcal min which is regarded as equivalent to 33% of the Vo? max of a

young, healthy, adult male (Garg and Ayoub, 1980), in their examination of

the metabolic load associated with acceptable loads at different lifting

frequencies as determined by a psychophysical technique. With a lifting

frequency of 3 lifts min , the average load lifted in a free style lift was

approximately 23 kg. However, the metabolic costs associated with this load

and those for stooped or bent knee lifting, were significantly lower than

5 Kcal min ; indicating, as was suggested by the authors, that muscle

strength rather than aerobic capacity appeared to be the limiting factor. A

similar result was observed at 6 lifts min for free style or stooped back

lifting, but not for bent knee lifting (presumably reflecting greater lower

limb strength). At the highest frequency (12 lifts min ) the metabolic

cost of bent-knee lifting was significantly higher than 5 Kcal min and

subjects were willing to lift far less than with other techniques. This was

23

considered to reflect the greater proportion of body weight being raised

with each shift. Free style lifting was found to have the lowest metabolic

cost per unit work, and a frequency of 9 lifts min was most efficient.

The authors did not derive any lifting limits based on their data. They did

however draw attention to the potential errors which could arise if data for

one lifting frequency were extrapolated to others.

Lind and Petrofsky (1978) examined the relationships between aerobic

capacity (Vo_ max) and fatigue in more detail. They found that fatigue

occurred above approximately 50% Vo. max (±10%) as measured for that lifting

load. This was regarded as an Important consideration because, for example

in lifting light boxes (7 kg), fatigue could occur at work loads greater

than 33% of the Vo. max as determined on a cycle ergometer.. j- - ,

Garg and Ayoub (1980) published a figure illustrating the effect of

lifting technique on physiological cost based on regression equations

published by Garg et al (1978). The figure showed that, for rates above 5

lifts min , at least twice as much could be lifted at a physiological cost

of 5 Kcal min using the stooped back technique than could be lifted using

a bent knee lift. A comparison of published lifting limits derived

according to psychophysical and physiological criteria (Garg and Ayoub,

1980) showed that those based on psychophysics were lower at low lifting

frequencies and higher at high frequencies.

In conclusion, estimates of the physiological limits which should be

placed on lifting work vary from 30-50% Vo. max. Limits in terms of energy_1 i

expenditures of 4.2 to 5.2 Kcal min have also been suggested (Astrand and

Rodahl, 1977; Garg et al 1978) which, from data published by Astrand and

Rodahl (1977) appear to be approximately equivalent. Some of the

"disagreement between authors may be a function of the manner in which the

maximum oxygen uptake (aerobic capacity) is determined, and it has been

suggested (Petrofsky and Lind, 1978) that the higher values are correct if

maximum aerobic capacity is determined for the task under examination rather

than on a cycle ergometer.

24

Snook et al (1970) suggested that whereas muscle strength was the

limiting psychophysical factor for lower frequencies/higher loads,

physiological fatigue was the dominant factor at higher frequencies/lighter

loads. However, although the observations of Garg and Saxena (1979) cited

above support this theory for lower frequencies, the influence of

physiological fatigue at higher lifting frequencies is more complicated.

Thus Lind and Petrofsky (1978) reported local muscle fatigue as a limiting

factor with low loads/high frequencies. The same authors published values

for extended work (4 hours) which showed subjects working at an average of

54% (±7%) of their aerobic capacity measured on the same task. This was

equivalent to an oxygen consumption for these subjects of 1.65 litres min

or an energy expenditure of 8.25 Kcal min , substantially above the 5 Kcal

min limit cited above. A limit of 50% Vo_ max may well therefore be a

realistic limit for repetitive, moderate weight lifting although for

heavier, less frequent loads or very light rapid lifting, local muscle

fatigue may restrict safe lifting capacity below such a limit. However, the

application of such a limit to industry would require the determination of a

lengthy series of task related 'norms' for aerobic capacity.

3. A 'HYBRID' CRITERION

NIOSH have recently published a work practices guide for manual lifting

(NIOSH, 1981). This examines lifting limits based on epidemiological

(injury rates), biomechanical (spinal forces), metabolic and psychophysical

criteria, and develops a hybrid series of limits based on these. It does

not make use of the limits derived by Davis and Stubbs (1977 et seq)

although it does briefly refer to them. The limits are divided into three

zones': (1) 'Acceptable Lifting Conditions', which is divided by the 'Action

Limit1 from 2) 'Administrative Controls Required1 within which either

administrative (personnel selection or training) or engineering (job

redesign) controls are required. This in turn is separated from the

'Hazardous Lifting Conditions' zone by the 'Maximum Permissible Limit*. The

Hazardous Lifting Zone is regarded as hazardous to any individual and such

jobs require engineering controls.

25

The three zones are defined by equations representing the Action Limit

(AL) and the Maximum Permissible Limit (MPL).

AL (Kg) = 40 (15/H) (1 - .004 (V - 75 )) (.7 + 7.5/D) (1 - F/Fmax)

MPL - 3 (AL)

where H = horizontal location (cm) forward of midpoint between

ankles at origin of lift; 15 = H = 80

V = vertical location (cm) at origin of lift; 0 = V = 175

D = vertical travel distance (cm) between origin and

destination of lift; 25* = D• - 200-V

F = average frequency of lift (lifts min ); .2 = F = max

0F = maximum frequency which can be sustainedmax

* For travel < 25, D - 25

+ For frequency < .2, F = 0

0 F varies with V and lifting periodmax

The Action Limit Loads can be safely lifted ('represents nominal risk1)

by over 99% of men, the MPL by 25%. The equation for the AL does not

require any data regarding the workforce. As F approaches F , the

frequency component in the equation approaches zero. Therefore, at F ,1 maxAL =-0 ie the equation does not consider any lifting at a frequency of F

-1 -i(12-18 lifts min ) to be acceptable. Varying lifting rates below 1 min

has little effect on AL as F approaches 1 at such rates.

A comparison of the Action Limits obtained using the equation with the

limits published by Snook shows some interesting differences. In the

26

examples the data of Snook (1978) have been extrapolated to the 99th

percentile (2.576 s.d. assuming a normal distribution) to facilitate

comparison with the NIOSH values. At all but the fastest.rate cited by

Snook (12 lifts min ) the NIOSH equation gives a much higher lifting limit

for lifting a compact object from the floor to knuckle height (typically

approximately 15 kg greater), a pattern which is repeated for shorter

lifting differences. However, with a larger object requiring to be held

further away from the body (hands 38 cm away rather than 18 cm away) the

values published by Snook are higher by 2-3 kg. A similar pattern emerges

with lifting in higher lifting zones (shoulder to reach) although the

differences are less marked (seldom more than 7 kg) and the faster lifting

rates follow the general pattern.

The NIOSH criteria apparently therefore place greater emphasis on the

biomechanical stresses imposed on the body, in particular the spinal cord,

when the centre of gravity of a load acts further away from the body, than

do the psychophysical limits developed by Snook. The latter limiting values

are referred to as acceptable limits rather than safe limits and it is

conceivable, particularly considering the insidious onset of much back pain,

that workers would be willing and able to lift loads which may have a

detrimental long-term effect on spinal discs or other structures associated

with the development of back pain.

The equations published by NIOSH are only applicable to smooth, two-

handed symmetric lifting in the sagittal plane, with good lifting

conditions. No reference is made to presumed level of training although the

workforce is assumed to be physically fit and accustomed to physical labour.

However, as training is specifically referred to as a form of administrative

control, it appears that no initial training is presupposed. No Indication

is given regarding the presumed effectiveness of subsequent training. Thus,

if the characteristics of a particular task fall between the AL and the MPL

indicating that administrative controls could be used, there is no way of

determining whether lifting training, in accordance with the procedure

defined in the NIOSH guidebook, can be regarded as rendering that task safe

for the trained workforce.

27

4. HEALTH AND SAFETY COMMISSION; REGULATIONS AND GUIDANCE

The Health and Safety Commission have published a consultative

document containing proposals for regulations and guidance on the manual

handling of loads (HSC, 1982). The draft regulations prohibit the manual

handling of any load likely to injure an individual because of '(a) its

weight, shape, size or lack of rigidity; or (b) the frequency with which he

handles loads; or (c) the conditions under which the load is to be handled*.

To facilitate compliance with these regulations the HSC have published a

guidance document which is intended to identify the sources of risks and

hazards involved in manual handling and to provide guidance about positive

steps which can be taken to minimise these risks. However, this document

has since been considerably revised and a new consultative document is

expected. It is not therefore appropriate to discuss the previous version

in detail.

':. V;- V

5. CONCLUSIONS

Many different limits to lifting have been proposed, based on a variety

of criteria. With the exception of the data reported by Ridd (1983), they

are restricted in their application to smooth lifting in good conditions and

are further limited to symmetrical, compact objects. Despite the initial

intrinsic appeal of limits derived on psychophysical principles they have a

potential inherent risk in that the epidemiology of back pain would suggest

that workers engaged in manual handling may willingly handle loads which may

be having a long-term detrimental effect. Furthermore, recent papers have

questioned the validity of the limits determined in this manner.

Other sets of guidelines, although based on a common criterion (eg,

-muscle strength), provide conflicting sets of values with no immediate way

==of .determining which, if any, are correct.

The guidelines recently published by the American National Institute of

Occupational Safety and Health (NIOSH, 1981) are a compromise between a

variety of approaches which may remove some of the deficiencies of the

individual methods (although they may equally compound them!). However, in

addition to the fact that they are only applicable to smooth, two-handed

28

lifts, the guidelines have further shortcomings which would require

clarification before they could be usefully employed in industry.

Furthermore, the apparently complex equation may well discourage its routine

industrial use.

The absence of any useable limits suggests that the best approach to

the assessment of manual handling tasks may include some form of direct

measurement of the strain imposed on the body as a result of manual

handling. Although there are reservations regarding the precise validity of

the assessment criterion currently employed (see section 2.1.2), the

measurement of intra-abdominal pressure, if only as a comparative measure,

would appear to be the most immediately practicable in an industrial setting

and worthy of further investigation.

ACKNOWLEDGEMENT

This report was prepared during the joint NCB/ECSC funded project

number 7247/12/014 (An Ergonomics Evaluation of the Haulage and

Transportation of Mining Supplies).

29

6. REFERENCES

ANDERSSON G B J, ORTENGREN R, NACHEMSON A (1977) Intradiskal

pressure, intra-abdominal pressure and myoelectric back muscle activity

related to posture and loading. Clinical Orthopaedics and Related Research;

129: 156-164.

ANDERSSON G B J, ORTENGREN R, NACHEMSON A (1978)

Studies of back loads in fixed spinal postures and in lifting. In:

Drury C G, ed. q.v: 26-33.

ANDERSSON G B J (1980) Biomechanics of the lumbar spine. In:

Pain S L, ed. Clinics in Rheumatic Diseases; 6 (1) Low back pain. London:

Saunders.

ASMUSSEN E, HEEBOLL-NIELSON K (1961) Isometric muscle strength of

adult men and women. COPENHAGEN: N.A.I.P. (Communications from the Testing

and Observation Institute of the Danish National Association for Infantile

Paralysis. No 11).

ASMUSSEN E E, POULSEN E, RASMUSSEN B (1965) Quantitative evaluation

of the activity of the back muscles in lifting. COPENHAGEN: N.A.I.P.

(Communications from the Testing and Observation Institute of the Danish

National Association for Infantile Paralysis. No 21).

ASTRAND P 0, CHRISTENSEN E H (1964) Aerobic work capacity. In:

Dickens F, Neie E, and Widdas W F, Oxygen in the animal organism. New York

: Pergamon; 295-325.

ASTRANp P.;Q,_RODAHL K (1977) Textbook of work physiology. 2nd ed.

New York : McGraw-Hill.

AYOUB M M (1977) Lifting capacity of workers. Journal of Human

Ergology, 6, 187-192.

30

AYOUB M M, DRYDEN R D, McDANIEL J W, KNIPFER R E, AGHAZADEH F (1978)

Modelling of lifting capacity as a function of operator and task variables.

In: Drury C G, ed. q.v. 120-130.

AYOUB M M, EL-BASSOUSSI M M (1978) Dynamic biomechanical model for

sagittal plane lifting activities. In: Drury C G, ed. q.v., 88-95.

BARTELINK D L (1957) The role of abdominal pressure in relieving the

pressure on the lumbar intervertebral discs. The Journal of Bone and Joint

Surgery; 39B: 718-725.

CARLSOO S (1978) Testing the back and lifting capacity. Scand. J.

of Rehab. Med.; 9: 164-8.

CARTER P J (1969) Manual lifting and carrying. Work Study and

Management Services; June: 362-365.

CIRIELLO V M, SNOOK S H (1983) A study of size, distance, height and

frequency effects on manual handling tasks. Human Factors; 25: 473-483.

DAVIS P R, TROUP J D G (1966) Effects on the trunk of erecting pit

props at different working heights. Ergonomics; 9: 475-484.

DAVIS P R, STUBBS D A (1977a) Safe levels of manual forces for young

males (1). Applied Ergonomics; 8: 141-150.

DAVIS P R, STUBBS D A (1977b) Safe levels of manual forces for young

males (2). Applied Ergonomics; 8: 219-228.

DAVIS P R, STUBBS D A (1978a) Safe levels of manual forces for young

males (3). Applied Ergonomics; 9: 33-37.

DAVIS P R, STUBBS D A (1978b) A method of establishing safe handling

forces in working situations. In: Drury C G, ed. q.v. 34-38.

DAVIS P R (1981) The use of intra-abdominal pressure in evaluating

stresses on the lumbar spine. Spine; 6: 90-92.

31

DRURY C G, PFEIL R E (1975) A task-based model of manual lifting

performance. Int. J. Prod. Res; 13: 137-148. International Journal of

Production Research.

DRURY C G (Ed) (1978) Safety in manual materials handling.

Cincinnati : N.I.O.S.H. DREW (NIOSH) Publication No. 78-185.

DRYDEN R D (1973) A predictive model for the maximum permissible

weight of lift from knuckle to shoulder height. PhD. Dissertation, Texas

Tech University, Lubbock, Texas. Cited in Ayoub et al (1978) q.v.

EIE N, WEHN P (1962) Measurements of the intra-abdominal presure in

relation to weight bearing of the lumbosacral spine. Journal of the Oslo

City Hospitals; 12: 205-217.

FISH D R (1978) Practical measurement of human postures and forces in

lifting. In: Drury CG, ed. q.v., 72-77.

GARG A, CHAFFIN D B, HERRIN G D (1978) Prediction of metabolic rates

for manual materials handling jobs. Am. Ind. Hyg. Assoc. J.; 39: 661-

674.

GARG A, SAXENA U (1979) Effects of lifting frequency and technique on

physical fatigue with special reference to psychophysical methodology and

metabolic rate. Am. Ind. Hyg. Assoc. J; 40: 894-903.

GARG A, AYOUB M M (1980) What criteria exist for determining how much

load can be lifted safely? Human Factors, 22: 475-486.

• ".:;;... GRAVELING R A (1984) Forces in awkward postures. Institute of

Occupational Medicine, (Ergonomics Report in preparation).

GRAVELING R A, SIMPSON G C, MABEY M H, FLUX B, HODGE C J, LEAMON T B

(1980) An investigation of stress on coalface workers and the temporal

variation of such stress. Edinburgh: Institute of Occupational Medicine

(IOM Report TM/80/7).

32

HEALTH AND SAFETY COMMISSION (1982) Proposals for Health and Safety

(Manual handling of loads) regulations and guidance. London : HMSO.

BUTTON W C, ADAMS M A (1982) Can the lumbar spine be crushed in heavy

lifting? Spine; 7: 586-590.

JORGENSEN K, POULSEN E (1974) Physiological problems in repetitive

lifting with special reference to tolerance limits to the maximum lifting

frequency. Ergonomics; 17: 31-39.

KNIPFER R E (1974) Predictive models for the maximum acceptable

weight of lift. PhD Dissertation. Texas Tech University, Lubbock, Texas.

Cited in Ayoub et al (1978) q.v.

LEGG S J (1981) The effect of abdominal muscle fatigue and training

on the intra-abdominal pressure developed during lifting. Ergonomics; 24:

191-195.

LIND A R, PETROFSKY J S (1978) Cariovascular and respiratory

limitations on muscular fatigue during lifting tasks. In: Drury C G, ed.

(1978) q.v., pps 57-62.

MARTIN J B, CHAFFIN D B (1972) Biomechanical computerised simulation

of human strength in sagittal-plane activities. Trans. Am. Inst. Ind.

Eng.; 4: 19-28.

McDANIEL J W (1972) Prediction of acceptable lift capability. PhD

Dissertation. Texas Tech University, Lubbock, Texas. Cited in Ayoub et al

(1978) q.v.

MITAL A (1983) The psychophysical approach in manual lifting - a

verification study. Human Factors, 25: 485-491.

MOLBECH S (1963) Average percentage force at repeated maximal

isometric muscle contractions at different frequencies. COPENHAGEN:

N.A.I.P. (Communications from the Testing and Observation Institute of the

Danish National Association for Infantile Paralysis. No 16).

33

NACHEMSON A (1960) Lumbar intradiscal pressure. Experimental studies

on post-mortem material. Acta Orthop Scand. Suppl. 43 cited in SCHULTZ, A.

et al (ibid).

NACHEMSON A (1965) The effect of forward leaning on lumbar

intradiscal pressure. Acta. Orthop. Scand; 36: 314-328.

NACHEMSON A (1966) The load on lumbar disks in different positions of

the body. Clinical Orthopaedic Surgery; 45: 107-122.

NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH (NIOSH) (1981) .

Work practices guide for manual lifting. Cincinnati: N.I.O.S.H. (DHHS

(NIOSH), Publication No 81-122.)

NICHOLSON A S, DAVIS P R, SHEPPARD N J (1981) Magnitude and

distribution of trunk stresses in telecommunication engineers. British

Journal of Industrial Medicine; 38: 364-371.

PETROFSKY J S, LIND A R (1978) Metabolic, cardiovascular and

respiratory factors in the development of fatigue in lifting tasks. Journal

of Applied Physiology; 45: 64-68.

POULSEN E (1970) Prediction of maximum loads in lifting from

measurements of muscular strength. Comm. from The Danish National Assoc.

for Infant. Paral. No. 31. Cited in Jorgensen and Poulsen (1974) q.v.

POULSEN E, JORGENSEN K (1971) Back muscle strength, lifting, and

stooped working postures. Applied Ergonomics; 2: 133-137.

POULSEN E (1978) Studies of back load, tolerance limits during

lifting of burdens. Scand. J. of Rehab. Med; 9: 169-172.

PYTEL J L, KAMON E (1981) Dynamic strength test as a predictor for

maximal and acceptable lifting. Ergonomics; 24: 663-672.

RIDD J E (1983) Spatial restraints and intra-abdominal pressure.

Seminar on Prevention of Low Back Pain. Luxembourg: ECSC.

34

RODGERS S H (1978) Metabolic indices in materials handling tasks.

In: Drury C G, ed. (1978) q.v., pps 52-56.

t*

SCHULTZ A, ANDERSSON G, ORTENGREN R, HADERSPECK K, NACHEMSON A (1982)

Loads on the lumbar spine. Validation of a biomechanical analysis by

measurements of intradiscal pressures and myoelectric signals. J of Bone

and Joint Surgery; 64-A; 713-720.

SNOOK S H, IRVINE C H (1967) Maximum acceptable weight of lift.

American Ind. Hyg. Assoc. Journal; 28: 322-329.

SNOOK S H, IRVINE C H, BASS S F (1970) Maximum weights and work loads

acceptable to male industrial workers. Am. Ind. Hyg. Assoc. Journal; 31:

579-586.

SNOOK S H (1978) The design of manual handling tasks. Ergonomics;

21: 963-985.

STUBBS D A (1973) Manual handling in the construction industry.

Construction Industry Training Board Report, University of Surrey. .

STUBBS D A (1981) Trunk stresses in construction and other industrial

workers. Spine; 6: 83-89.

TROUP J D G (1979) Biomechanics of the vertebral column.

Physiotherapy; 65: 283-244.

35

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