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NIOSHequationhorizontaldistancesassociatedwiththeLibertyMutual(Snook)liftingtableboxwidths.ARTICLEinERGONOMICSJULY1997ImpactFactor:1.61DOI:10.1080/001401397187946Source:PubMed

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JimRPotvinMcMasterUniversity76PUBLICATIONS1,118CITATIONS

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NIOSH equation horizontal distances associated with the

Liberty Mutual (Snook) lifting table box widths

J. R. POTVIN and L. R. BENT

Biomechanics Laboratory, Department of Human Biology and Nutritional

Sciences, University of Guelph, Guelph, Ontario, Canada NIG 2W1

Keywords: Lifting; Psychophysics; NIOSH equations; Prevention and control;Evaluation methodology.

A study was conducted to determine the NIOSH equation horizontal distances

(dH ) associated with the three different box widths (34, 49 and 75 cm) and lift

starting heights ( oor, knuckle and shoulder) used in the psychophysically basedLiberty Mutual lifting tables (Snook 1978, Snook and Ciriello 1991). Data werecollected with 12 male and 12 female subjects and three repetitions were

performed for each of the nine lifting conditions. No gender effects were observed

so male and female data were pooled. The value of dH was positively related tobox width but there was also a signi cant interaction between box width and

starting height. When pooled across lift heights the average values of dH were 44,49 and 57 cm for the 34, 49 and 75 cm box widths, respectively. When pooled

across box widths the average values of dH were 52, 45 and 52 cm for the oor,knuckle and shoulder height lifts, respectively. A knowledge of the dH associated

with each box width will allow for direct comparisons to be made between theNIOSH and Liberty Mutual outputs. This will facilitate further validation of the

NIOSH equations. A variable (GAP) was calculated to indicate the horizontaldistance from the ankles to the edge of the box. Previously, this GAP has been

assumed to remain constant and values of 15, 20 and 25 cm have been proposed.The GAP was observed to have an overall mean of 23 6 cm with individualcondition means ranging from 14 6 cm to 31 2 cm. When pooled acrossconditions the mean GAP values were equal to dH minus half the box width.

1. Introduction

Many efforts have been made to provide criteria for establishing acceptable loads

under various sagittal plane lifting conditions. Two methods commonly used for this

purpose are: (1) the Liberty Mutual (LM) lifting tables based on a series of

psychophysical studies by Snook and colleagues (Snook 1978, Snook and Ciriello

1991) and, (2) the NIOSH lifting equations developed as an integration of

biomechanical, psychophysical, physiological and epidemiological data (NIOSH

1981, Waters et al. 1993). Both methods use measures that account for the load s

vertical location and displacement as well as the frequency of lifting. NIOSH (1981)

have identi ed the horizontal distance of the load centre of mass from the body (dH )

as the critical determinant of low-back injury risk owing to its effect on the lumbar

moment and subsequent joint compression forces. Consequently, the two methods

have a fourth measurement to account for the horizontal location of the load

although they do this in different ways. Snook and colleagues (1978, 1991)

manipulated box widths in their studies to alter the horizontal location of the load.

Therefore, to apply the LM lifting tables to various lifting conditions, some estimate

ERGONOMICS, 1997, VOL. 40, NO. 6, 650 655

0014 0139/97 $12 00 1997 Taylor & Francis Ltd

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of the box width must be made. Conversely, the NIOSH equations use a direct

measurement of dH as the distance from the centre of mass of the load to the centre

of the ankle joints.

A dif culty in using the LM lifting tables is that the dH associated with a load is

not always determined by the size of the load lifted. For example, when lifting over

an obstacle the dH will be large even for a small box width. In such a case, it would be

more appropriate to recommend safe loads based on table values corresponding to a

box width larger than that of the actual load. In addition, the discrepancy in the

methods used by the NIOSH equations and the LM tables, to account for the

horizontal location of the load, makes it dif cult to compare the output from each

method for similar lifting conditions. Such comparisons are essential to ongoing

efforts to validate the recently proposed NIOSH equation (Waters et al. 1993). For

the original NIOSH equation (NIOSH 1981) it was assumed that the dH associated

with a LM box was half the width plus a constant distance of 15 cm. Subsequently,

the revised NIOSH equation used constant values of 25 and 20 cm when the load

was below and above 75 cm, respectively. Garg (1989) tested 13 male subjects and

demonstrated that these constant values were too low when loads were lifted at oor

and knuckle height and found that the most appropriate values changed with lifting

height.

The purpose of the current study was to determine the NIOSH dH associated

with each combination of the three box widths and three starting heights in the LM

lifting tables. It is anticipated that the data from this study will be useful for: (1)

facilitating use of the LM tables when dH is not constrained primarily by box width,

(2) allow for direct comparisons to be made between the NIOSH equation and LM

lifting table outputs for similar lifting conditions, and (3) determine the validity of

using half the width plus a constant distance when converting box width to dH .

2. Methods

2.1. Subjects

Twenty-four healthy subjects (12 females, 12 males) participated in the experiment.

The mean height was 1 653 6 0 052 m (female), 1 794 6 0 067 m (male) and1 723 6 0 092 m (overall). The mean age was 23 5 6 2 4 years (female), 23 9 6 2 6years (male) and 23 7 6 2 5 years (overall). All subjects were informed of theprocedures and risks involved with the protocol and each signed an informed

consent form. None of the subjects reported any history of low-back pain or injury.

2.2. Tasks

Three boxes were constructed with the same dimensions as those used in the LM

lifting tables (Snook 1978, Snook and Ciriello 1991). Each box had a length (lateral

span) of 48 cm, a height of 14 cm and handles with a height of 7 5 cm from the base.The boxes differed only in their widths (frontal span) which were 34, 49 and 75 cm.

Each box was loaded such that its total mass was 5 kg.

Subjects were given a warm-up period to become accustomed to each lifting

condition. They then performed lifts with each of the three boxes at the three

different handle starting heights from the LM tables: (1) 7 5 cm (box on the oor),(2) knuckle height, and (3) shoulder height. The knuckle and shoulder heights were

adjusted to the dimensions of each individual subject. The order of presentation of

the starting heights was randomized for each subject. Within a height, the order of

box widths was randomized. Three repetitions were performed for each of the nine

651NIOSH equation horizontal distances

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lifting conditions (n = 27 lifts per subject). Subjects were instructed to approach the

box with at least three strides, grasp the handles in the centre and lift the box

symmetrically in the sagittal plane. They were provided with no other instructions as

to lifting technique.

2.3. Data analysis

Each lift was recorded on videotape (20 Hz) with a camera placed 6 m lateral to the

subject s right side. The image of a metre stick was recorded so that all data could be

scaled. Markers on the ankle (lateral malleolus) and the knuckle (3rd metacarpal)

were digitized to determine the NIOSH dH (cm) at the start of each lift (NIOSH

1981, Waters et al. 1993). The dH value directly attributable to the width of the box

(W ) can be calculated as half of W (the distance from the edge of the box to the

knuckle). Generally dH is larger than12W and this discrepancy was termed the `GAP .

The relationship between these variables is indicated below:

GAP 5 dH 21

2W

where GAP = distance from the ankle to the edge of the box (cm); dH = horizontal

distance from the ankle to the knuckle (cm); and W = width of the box being lifted

(cm).

2.4. Statistics

A mean value was calculated with the dH values measured from the three repetitions

performed with each starting height/box width combination. The coef cients ofvariation (CV) were calculated and pooled within the three repeat measures of dH for

each individual condition/subject. CV values were also calculated between subjectswith the means and standard deviations for each condition. A 3-way ANOVA with

repeated measures (1 within subjects was gender, 2 between subjects were box size

and height) with the GAP variable used as the dependent measure. A post hoc

analysis was performed on all signi cant main effects and interactions. The digitizing

procedure was repeated a second time for one subject so that digitizing reliability

could be assessed.

3. Results

3.1. Starting heights

The average knuckle heights were 0 716 6 0 026 m (females), 0 773 6 0 032 m (males)and 0 746 6 0 042 m (overall). The average shoulder heights were 1 295 6 0 055 m(females), 1 434 6 0 063 m (males) and 1 362 6 0 091 m (overall). The starting heightof the lifts from the oor were always 7 5 cm as this was the height of the handlesfrom the base of each box.

3.2. GAP distances

For the GAP distance, there were signi cant main effects of each box width and

starting height and a signi cant interaction effect (all p

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As expected, the average dH increased with increasing box width (table 2). The

observed changes in dH were in contrast to those of Garg (1989) who noted

substantial increases in dH as the starting height increased from oor to knuckle

level. At oor level, the results of Garg for 38 and 51 cm wide boxes were

comparable to those of the current study for 34 and 49 cm box widths. There were,

however, large discrepancies observed when dH was compared at knuckle height. As

the load was moved from oor to knuckle level, Garg observed average increases in

dH of 9 3 and 11 1 cm for the 38 and 51 cm boxes, respectively. Conversely, undersimilar conditions in the current study, the dH decreased by 6 2 and 7 5 cm,respectively. It is dif cult to explain these discrepancies at knuckle height as there do

not appear to be substantial differences between the task characteristics of the two

studies. It should be noted that while the CV values for the current study s 34 and

49 cm lifts were 11 1 and 8 1% , respectively, Garg s CVs for similar boxes were 15 8and 14 8% . This may indicate some differences in the reliability of the methods usedto quantify the dH as box width was changed.

The current values can be used to facilitate use of the LM lifting tables when the

box width is not the only factor determining the dH of the load from the body. In

many cases, small loads must be held at a distance from the body because of

obstacles to lifting (e.g. lifting from a bin or the back of a table). Therefore, always

associating the smallest box size in the LM lifting tables (34 cm) with a small load

may lead to a MAWL that overestimates the actual safe load. Substituting the

proposed dH values for the current box widths in the LM lift tables would allow for a

more accurate account of the load dH regardless of load size or lifting obstacles. Such

a substitution would result in both the NIOSH equations and the LM lifting tables

having similar inputs so that their outputs could be compared. This would further

facilitate current efforts to validate the NIOSH equation s psychophysical basis.

The results of the GAP variable provided direct evidence that the subjects moved

closer as the boxes became wider (table 1). The average GAP was observed to

decrease from 27 3 cm with a 34 cm box width to 19 0 cm with a 75 cm box width.This may have been done to minimize the biomechanical implications of an increased

obstacle size. Both NIOSH equations suggest that increases in dH will result in

substantial decreases in the acceptable load or, alternatively, increases in the injury

risk associated with a particular load (NIOSH 1981, Waters et al. 1993). The average

GAP magnitudes also appeared to be affected by the starting height, with overall

average values being 18 7 cm at knuckle height and approximately 26 cm at both oor and shoulder heights. This result may indicate that subjects were able to move

Table 2. Average NIOSH equation horizontal moment arms (cm) associated with each

combination of box width and lift starting height. The data had been pooled acrossgenders (n= 24 for each cell, n= 72 for each column and row mean, and n= 216* for the

overall mean).

Snook table box width

Lift startingheight 34 cm 49 cm 75 cm Mean

Shoulder

Knuckle

FloorMean

48 239 245 444 3

51 543 851 348 9

57 052 160 556 5

52 245 052 449 9*

654 J.R. Potvin and L.R. Bent

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the load in close to the body at knuckle height but some larger clearance was needed

when subjects were required to reach the hands down to oor level or up to shoulder

level. The lower dH values observed at knuckle height were consistent with the

NIOSH equations assumption that this is the optimal vertical height for lifting

(NIOSH 1981, Waters et al. 1993). The average GAP of 23 6 cm was in the range of20 25 cm assumed by Waters et al. (1993) and 25 cm assumed by Chaf n and Page

(1994), however it is substantially larger than the value of 15 cm used by NIOSH

(1981). The most recent NIOSH equation appeared to be correct in assuming that

the GAP was greater when V was below knuckle height (25 versus 20 cm). However,

the current results indicate that a value of 20 cm should not be set for all V>75 cmas the average GAP was found to be 25 9 cm at shoulder height. It is proposed thatthe GAP values provided in table 1 are more appropriate than the constant values of

15, 20 or 25 cm that have been used to date.

5. Conclusions

It was concluded from the current study that female and male subjects adopted

similar dH in response to changing box widths and relative lift starting heights. It is

proposed that the dH values determined in this study may be substituted for the box

widths currently in the LM lifting tables. This will allow the psychophysically based

tables to be applied under more varied lifting conditions than is currently possible. In

addition, such a substitution would result in similarly measured inputs for the

NIOSH equation and the LM lifting tables. This would facilitate a comparison

between the output of both methods for a variety of lifting conditions. Although the

average GAP was 23 6 cm for all conditions tested, there was a large range (16 6 cm)of GAP observed between the conditions. This observation disagrees with the

previous assumption of a constant GAP made during the development of the

NIOSH equations.

ReferencesCHAFFIN, D. B. and PAGE, G. B. 1994, Postural effects on biomechanical and psychophysical

weight-lifting limits, Ergonomics, 37, 663 676.

GARG, A. 1989, An evaluation of the NIOSH guidelines for manual lifting, with special

reference to horizontal distance, American Industrial Hygiene Association Journal, 50,157 164.

NIOSH 1981, Work practices guide for manual lifting. Technical report, DHHS (NIOSH)Publication No. 81, 122.

POTVIN, J. R., NORMAN, R. W., ECKENRATH, M. E., MCG ILL, S. M. and BENNETT , G. W. 1992,Regression models for the prediction of dynamic L4/L5 compression forces duringlifting, Ergonomics, 35, 187 201.

SNOOK , S. H. 1978, The design of manual handling tasks, Ergonomics, 21, 963 985.

SNOOK , S. H. and C IRIELLO, V. M. 1991, The design of manual handling tasks: revised tables ofmaximum acceptable weights and forces, Ergonomics, 34, 1197 1213.

WATERS, T. R., PUTZ-ANDERSON, V., GARG, A. and FINE, L. J. 1993, Revised NIOSH equationfor the design and evaluation of manual lifting tasks, Ergonomics, 36, 749 776.

655NIOSH equation horizontal distances