Ergonomic

Risk Factors

A study of heavy earthmoving machinery operators

By N. Kumar Kittusamy

N. Kumar Kittusamy, Sc.D., is a research

engineer at NIOSH’s Spokane (WA) Research

Lab. He conducts research related to safety and

health issues in mining and construction. Prior

to joining NIOSH, he was an assistant professor

in the Dept. of Safety Technology at Marshall

University, where he was also coordinator of

the mine safety and health program. Kittusamy

holds a B.S. in Industrial Engineering and an

M.S. in Public Health, both from the University

of South Florida, and an Sc.D. in Ergonomics

from the University of Massachusetts Lowell.

ALTHOUGH MANY STUDIES HAVE SHOWN an

association between operating heavy construction

equipment and symptoms of musculoskeletal disorders (MSDs), little research has been performed that

systematically characterizes the exposure of operating engineers to ergonomic hazards. This study evaluated: 1) vibration at the seat/operator interface;

2) transmissibility of vibration in the Z-axis; 3) psychophysical ratings of vibration level and vibration

discomfort; and 4) postural requirements of the job.

Results indicate that the digging operation had higher levels of total weighted acceleration than high- or

low-idling conditions. Transmissibility data showed

that the seat amplified vibration, particularly in the

lower frequencies. Seats demonstrated that they may

not be sufficient in protecting operators from longterm effects of vibration exposure. High positive correlation was found among subjective ratings

(vibration discomfort and vibration level), but moderate positive correlation was found between subjective ratings and quantitative vibration levels.

Postural evaluations revealed that the operators were

required to assume awkward postures of the neck,

shoulder and trunk while performing their jobs.

Work-related injuries and illnesses pose a continuing threat to the health and well-being of American

workers. The construction industry has been historically recognized as having higher rates of fatality,

injury and illness than other industries (McVittie

285+; BLS). In 1994, an estimated 218,800 lost-workday injuries were reported in the construction industry (BLS). Construction also

had the second-highest incidence rate for sprains and

strains. Although the industry

has made progress since then,

injuries and illnesses, including MSDs, continue to be

cause for concern.

Operating engineers (also

known as hoisting and portable engineers) operate and

maintain heavy construction

equipment, such as cranes,

bulldozers, front-end loaders, rollers, backhoes and

graders; they may also work as surveyors or mechanics. This equipment is used to: 1) build roads, bridges,

tunnels and dams; 2) construct buildings and power

plants; 3) remove earth materials and grade earth surfaces, and to replace concrete, blacktop and other

paving materials; and 4) construct drainage systems,

pipelines and related tasks, such as blasting (Stern and

Haring-Sweeney 51+). An estimated 487,000 operating engineers (55 percent union, 45 percent nonunion)

are employed in the U.S. and Canada, most of whom

are exposed to whole-body vibration, albeit in concert

with other occupational risk factors.

Past studies have shown that MSDs affect construction equipment operators due to awkward postures (e.g., static sitting), whole-body vibration, work

intensity, high-resistance levers and repetitive motions

(Kittusamy, et al; Buchholz, et al 23+). It is believed

that reducing ergonomic exposures such as vibration

and postural stress may be an important factor in

improving operator health, comfort and efficiency.

Study Methodology

This study evaluated workers employed in the

construction of the Central Artery/Third Harbor

Tunnel (CA/T) in Boston. Eight journey-level (experienced) operators (seven males, one female) employed

by two major contractors were studied; however, only

six participated in the postural portion of the study

because environmental conditions at the site were

deemed unsafe for standing and videotaping the tasks

being performed). Operators’ ages ranged from 33 to

58 years; experience ranged from 11 to 40 years; height

ranged from 5’5” (165 cm) to 6’ (183 cm); and weight

ranged from 115 lbs. (52 kg) to 284 lbs. (129 kg) (Table

1). Operators were briefed about the study and each

signed an informed consent form.

Each operator used a different piece of earthmoving equipment including a wheeled loader, wheeled

backhoe/loader, wheeled excavator and a crawler

excavator. Equipment was grouped into three size categories: small, medium and large. Equipment was

engaged in static tasks (i.e., low- and high-idling) and

dynamic tasks (i.e., digging-related).
www.asse.org OCTOBER 2002 PROFESSIONAL SAFETY 39

Table 1

Background Data of Operators & 

Equipment Evaluated in the Study

Operator Characteristics Equipment Information

Height Weight Years

Operator* (cm) (kg) Age Exp. Size Type

1 165 52 33 11 Small Loader/Backhoe

2 178 129 49 30 Small Loader/Backhoe

3 183 86 38 17 Small Loader/Backhoe

4 165 70 58 36 Medium Excavator

5 178 100 56 40 Medium Excavator

6 168 86 35 15 Large Excavator

7 170 66 35 12 Large Excavator

8 168 84 56 30 Large Loader 

Mean 171.8 84.2 45.0 23.9

SD 6.8 23.5 10.8 11.4

Var 46.0 552.5 117.1 130.7

*OE #1 is female, all others are male operators; OE #1 & OE #8 did not participate in postural studies.

Whole-body vibration was

assessed for each task at the

seat/operator interface using a

triaxial piezoresistive seat pad

accelerometer (Model VT-3) and

at the floor level using a single

axis piezoresistive accelerometer (Model 7265A-HS), both

from Endevco Corp. A field

computer system (Model 2100

FCS) from Somat Corp. was

used to filter and store data

(Figure 1). 

Figure 1

Instrumentation Used for Data Collection

S = seat; F = floor; S & F denote the location of accelerometer.

*Ch1 = Z axis—floor; Ch2 = Y axis—seat; Ch3 = Z axis—seat; Ch4 = X axis—seat

Vibration measurements were performed in accordance with ISO 2631 (ISO;

ACGIH 126+). Test equipment

was calibrated and mounted

according to manufacturer

guidelines. Vibration data were

sampled at a rate of 500Hz and

filtered using a Butterworth filter with a low-pass break frequency of 100Hz.

Vibration data were collect-
40 PROFESSIONAL SAFETY OCTOBER 2002 www.asse.org

ed on 50 tasks performed by all operators. Of these,

16 (four low-idling, four high-idling, two chipping

concrete, four digging and two driving) were perfomed by small-size equipment; 18 (three lowidling, three high-idling, 10 digging and two

spreading rocks) were performed by medium-size

equipment; and 16 (three low-idling, three highidling, eight digging and two loader functions) were

performed by large-size equipment. Results were

summarized as 1) total vector sum (X, Y and Z axes)

at the seat; 2) weighted acceleration in the Z axis at

the seat; and 3) nonweighted acceleration in the Z

axis at the seat and cab floor. Static tasks were evaluated for a period of two minutes each, while

dynamic tasks were evaluated for periods ranging

from three to five minutes.

After each task, operators rated their perceived

vibration level and discomfort. They were asked to

rate the amount of vibration felt from the equipment

through the seat and the level of discomfort felt due to

this vibration by placing a straight vertical line on a

continuous visual analog scale (VAS). The VAS consisted of a four-inch (10 cm) line with word anchors on

each extreme (McDowell and Newell). For the vibration discomfort scale, the extremes were

“no discomfort” and “intolerable discomfort”; for the vibration level scale, the

extremes were “no vibration” and “high

vibration.” Rating scales were scored for

each task by measuring the distance from

the left end of the VAS to the operator’s vertical line and obtaining a value from 0 to 10. 

Perceived ratings were collected from

operators who performed 36 different tasks

(20 static, 16 dynamic). Of these tasks, 12

(four low-idling, four high-idling, two digging, one driving and one chipping concrete) were performed by small-size

equipment; 13 (three low-idling, three

high-idling, five digging and two spreading rocks) were performed by medium-size

equipment; and 11 (three low-idling, three

high-idling, four digging and one loader

function) were performed by large-size

equipment. The evaluation period for the

vibration measurement and the work time

considered for the psychophysical ratings

were identical. This was done to allow for a

more-direct comparison of perceived ratings and quantitative vibration level.

Postural recordings of the operator performing dynamic tasks (i.e., digging-related) were obtained. Then, postural

analysis was performed by observing the

videotaped job in simulated real-time

(Figures 2 and 3). 

Figure 2

Standard Postures Used to Classify 

Trunk, Shoulder & Neck Position

Adapted from Keyserling, 1986.

Trunk Posture

Shoulder Posture Neck Posture

Flexion/Extension

Mild is 20° to 45°

Severe is > 45°

Bending

Bent is > 20°

Twisting

Twist is > 20°

Flexion/Abduction

Mild is 45° to 90°

Severe is > 90°

Flexion/Extension

Mild is 30° to 45°

Severe is > 45°

Bending

Bent is > 20°

Twisting

Twist is > 20° Thirteen tasks were

taped and analyzed using the computeraided system developed by Keyserling

(569+). Interobserver variability was eliminated by having an experienced analyst

perform all analyses. To avoid the need to

observe and analyze multiple joints simultaneously, a tape was played three different times with different joints—such as trunk, left or

right shoulder, and neck—studied during each playback. Since it was not possible to videotape both

shoulders, at least one shoulder was videotaped for

each job. In addition, at least one cycle of each job

was recorded; where feasible, additional cycles were

taped and analyzed.

Results & Discussion

Whole-Body Vibration

Examination of the 1/3-octave band values in the

range of 1 to 80Hz revealed that all equipment demonstrated a marked increase in amplitude in the critical

frequency range for the Z axis (4 to 8 Hz), and for the

X and Y axes (1 to 2 Hz) (Figures 4, 5 and 6). When

total vector sum accelerations were compared for the

different size equipment and tasks, it showed significance for size, tasks and interaction (p< 0.05). In addition, the mean values for digging-related tasks were

higher than low- or high-idling tasks. The total vector

sum value for the digging tasks were always higher

than the 0.5 m/s2 eight-hour action level established

by the European Commission (ACGIH 126+). However, total vector sum accelerations for low- or high-
www.asse.org OCTOBER 2002 PROFESSIONAL SAFETY 41

idling tasks were consistently lower than

the limit set by the European Commission.

A comparison of the total transmissibility ratio was performed for the different size

equipment and tasks. The transmissibility

of the seating system was not significant for

size, tasks and interaction. However, higher

transmissibility was noted in the lower frequencies for all tasks (Figure 7).

Figure 3

Principle Steps in Evaluating

Equipment Operator Posture

Playback 

#1

Videotape Job

at Worksite

Shoulder

Analysis

Neck

Analysis

Trunk

Analysis

Playback 

#3

Playback 

#2

Generate

Posture Profiles

Computer-Aided

Posture Analysis

to laboratory

for analysis

Psychophysical Rating

Before performing the correlation

among the variables, data from similar

tasks performed by the same operator and

equipment were averaged. This was done

to smooth the data so that each operator

would have one data point representing

each task (e.g., low- and high-idling). Then,

quantitative vibrations were logged and

compared to the ratings. This was done

because visual examination revealed a logarithmic relationship in the way the operators rated the ratings when compared to the

quantitative vibration values. Correlation

was performed between several factors of

concern (e.g., rating of vibration level, rating of vibration discomfort, quantitative

vibration-vector sum, quantitative vibration-weighted acceleration in Z axis, and quantitative

vibration-nonweighted acceleration in Z axis).

Another correlation was performed that encompassed all tasks observed for this study (static,

dynamic and miscellaneous). This analysis revealed

1) a high positive correlation between ratings of

vibration level and vibration discomfort (r=0.70;

p<0.05); 2) a moderately high positive correlation

between ratings of vibration level and any type of

quantitative vibration (p<0.10); and 3) a positive

association between vibration discomfort and any

type of quantitative vibration (p>0.10).

A two-way analysis of variance (ANOVA) was

conducted using size of equipment (small, medium,

large) and task performed (low-idling, high-idling,

digging) as the independent variables and rating of

vibration level as the dependent variable. A significant main effect was only found for task (p<0.05). An

examination of the means (0.75 for low-idling, 1.1 for

high-idling and 2.26 for digging) revealed that operators performing digging tasks rated higher levels of

vibration than other tasks. A post-hoc comparison

using Scheffe’s test did not find any significant differences between the tasks. However, the largest

difference in means were recognized between lowidling tasks and digging tasks and between highidling tasks and digging tasks.

A two-way ANOVA was conducted using equipment size and task performed as independent variables and rating of vibration discomfort as the

dependent variable. A significant main effect was

only found for size of equipment (p<0.05). Examination of the means for size—0.61 for small, 0.77 for

medium, 2.5 for large—indicates that operators

using large-size equipment reported higher levels of

vibration discomfort. A post-hoc comparison using

Scheffe’s test found a significant difference between

1) small- and large-size equipment; and 2) mediumand large-size equipment.

High positive correlation was observed between

the ratings of vibration level and vibration discomfort. Moderate correlation was evident between the

ratings (vibration level or vibration discomfort) and

the quantitative vibration. Higher ratings were seen

for both dynamic tasks and larger equipment. This

can be explained by the inherent nature of the

dynamic job and the size of equipment. Larger

equipment is involved in more-demanding tasks.

Postural Analysis

The percentage of time an operator assumed a neutral posture was calculated using the computer-aided

system. Only dynamic jobs were studied (n=15). The

percentage of time the back was in a neutral posture

ranged from 32 to 100 percent (Figure 8); the shoulder

(left or right) from 29 to 99 percent (Figure 9); and the

neck from 55 to 97.5 percent (Figure 10).

This showed that operators were required to

assume awkward postures in the course of performing their jobs. In general, these postures were

notable for the neck, shoulder and back (descending

order). Deviation of the back can be explained by the

inherent nature of the job—workers must bend over

to see the ground that they are digging or moving.

Deviation of the shoulder can be explained by the

requirements of operating various controls (i.e.,

levers and gears) located inside the cab. Deviation of

the neck was mainly due to the operator maintaining eye contact with the work, which was located at

or below ground level.
42 PROFESSIONAL SAFETY OCTOBER 2002 www.asse.org

Figure 4

Whole-Body Vibration from

Medium-Size Excavator*

*Whole-body vibration from a medium-size excavator (OE #4) in the transverse (X-axis) compared

to the ISO/TLV limit.

Figure 5

Whole-Body Vibration from Medium-Size Excavator*

*Whole-body vibration from a medium-size excavator (OE #4) in the transverse (Y-axis) compared to the ISO/TLV limit.

(article continues on page 45)

Conclusion

This study was performed

to evaluate ergonomic exposures among operators of

heavy construction equipment. Results reveal that

operators performing dynamic tasks are exposed to

whole-body vibration higher

than the allowable limit

established by the European

Commission. Seats can be

improved to attenuate the

levels of vibration at the

lower frequencies. Job location (at, above or below

ground) and operator placement within the cab makes it

difficult for him/her to

assume a neutral posture

while working. This can be

addressed by designing cabs

that consider both factors.

Use of psychophysical ratings to evaluate vibration dis-
www.asse.org OCTOBER 2002 PROFESSIONAL SAFETY 43

Figure 6

Whole-Body Vibration from

Medium-Size Excavator*

*Whole-body vibration from a medium-size excavator (OE #4) in the longitudinal (Z-axis) compared to the

ISO/TLV limit.

Figure 7

Acceleration Transmissibility for Medium-Size Excavator*

*Acceleration transmissibility for a medium size excavator (OE #4) in the longitudinal (Z-axis).

Studies have

shown that

MSDs affect

construction

equipment

operators due

to tasks

that require

awkward

postures and

expose them

to whole-body

vibration.
44 PROFESSIONAL SAFETY OCTOBER 2002 www.asse.org

On this site,

operators were

required to

assume awkward postures

in the course

of performing

their jobs.

These postures

were notable

for the neck,

shoulder

and back.

Figure 8

Percentage of Cycle Back in Neutral

Posture for Specific Digging Tasks

Figure 9

Percentage of Cycle Time Shoulder in Neutral Posture

for Specific Digging Tasks
www.asse.org OCTOBER 2002 PROFESSIONAL SAFETY 45

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comfort and vibration level appears to be a useful tool

for quantifying operator perceptions (Kittusamy and

Buchholz). However, more field application of this

process is needed to evaluate its full potential.

Recommendations

Engineering controls are the preferred approach as

they focus on workstation/job design or redesign to

accommodate the operator. When these controls are

not feasible or during their implementation, administrative controls may be used to limit exposures.

Whole-Body Vibration

1) Design and select seats based on the transmissibility characteristics and not just on the immediate

comfort of the operator.

2) Design and select seats that will adequately

dampen vibration at all frequencies, but importantly

in the lower frequencies (1 to 8 Hz).

3) Properly maintain equipment to reduce wear

and tear that could result in increased vibration.

4) Limit the speed of the equipment when driven,

especially over bumpy or irregular surfaces.

5) Workers should not jump off equipment when

exiting, since this introduces a shock to the body that

has just been vibrated for several hours.

Awkward Posture

1) Redesign cabs to accommodate better upward

and/or downward visibility.

2) Have a coworker guide the operator (hand signals) when visibly challenging jobs are performed.

3) Install mirrors to provide better visibility (sideways and below ground). 

Figure 10

Percentage of Cycle Neck in Neutral Posture 

for Specific Digging Tasks

(continued from page 42) References

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Cincinnati: ACGIH, 2000.

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and Illnesses Resulting in Absences from Work in 1994.”

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2631/1. Geneva, Switzerland: ISO, 1985.

Keyserling, W.M. “Postural Analysis of the Trunk and

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Equipment.” Presentation at INTER-NOISE Conference, Ft.

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Acknowledgments

This research study was funded in part by the

ASSE Foundation and the National Institute

for Occupational Safety and Health through

the Center to Protect Workers’ Rights.