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ORIGINAL ARTICLE

Gender differences in psychophysically determined maximumacceptable weights and forces for industrial workers observedafter twenty years

Vincent M. Ciriello Rammohan V. Maikala

Patrick G. Dempsey Niall V. OBrien

Received: 18 May 2010 / Accepted: 1 October 2010 / Published online: 16 October 2010

Springer-Verlag 2010

Abstract

Purpose In the year 1991, manual materials handling

guidelines were published by Liberty Mutual Research

Institute for Safety. In these guidelines, maximum accept-

able weights (MAWs) and forces (MAFs) for lifting,

lowering, pushing, pulling, and carrying were derived from

studies conducted in a 20 year span before the above

publication date. The question is whether the present

generation of workers has retained the same gender dif-

ferences and absolute values in psychophysically deter-

mined MAWs and MAFs as those reflected in the

guideline.

Methods Twenty-four female industrial workers per-

formed 20 variations of lifting, lowering, pushing, pulling,

and carrying. A psychophysical methodology was used

whereby the workers chose a workload they could sustain

for 8 h without straining themselves or without becoming

unusually tired, weakened, overheated or out of breath.

Results In females, MAWs of lifting, lowering, and car-

rying averaged 53% of the present-day male values, similar

to the 55% in the guideline. MAFs of pushing and pulling

were 83 and 86% of the present-day male values but

slightly higher than the 73 and 78% in the guideline,

respectively for initial and sustained forces.

Conclusions The similarity of gender differences between

the guideline and the present findings was coupled with

dramatic decreases in MAWs of lifting, lowering, and car-

rying. Such decreases may reflect a new psychophysical set

point; however, considerations about adjusting existing

guidelines on lifting, lowering, and carrying may not be

appropriate until additional data from other sources inside

and outside the US confirm the present findings.

Keywords Psychophysics Manual materials handlingguidelines Lifting and lowering Pushing and pulling Carrying Ergonomic redesign

Introduction

When manual material handling (MMH) tasks are designed

to be within the acceptable limits for a high percentage of

the industrial population, there is a twofold advantage of

accommodating the workplace for workers with and

without low back disability (Snook et al. 1978; Benson

1986, 1987; Snook 1987; Ciriello and Snook 1999; Ciriello

et al. 1999). This is important because MMH is the most

frequent (36% of all claims) and costly (35% of total cost)

category of compensable loss (Leamon and Murphy 1994;

Murphy et al. 1996; Dempsey and Hashemi 1999). MMH

tasks are also associated with the largest proportion

(6370%) of compensable low back disability (Snook et al.

1978; Bigos et al. 1986; Murphy and Courtney 2000). To

establish acceptable workloads in MMH, investigators

have used a variety of work evaluation approaches

including physiological, biomechanical, subjective, obser-

vational, focus groups, psychophysical, postural analysis,

and a combination of the all these (Kemper et al. 1990;

Kivi and Mattila 1991; Waikar et al. 1991; Burdorf et al.

1992; Waters et al. 1993; de Looze et al. 1994; Winkel and

Mathiassen 1994; Straker et al. 1996; Van der Beek et al.

2005; Bust et al. 2005).

By relating human sensation to a physical stimulus

which is the basis for the psychophysical technique,

V. M. Ciriello (&) R. V. Maikala P. G. Dempsey N. V. OBrien

Liberty Mutual Research Institute for Safety,

71 Frankland Road, Hopkinton, MA 01748, USA

e-mail: [email protected]

123

Int Arch Occup Environ Health (2011) 84:569575

DOI 10.1007/s00420-010-0589-0

Snook and Ciriello (1991) have developed maximum

acceptable weights (MAWs) and forces (MAFs). These

MAWs and MAFs can be easily applied in field situations

compared to other techniques mentioned above and have

been used extensively to redesign workplaces (Benson

1986, 1987; Ciriello and Snook 1999; Ciriello et al. 1999).

The development of these MMH guidelines was derived

from the studies conducted during a 21 year span prior to

their publication in 1991. However, it is not known whe-

ther the present generation of female workers has retained

the same gender differences in psychophysically deter-

mined weights and forces as those reflected in the 1991

guideline. The present-day male equivalents that were used

for the comparisons were from a recent study (Ciriello

et al. 2008). Therefore, the current study investigated

gender differences in typical lifting, lowering, pushing

pulling, and carrying tasks with present-day local industrial

workers and compared the results to the gender differences

observed in the study by Snook and Ciriello (1991). The

second question of interest was to investigate the extent of

change in absolute values of MAWs and MAFs. The third

question of this investigation was whether the relationship

between task variables (frequency, height of lifting or

lowering, lifting vs. lowering, pushing vs. pulling) have

remained similar to those reported in earlier studies. With

the above questions answered, we can assess the extent to

which the industrial population has changed in regard to

males versus females, variable effects changes, and abso-

lute value changes, indicating a shift in the psychophysical

set point. This information may provide a starting point for

modifications to future guidelines.

Subjects and methods

Subjects

Twenty-four female industrial workers were recruited by

newspaper advertisements, and their employment history

was checked to ensure that they had enough industrial

experience to make good psychophysical judgments. They

were then examined by a nurse-practitioner to ensure that

they had no serious cardiovascular problems and had not

experienced previous significant low back pain or muscu-

loskeletal problems of the extremities. Before participation,

subjects signed a written informed consent, which was

approved by our institutional review board. During the

experiment, the subjects were paid for every hour they

worked at the industrial factory rate which was reported in

the Wall Street Journal during the time of the experiment.

Participants shoulder, elbow, and knuckle heights were

measured with an GPM anthropometer to set the ranges for

the lifting and lowering tasks and the heights for pushing

and pulling. These measurements along with stature and

body mass were also compared with military and industrial

populations to ensure similarity with our subjects. The

subjects mean (SD) values for age, body mass, stature,

shoulder height, elbow height, and knuckle height were

40.4 (9.65) years, 73.7 (15.6) kg, 164.2 (6.4) cm, 134.4

(5.9) cm, 104.8 (4.3) cm, and 73.8 (3.6) cm, respectively.

The comparisons of the above measures with previous

studies yielded a median difference of ?10.5, ?1.3, ?0.7,

?2.2, and ?1.8%, respectively, for body mass, stature,

shoulder height, elbow height, and knuckle height (Ciriello

2004; Ciriello et al. 1990; Eastman Kodak and Human

Factors Section 1986; Gordon et al. 1989; Marras and Kim

1993; Snook and Ciriello 1974).

Industrial tasks

Subjects performed 20 variations of lifting, lowering,

pushing, pulling, and carrying. During lifting and lowering

tasks, a plastic box with external wood handles was used.

The external handles were 17.8 cm long 9 4.2 cm thick

and devoid of sharp edges. This box, which represented a

common small industrial tote box, had the following

dimensions: width, 33.4 cm; length, 56.2 cm; and depth,

16.0 cm. The width represents the box distance in the

sagittal plane away from the body, and the length repre-

sents the distance from the outside edge of the left handle

to the outside edge of the right handle. The handles were

placed midway in the width dimension. It is important to

note that these tasks and the specific box were also used as

a criterion in the 1991 guideline (Snook and Ciriello 1991).

Subjects performed lifting and lowering tasks on pneu-

matically activated shelves that automatically moved to a

specified vertical location and then returned the box to the

original location. The mechanisms of automated shelves

were described in a greater detail in Snook (1978). Briefly,

when the subjects slid the boxes off the shelf, the shelf

quickly moved to a new predetermined position in time for

the subject to place the box back on the shelf. The shelf

then moved back to the original position, thus completing

the lifting or lowering cycle. Lifting the boxes straight up

in a vertical plane was deterred by asking the subject to

imagine a rack of shelves above or below the box to be

lifted or lowered. In most cases, this instruction resulted in

some degree of body twisting during lifting and lowering.

Lifts and lowers had a vertical distance of 51 cm and

were studied within two ranges: between floor level and

knuckle height (low lift/lower), and between knuckle

height and shoulder height (center lift/lower). The lifting

and lowering tasks were performed at frequencies of 12,

4.3, and 1 min-1.

The carrying task required subjects to carry the box at

knuckle height for a distance of 4.3 m at frequencies of 4.3

570 Int Arch Occup Environ Health (2011) 84:569575

123

and 1 min-1. A combination task consisted of a lift, a

carry, and a lower. For example, the subject lifted the box

from the floor to knuckle height, turned 90 degrees, walked

2.1 m, turned 90 degrees again, and lowered the box to the

floor. The combination task was also performed at fre-

quencies of 4.3 and 1 min-1.

Dynamic pushing and pulling tasks were simulated on a

specially constructed treadmill (Snook 1978). During

pushing and pulling, the treadmill was powered by the

subjects as they pushed or pulled against a stationary bar. A

load cell on the stationary bar measured the horizontal

force being exerted. The force required to get the treadmill

belt moving is called the initial force. The force required to

keep the belt moving is called the sustained force. Subjects

controlled the resistance of the treadmill belt by varying

the amount of electric current flowing into the magnetic

particle brake that transferred resistance to the drum around

which the belt travels. The current controller was devoid of

visual cues and positioned within arms length of the

subject. Subjects turned the control knob clockwise to

increase the resistance and counterclockwise to decrease

resistance. The control knob could be adjusted before,

during, or after each push. Pushing and pulling tasks were

performed for a distance of 7.6 m and at frequencies of 4

and 1 min-1. Subjects pushed and pulled at a height mid-

way between knuckle and elbow height, an average of

89.3 cm for the group.

Experimental procedure

The psychophysical methodology, as described previously

(Ciriello and Snook 1983; Snook and Ciriello 1991;

Ciriello et al. 1993), was used in this experiment. In brief,

subjects were instructed to adjust the amount of weight or

force until it represented the maximum they could handle

for 8 h without straining themselves or without becoming

unusually tired, weakened, overheated, or out of breath.

During lifting, lowering, and carrying tasks, subjects varied

the weight of the tote boxes by adding or removing steel

shot. In an attempt to minimize visual cues, the boxes

contained false bottoms that could hold up to 11 kg. Sub-

jects were aware of the false bottoms but never knew how

much weight they contained. The amounts of weight in the

false bottom were randomly varied.

All subjects dressed in surgical type scrub suits to

control for heat dissipation. They were also provided with

similar type shoes. The coefficient of friction (COF)

between the shoe sole material and the treadmill belt were

determined by a Brungraber Slip-tester (Model mark II)

and resulted in a COF of 0.86. Three training sessions were

conducted to gradually condition the subjects to the dif-

ferent tasks and to enable them to gain experience in

adjusting weight and force. The training progressed as

follows: on day 1 subjects performed six 10 min tasks; on

day 2, six 20 min tasks; and on day 3, six 30 min tasks. On

days 4 through 8, subjects performed five 40 min tasks in

4 h with 10 min breaks between tasks.

At the beginning of each task, subjects were given a box

weight or treadmill force that was randomly selected and

alternately high (1827 kg) or low (211 kg). During the

next 20 min, subjects adjusted the weight or force according

to instructions. At the end of 20 min, subjects received a

new random weight or force and began the adjustment

process again. During the 4 h test session, 10 weight or

force adjustments were made by each subject (two for each

task). The experiment required each subject to work 2 days

per week for 4 weeks to complete the above schedule.

Data analysis

The dependent variables were MAWs and MAFs, and

independent variables were frequency, height of lifting and

lowering, lifting versus lowering, pushing versus pulling,

and combination versus individual tasks. Effects of experi-

mental conditions were analyzed using one-way and two-

way analyses of variance (ANOVA) with repeated measures.

If significant differences were present (P \ 0.05), theNewman-Kuels and Tukey tests were used for post hoc

comparison. Comparisons between the comparable data in

this experiment and the criterion values cited in Snook and

Ciriello (1991) were analyzed with t tests for independent

samples. Statistica (version 5.5) was used for all statistical

analyses (StatSoft Inc.; Tulsa, OK).

Results

Gender differences

The MAWs of lifting, lowering, and carrying for females

averaged 53% of the present-day male values (Ciriello et al.

2008) (Table 1). MAFs of pushing and pulling, for females

averaged 83 and 86% of the present-day male values,

respectively for initial and sustained forces. The lifting,

lowering, and carrying comparisons between males and

females were all significant (P \ 0.0001). Pushing andpulling comparisons between males and females were not

consistently statistically significant (see Table 2). In the

1991 guideline, MAWs of lifting, lowering, and carrying for

females averaged 55% of the male values (Snook and

Ciriello 1991) and MAFs of pushing and pulling averaged

73 and 78% of the male values, respectively, for initial and

sustained forces (Tables 1, 2). Besides similar gender dif-

ferences observed in the 1991 guideline and the present-

findings, female MAWs of lifting, lowering, and carrying,

Int Arch Occup Environ Health (2011) 84:569575 571

123

averaged 67% of the 1991 female values (Snook and Ciriello

1991). Comparisons with criterion tasks from the 1991

guideline were all significant (see Table 1). Female MAFs

of pushing and pulling averaged 107 and 91% of the 1991

female values for initial and sustained forces. The initial and

sustained MAFs of the 7.6 m pushing task at the slow rate

was the only pushing or pulling criterion task from the 1991

guideline that could be compared statistically (Table 2).

Effect of frequency on tasks

MAWs and forces of all but two tasks were significantly

affected (P \ 0.01) by frequency (Tables 1, 2). Theexceptions were between the 4.3 and 1 min-1 frequencies

for both the low-lower, and carrying tasks.

Effect of height on lifting and lowering

MAWs were significantly affected (P \ 0.05) by the heightrange for both the lifting and the lowering tasks at a fre-

quency of 12 min-1 (Table 1). At frequencies of 4.3 and

1 min-1 for both lifting and lowering, MAWs were not

significantly (P [ 0.05) effected by height.

Lifting versus lowering

MAWs of lowering was significantly greater than MAWs

of lifting in the center range (P \ 0.02) at 12 min-1

(Table 1). All other MAW comparisons of lifting and

lowering were not significantly different (P [ 0.05).

Pushing versus pulling

For both frequencies investigated, the duration of pulling

was longer (P \ 0.01) than during pushing. Initial andsustained MAFs of pulling were significantly lower

(P \ 0.05) than initial and sustained MAFs of pushing atthe 4 min-1 frequency (Table 2). At the slower frequency

of 1 min-1, the initial MAF of pulling was lower

(P \ 0.05) than the initial MAF of pushing. However,there was no significant difference between pushing and

pulling in the sustained forces at this frequency.

Table 1 Maximal acceptable weights for females performing lifting, lowering, carrying, and a combination tasks with small boxes at variousfrequencies and heights

Task Frequency (tasks/min) Weight (N) MAW comparisons (%)

(F/M)a (F/M)b (F/F)c

X SD 010/008 091/091 010/91

Low lift 12 56.9 23.5 50 65 53

4.3 88.3 24.5 55 58 64

1 100.0 28.4 49 52 641

Center lift 12 68.6 26.5 56 67 70

4.3 90.2 23.5 54 57 71

1 104.9 29.4 55 50 762

Low lower 12 63.7 18.6 52 53 65

4.3 94.1 26.5 53 54 69

1 101.0 27.5 53 48 643

Center lower 12 76.5 24.5 55 55 71

4.3 87.3 19.6 49 48 69

1 104.0 33.3 55 47 764

Carry 4.3 99.0 27.5 58 60 67

1 101.0 31.4 46 59 545

Combination 4.3 84.3 29.4 59

1 100.0 26.5 53

53d 55d 67d

a Females from this experiment/males from Ciriello et al. (2008). All comparisons significant (P \ 0.0001)b Females/males from Snook and Ciriello (1991)c Females from this experiment/females from Snook and Ciriello (1991). Significance for comparison 1 P \ 0.0001, 2 P = 0.0032,3 P \ 0.0001, 4 P = 0.0001, 5 P \ 0.0001d Average of the column values


123

Combination tasks

MAWs of the individual lowering and carrying tasks,

which make up the two of the components of the combi-

nation task, were greater (P \ 0.05) than the combinationtask for the 4.3 min-1 frequency. The MAWs of the lifting

task at that frequency were not significantly different from

the combination task. MAWs of the individual lifting,

carrying, and lowering tasks at the 1 min-1 frequency were

all non-significant to that of the combination task.

Discussion

In a typical psychophysical approach, participants undergo

a combination of adjusting the workload and tracking their

ability to sustain the selected load for a prolonged period

without strain, thereby correlating sensory input into an

acceptable response in terms of either force or workload.

The main goal of the present experiment was to replicate

this psychophysical approach at a time distant from the

original investigations in order to detect changes in gender

differences in determined MAWs and MAFs. The present

findings clearly indicate that gender differences were

similar for lifting, lowering, and carrying compared to

values of Snook and Ciriello (1991); however, gender

differences were reduced for females in MAFs of pushing

and pulling. In other words, female performance as a per-

centage of the male performance increased (Tables 1, 2).

The results also indicate that MAWs for the industrial

females in this study decreased markedly compared to

females in the 1991 guideline. These lower MAWs may

represent a decrease in the female industrial work forces

set point for performance based on an acceptance for a

lower burden on the musculoskeletal system. We have

attempted to replicate this study with a similar protocol to

that of previous experiments, yet the decreases in MAWs

of lifting, lowering, and carrying were substantial. It is

interesting to note that MAFs of pushing and pulling did

not decrease in the same magnitude as lifting, lowering,

Table 2 Maximal acceptable forces for females performing a 7.6-m pushing and a 7.6-m pulling task at two frequencies

Task Frequency (tasks/min) X SD MAF comparisons (%)

(F/M)c (F/M)d (F/F)e

010/008 091/091 010/91

Push

Initial force (N)a 4 208.6 45.4 751 77 106

Sustained force (N)b 107.6 26.2 852 86 91

Task duration (s) 8.8 1.6

Initial force (N) 1 263.8 62.3 813 61 1171

Sustained force (N) 145.9 44.5 864 68 992


Pull

Initial force (N) 4 190.4 38.7 915 83 102



Initial force (N) 1 233.5 58.3 837 70 104



83f 73f 107f

86g 78g 91g

a Amount of force needed to get the treadmill movingb Amount of force needed to maintain treadmill movementc Females from this experiment/males from Ciriello et al. (2008). Significance for comparison 1 P \ 0.0001, 2 P = 0.0229, 3 P = 0.0030,4 P = 0.0870, 5 P = 0.1818, 6 P = 0.1015, 7 P = 0.0358, 8 P = 0.0642d Females/males from Snook and Ciriello (1991)e Females from this experiment/females from Snook and Ciriello (1991). Significance for comparison 1 P = 0.0155, 2 P = 0.9273f Average of initial forces in the columng Average of sustained forces in the column


123

and carrying, a trend observed in a earlier study with

females (Ciriello 2005), indicating the female population is

maintaining performance levels established in our 1991

guidelines (Snook and Ciriello 1991) for pushing and

pulling.

Our recent study on males (Ciriello et al. 2008) reported

that MAWs of lifting, lowering, and carrying, averaged

69% of the male 1991 guideline values (Snook and Ciriello

1991), and this observation was similar to findings in a

earlier study with males (Ciriello 2001) and to the present

study with females. More importantly, males in Ciriello

et al. (2008) reported MAFs of pushing and pulling aver-

aging 82% of the guideline values, whereas females

reporting in the present study have maintained MAFs

levels of the 1991 guideline. Ergonomic strategies call for

designing tasks to be acceptable for 75% of the female

industrial worker. The noted convergence of MAFs of

pushing and pulling for males and females in this study

may actually simplify ergonomic recommendations for

pushing and pulling tasks, thereby representing less risk for

musculoskeletal problems.

There is still a question concerning the significant

decrease in MAWs of lifting, lowering, and carrying in the

present study compared to the guideline of Snook and

Ciriello (1991). In our comparable male study (Ciriello

et al. 2008), we suggested that modification to a less

stringent subject recruitment strategy may have been a

relevant factor in the observed decrease in MAWs. How-

ever, Singh et al. (2009) reported MAWs which were very

similar to our male values. Their experimental methodol-

ogy was comparable to the present study, and 28 of their 30

subjects were documented as manual material handlers.

The experiment of Singh et al. was also conducted in

another area of the country. This confirmation of our results

with males give some credence to the secular changes in

performance observed in our female industrial workers.

In earlier studies, we also documented maximum vol-

untary contraction (MVC) strengths (Ciriello and Snook

1978; Ciriello et al. 1990, 1993) along with anthropometry

in order to compare our sample with other industrial

samples. In this study, we lack the information on MVC,

which would have been helpful in comparing the percent of

MVC chosen for MAWs and MAFs. In terms of anthro-

pometrics, the shoulder, elbow, and knuckle height and

stature measurements taken in the present experiment were

very similar to our earlier studies (Ciriello et al. 1990;

Ciriello 2004; Snook and Ciriello 1974) and other indus-

trial (Eastman Kodak and Human Factors Section 1986;

Marras and Kim 1993) and military populations (Gordon

et al. 1989). However, the body mass of our sample was

considerably higher than the above-reported studies with a

median difference of ?10.5%. However, without specific

information about % body fat, body segment distribution of

the increased mass, and muscle composition of the seg-

ments, we cannot assign the changes in psychophysically

chosen weights to body mass changes. Interestingly, in a

recent comprehensive study that looked at only body mass

index (BMI) as a factor in selecting MAWs, Singh et al.

(2009) concluded that a higher BMI does not reduce

MAWs of lift.

Although MAWs have decreased for lifting, lowering,

and carrying, variable effects, such as frequency, heights,

lifting versus lowering, pushing versus pulling and tasks in

combination, have remained relatively constant. Frequency

was significant in this experiment and has been a signifi-

cant factor in all of our previous papers for both males and

females (Snook 1971; Ciriello and Snook 1983; Ciriello

et al. 1990, 2008; Ciriello 2003, 2007). The results on the

effects of heights on lifting and lowering were similar to

earlier studies, which reported no height effects for males

and females (Snook 1971; Ciriello 2001, 2005; Ciriello

et al. 1993, 2008), but contrary to other studies which

found significant height effects for males and females

(Ciriello and Snook 1983; Ciriello et al. 1990; Snook and

Ciriello 1974). MAWs of lowering were consistently

greater than MAWs of lifting as reported in previous

studies (Ciriello and Snook 1983; Ciriello et al. 1990). For

pushing versus pulling, the results are the same as those

reported in our male study (Ciriello et al. 2008). In previ-

ous experiments of comparing pushing and pulling at the

similar 1 min-1 frequency and 7.6 m distance, both initial

and sustained MAF were non-significant (Ciriello et al.

1990; Ciriello 2002, 2004). And lastly, our previous studies

of combination tasks, which included lifting, carrying, and

lowering, concluded that the limiting factor of the combi-

nation task was the individual task with the smallest dif-

ference in MAW to the combination task (Ciriello et al.

1990, 1993). In the present study, this rule applies to both

frequencies. The above findings are encouraging and imply

that participants are fairly consistent in reaction to vari-

ables when choosing their respective MAWs. Future

guidelines for lifting, lowering, pushing, pulling, and car-

rying may reflect the findings from this study and might

take into account the knowledge that lifting is similar to

lowering, low and center lifting and lowering are similar,

and sustained forces for pushing and pulling are also

similar.

It was concluded that considerations about adjusting

existing guidelines on lifting, lowering, and carrying

may not be appropriate until these findings are con-

firmed by replicating these psychophysical experiments

on a wide variety of subject pools by other investigators

in different areas of the US and different countries.

However, adjustments to pushing and pulling guidelines

may have less merit based on the evidence available at

this time.


123

Acknowledgments The authors gratefully acknowledges the datacollection assistance of Susan OBrien and Amanda Rivard.

Conflict of interest The authors declare that they have no conflictof interest and will receive no benefits in any form from a commercial

party related directly of indirectly to the subject of this manuscript.

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123

Gender differences in psychophysically determined maximum acceptable weights and forces for industrial workers observed after twenty yearsAbstractPurposeMethodsResultsConclusions

IntroductionSubjects and methodsSubjectsIndustrial tasks

Experimental procedureData analysis

ResultsGender differencesEffect of frequency on tasksEffect of height on lifting and loweringLifting versus loweringPushing versus pullingCombination tasks

DiscussionAcknowledgmentsReferences

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