External L5–S1 joint moments when lifting wire mesh screen used

to prevent rock falls in underground mines

Sean Gallagher a,*

* Corresponding author. Tel.: þ1 412 386 6445; fax: þ1 412 386 6764.

E-mail address: sfg9@cdc.gov (S. Gallagher).

Christopher Mark a, Craig S. Compton a,

aNational Institute for Occupational Safety and Health, PO Box 18070, Pittsburgh, PA 15236, USA

, Susan Kotowski b, Kermit G. Davis b,

Ronald L. Huston b, John Connelly b

bUniversity of Cincinnati, 2600 Clifton Ave., Cincinnati, OH 45221, USA

abstract

Bolting large sheets of wire mesh screen (WMS) to the roof of underground mines prevents injuries due

to rock falls. However, WMS can be heavy and awkward to lift and transport, and may result in significant

spinal loading. Accordingly, six male subjects (mean age 1⁄4 45.8 years þ 7.5 SD) were recruited to lift WMS

in a laboratory investigation of the biomechanical demands. Biomechanical modeling was used to

estimate external moments about L5–S1 for sixteen lifting tasks, using two sizes of WMS. Full-size WMS

involved a two-person lift, while half-size WMS involved a one-person lift. Lifts were performed under

168 cm and 213 cm vertical space. Restriction in vertical space increased the maximum L5–S1 extensor

moment from 254 to 274 Nm and right lateral bending moment from 195 to 251 Nm. Lifting full sheets of

screen (as opposed to half sheets) resulted in an average 33 Nm increase in L5–S1 extensor moment. The

L5–S1 extensor moment was increased by an average of 44 Nm (18%) when lifting screens positioned flat

on the floor compared to an upright position.

Relevance to industry: Large flexible materials are commonly lifted in industrial work environments, and

may involve the efforts of two or more workers. The current study examines the low back loading

associated with lifting large flexible screens and presents recommendations to reduce spine loading.

1. Introduction

Underground coal miners are frequently exposed to poor roof

conditions that put them at increased risk of injury due to rock falls.

According to data compiled by the Mine Safety and Health

Administration (MSHA), for the years 1998–2007 the number of

roof fall injuries ranged from a low of 427 per year to a high of 709

per year (MSHA, 2008). A detailed analysis of rock fall injuries for

1997 determined that nearly 100% occurred where the miners

could have been protected by roof support (Bauer and Dolinar,

2000). When wire mesh screen (WMS) was installed on the mine

roof, on the other hand, analysis of injury data indicated that the

number of rock fall injuries was reduced to less than 15% of that

experienced prior to its installation (Robertson et al., 2003).

The reason WMS is more effective than other rock fall control

techniques is that it covers more surface area of the roof – close to

100% protection can be achieved. However, in the process of

achieving increased rock fall protection, musculoskeletal hazards

may be introduced due to lifting and handling WMS. WMS handling

can be a challenge for operators because it often requires overhead

lifts and awkward postures, and the screen is unwieldy due to its

large size and flexible nature (Fig. 1). In addition, WMS can be fairly

heavy depending on its size, the number of reinforcing wires, and

the gage of the steel. Typical full-size sheets of WMS have a mass of

about 14.5 kg, but at least one mine in the Western U.S. installs 8-

gauge steel sheets that are 6 m long and 1.5 m wide, with a mass of

approximately 23 kg (Robertson et al., 2003). Due to the size and

weight of WMS, large load moments can be introduced, the

magnitude of which will depend on the orientation of the screen.

Given the superior rock fall protection afforded by WMS, the

National Institute for Occupational Safety and Health (NIOSH)

supports its widespread use in the underground coal mining

industry. Recognizing that the biomechanical stresses of handling

WMS could negatively impact achievement of this aim, it was

considered important to obtain an improved understanding of these

stresses so that better handling techniques could be recommended.

Accordingly, the purpose of the present exploratory research study

was to use a bottom-up biomechanical model to derive moments

about the L5–S1 joint during manual lifting of WMS under a variety

of conditions. Lifting tasks involved handling two sizes of WMS,

various initial and final orientations of the WMS, and different
vertical space restrictions, all of which were chosen to simulate

WMS handling practices observed in underground coal mines.

Fig. 1. Miners lifting wire mesh screen positioned on the mine floor (A) vs. stored upright against the mine wall, or ‘‘rib’’ (B).

2. Methods

2.1. Subjects

Six male subjects (mean age 1⁄4 45.8 years ± 7.5 SD; mean

height 1⁄4 175.8 cm ± 9.4 SD; mean body mass 1⁄4 84.0 kg ± 16.2 SD)

volunteered to serve as test subjects in this study. Male subjects

were used owing to the fact that roof bolters in underground coal

mines (workers who would install WMS) are almost exclusively

male. Subjects operated under terms of informed consent and were

tested in accordance with procedures approved by the National

Institute for Occupational Safety and Health (NIOSH) Human

Subjects Protection Board. All participants had experience in the

underground mining environment, but none had specifically performed WMS lifting in an underground setting. None of the

subjects reported a significant history of low back disability (i.e.,

had experienced no lost-time due to low back pain), and were

asymptomatic at the time of testing.

2.2. Experimental design

A completely randomized within-subjects experimental design

was used to evaluate the biomechanical demands of lifting full-size

and half-size WMS. Lifting was performed under two vertical

workspace conditions (168 cm and 213 cm). These vertical

workspace conditions were chosen to evaluate the effects of lifting

in high-seam (>183 cm) coal mine conditions as opposed to

a vertical height more characteristic of a mid-seam (122–183 cm)

coal mine where stooping would be required. Two sizes of the wire

mesh screen were tested. These included full-size WMS

(4 m x 1.53 m, 8-gage steel wire, 14.5 kg) and half-size WMS

(1.5 x1.5 m, 8-gage steel wire, 6 kg). Examples of lifting WMS in the

motion analysis laboratory can be seen in Fig. 2. As shown in Fig. 2A,

the subject lifted one side of the screen while other side was lifted

by a member of the research team (always the same member) when

lifting full-size WMS. The half-size WMS was performed as a oneperson lift (Fig. 2B).

Fig. 2. Examples of lifting wire mesh screen in the Motion Analysis Laboratory. (A) Shows an overhead lift of a full-size WMS, while (B) shows subject lifting a half-size WMS from

the floor.

Dependent measures consisted of minimum,

average and peak three-dimensional external moments (for X, Y,

and Z axes) about the L5–S1 joint based upon calculations derived

from a biomechanical model described in detail below.

A priori orthogonal contrasts were developed to test effects of

interest for sixteen lifting conditions. Contrasts are identified in

Table 1. Each of these contrasts sum to zero and each contains

a unique piece of information regarding the data collected. As an

example of how such contrasts are analyzed, the first row in the

table represents a comparison representing the effect of different

seam heights (168 cm vs. 213 cm) on L5–S1 external moments.

Since there are 6 trials at the 168 cm height and 10 at the 213 cm

height, values are multiplied by 5 (168 cm) or -3 (213 cm) to

provide a fair comparison regarding the effect of seam height. As

these contrasts were established before data collection, t-tests were

computed using a per contrast Type I error 1⁄4 0.05 (Kirk, 1995). All

t-tests reported below have 75 degrees of freedom.
Table 1

A priori orthogonal contrasts used to analyze data from the experiment. [Note: Seam height represents vertical space constraints, FLR 1⁄4WMS laying flat on floor at start of lift,

RIB 1⁄4WMS leaning against wall (rib) at start of lift, L 1⁄4 subject facing screen and lifting left side of full-size WMS, R 1⁄4 subject facing screen and lifting right side of full-size

WMS, OV 1⁄4 Lifting full-size WMS to an overhead-carry position, SI 1⁄4 lifting full-size WMS to side-carry position].

168 cm seam height 213 cm seam height

Half-size

WMS

Full-size WMS Half-size

WMS

Full-size WMS

FLR RIB FLR RIB FLR RIB FLR RIB

L R L R L R L R

OV SI OV SI OV SI OV SI

168 cm vs. 213 cm

Half-size WMS vs. Full-size WMS

FLR vs. RIB

Seam Height x FLR/RIB

Left side vs. Right side (Full-size WMS)

Overhead Lift vs. Side Lift (Full-size WMS)

Left/Right x Overhead/Side (Full-size WMS)

Left/Right x FLR/RIB (Full-size WMS)

5

4

1

5

0

0

0

0

5

4

-1

-5

0

0

0

0

5

-2

1

5

1

0

0

1

5

-2

1

5

-1

0

0

-1

5

-2

-1

-5

1

0

0

-1

5

-2

-1

-5

-1

0

0

1

-3

4

1

-3

0

0

0

0

-3

4

-1

3

0

0

0

0

-3

-1

1

-3

1

1

1

1

-3

-1

1

-3

1

-1

-1

1

-3

-1

1

-3

-1

1

-1

-1

-3

-1

1

-3

-1

-1

1

-1

-3

-1

-1

3

1

1

1

-1

-3

-1

-1

3

1

-1

-1

-1

-3

-1

-1

3

-1

1

-1

1

-3

-1

-1

3

-1

-1

1

1

2.3. Motion analysis system/force plates

A ten-camera motion analysis system (Motion Analysis Corporation, Santa Rosa, California) was used to obtain kinematics of

subjects performing the lifting tasks. Reflective markers were

attached using a modified (i.e., no markers on the greater

trochanters) Helen Hayes marker set, as depicted in Fig. 3 (Davis

et al., 1991).

Fig. 3. Modified Helen Hayes marker set used to obtain motion data in the study.

Motion data were recorded at a rate of 60 frames/s

using digital video cameras. A set of five reflective markers was

placed on each of the wire mesh screens to track the motion of the

object as it was lifted. Analog data from a pair of force plates

(Advanced Mechanical Technology (AMTI), Inc., Watertown, MA)

were collected synchronously with the motion data and stored in

the motion capture data files, as was a digital video representation

of the lifting sequence. All subjects started in a standardized position (a ‘‘T-pose’’, standing up with arms held out to the side), facing

the same direction, prior to the start of data collection.

2.4. Biomechanical model

Biomechanical models employing both bottom-up and topdown approaches were used in to analyze the data. However, the
bottom-up approach was considered superior due to the fact that

hand forces on the WMS were not captured and had to be assumed

to be equal, which was difficult to justify. Only results from the

bottom-up analysis are presented. The 14 body segments and local

coordinate system for L5–S1 are shown in Fig. 4.

13

12

2

1

3

4

5 8

7

6

9

Z 

Y 

X 

L5-S1

10

11 14

Fig. 4. Fourteen body segments used in link segment model and orientation of local

coordinate system for L5–S1.

Mass distributions

for each body segment were based upon data provided by Dempster (1955), as corrected for fluid loss by Clauser et al. (1969). Threedimensional forces, moments, and center of pressure data were

obtained from AMTI force plates and were used to calculate

moment estimates for L5–S1. Axes established for the force plates

used the ‘‘right hand’’ rule and are illustrated in Fig. 5.

X 

Z 

Y 

O 

Fig. 5. Orientation of axes with respect to force plates. Subjects were facing in the

positive X direction when the lift was initiated.

1 2 

The position

estimate for L5–S1 was operationally defined as a point lying 40% of

the distance (posterior to anterior) from sacral motion analysis

marker (V.SACRAL) to a point bisecting the line connecting markers

on the right and left anterior superior iliac spines (R.ASIS and

L.ASIS). Moments about L5–S1 were calculated based on the ground

reaction forces and the positions of force application and the

position of L5–S1 were corrected by subtracting moments about

the left and right thighs, shanks, and feet. The total force (including

calculated inertial forces for each body segment) of these segments

was used in this correction. Inertial forces were calculated using

a dynamic model described by Huston et al. (1976). Equation (1)

demonstrates the bottom-up model used to calculate MY (extension

moment) at L5–S1:

MY L5—S1 1⁄4 (xfp1 - xL5—S1)x fzfp1 þ

(

xfp2 - xL5—S1)x fzfp2

-

(

zL5—S1 x fxfp1)

-

(

zL5—S1 x fxfp2)- MY r:thigh

- MY r:shank - MY r:foot - MY l:thigh - MY l:shank

- MY l:foot

(1)

In this equation, MY L5–S1 is the extension moment about L5–S1,

xfp is the location of the center of pressure for force plates 1 or 2

(per subscripts) in the x direction, xL5–S1 is the x coordinate of the

calculated position of L5–S1, fzfp is the measured force in the z axis

for force plates 1 or 2 (per subscripts), zL5–S1 is the z coordinate of

L5–S1, fxfp is the measured force in the x axis from force plate 1 or 2

(per subscripts), and MY r.thigh, MY r.shank, MY r.foot, MY l.thigh, MY l.shank,

MY l.foot are moments about the Y axis of the specified segments of

the lower extremities. Similar equations were developed to determine Mx and MZ for L5–S1. Since subjects pivoted from a positive X

facing position to a positive Y facing position (as they would to

begin carrying the screen), moments about L5–S1 were rotated

based on the position of the markers L.ASIS and R.ASIS so that

consistent moment estimates about the local coordinate system at

L5–S1 could be maintained.

2.5. Procedure

Upon arrival at the laboratory, subjects were explained the

purpose of the study, read and signed the informed consent form

and a standard photo release form. Subjects were asked to change

into clothes appropriate for the motion analysis study and were

fitted with the motion analysis markers. Calibration of the motion

analysis system capture volume was performed at the beginning of

each day of testing, and was repeated if necessary. Subjects were

instructed as to how the lifting tasks should be performed and were
allowed to practice until they felt comfortable with the tasks. Data

were obtained on 16 WMS lifting conditions for each subject. A

two-man lift was used for the full-size WMS, with data collected on

only one subject. For any given lifting condition, the subject would

perform one or more trials until the task was deemed to have been

performed successfully (i.e., subject remained on force plates

throughout the task, no difficulties encountered during the lift,

etc.). At the start of the lifting tasks, subjects stood on the two AMTI

force plates, with one foot on each plate facing in the positive X

direction (Fig. 5). Lifts started with the screen either flat on the floor

or at a near vertical orientation (as if leaning against a mine ‘‘rib’’).

Vertical space constraints (simulated mine seam heights) were

controlled by way of an adjustable roof that was raised or lowered

as necessary. Lifts of full-size screens in the 213-cm vertical space

consisted of lifting the screen to either an overhead-carry position

or to a side-carry position. In the limited 168-cm vertical space,

only a lift to the side-carry position was possible. Subjects alternated lifting on the right vs. left side of the screen for full-size WMS.

For half-size screens, lifts to only a side carry were examined, as

these screens are not observed to be carried overhead in the mining

environment. Starting positions for half-size WMS also consisted of

lying flat on the floor or leaning against a rib. Other than the

constraints listed above, the subject was free to use his own lifting

technique. The reader is referred to Table 1 for a full representation

of the lifting conditions. The order of presentation of the 16 lifting

conditions was completely randomized on a within-subjects basis.

Subjects were given two minutes rest between each trial (Caldwell

et al., 1974).

3. Results

Table 2 provides a summary of the external L5–S1 peak joint

moments observed for the ‘‘main effect’’ contrasts examined in this

study. Other significant results, involving interactive effects and

other joint moment measures are detailed below.

3.1. Effect of vertical space restriction

Compared to the 213 cm vertical space, restricted vertical space

(e.g., the 168 cm simulated mine seam height) was associated with

an increase in the peak L5–S1 extensor moment from 254 to

274 Nm (t 1⁄4 2.77, p < 0.01) and an increase in the average peak right

lateral bending moment from 195 to 251 Nm (t 1⁄4 5.54, p < 0.001).

The minimum L5–S1 extensor moment in the 168 cm vertical

workspace was 63 Nm greater than in the less restricted space

(t 1⁄4 4.24, p < 0.001).

3.2. Effect of WMS size

The difference in mass expected to be handled by the subject for

full-size WMS (two-person lift of a 14.5 kg WMS) and half-size

WMS (one-person lift of a 6 kg WMS) would be expected to average

1.25 kg. However, full-size WMS was associated with a peak L5–S1

extensor moment that was 26 Nm greater than the half-size WMS,

on the average (t 1⁄4 3.66, p < 0.001). The peak right lateral bending

moment was increased 7 percent (205.1 vs. 219.6 Nm) when lifting

the full-size WMS (t 1⁄4 2.14, p < 0.05).

3.3. Effects of initial WMS orientation

Lifting WMS from a floor-lying orientation significantly

increased both peak (t 1⁄4 6.19, p < 0.001) and average (t 1⁄4 7.74,

p < 0.001) extensor moments compared to when the screens were

oriented vertically. Table 2 provides the increased peak extensor

moment observed when lifting from the floor vs. the rib. The L5–S1

peak extensor moment exhibited an increase of 44 Nm (18%

increase) when lifting the screen from the floor. The average

extensor moment was 32% higher (95 vs. 125 Nm) when the WMS

was lifted from the floor.

Table 2

Averaged peak external L5–S1 joint moments (Nm) and standard errors for specified

contrasts. (Note: * p < 0.05, *** p < 0.001.)

Contrast Mx MY MZ

168 cm vs. 251.1 ± 13.2*** 274.5 ± 12.7*** 30.1 ± 2.4

213 cm 194.9 ± 6.9 254.2 ± 8.7 25.3 ± 1.7

HALF-SIZE WMS vs. 205.2 ± 14.1* 242.0 ± 13.2*** 26.3 ± 2.3

FULL-SIZE WMS 219.6 ± 8.1 268.4 ± 8.4 27.3 ± 1.7

FLOOR vs. 216.3 ± 8.8 283.8 ± 7.6*** 28.4 ± 1.9

RIB 215.7 ± 11.2 239.8 ± 11.6 25.8 ± 2.1

RIGHT vs. 230.6 ± 13.1 281.5 ± 12.5 29.4 ± 2.3

LEFT 208.5 ± 9.8 255.2 ± 11.3 25.3 ± 2.6

OVERHEAD vs. 192.3 ± 9.3 275.2 ± 13.4 26.7 ± 2.9

SIDE 206.0 ± 12.6 245.0 ± 14.2 25.1 ± 2.9

3.4. Interaction contrasts

Three contrasts tested for significant interactions, as specified in

Table 1. Only one interaction contrast was found to have a significant effect on the peak extensor moment. Specifically, the interaction between Overhead/Side lift by Left/Right side positioning of

the subject during the lift was found to be significant (t 1⁄4 2.46,

p < 0.05). Fig. 6 shows the nature of the interaction, which

appeared to be the result of a smaller disparity between Overhead

and Side lifts when the subject was located on the right side of the

full-size WMS compared to when the subject was positioned on the

left.

150

170

190

210

230

250

270

290

310

Side Overhead

Peak Extensor Moment (Nm)

Left

Right

Fig. 6. Interaction between lifting to an overhead vs. side-carry position and being

positioned on the left vs. right side of the full-size WMS on the peak extensor moment

about L5–S1. Error bars represent the standard error of the mean.

4. Discussion

Wire mesh screens provide an effective method with which to

prevent falls of rock in a mine. However, these screens are large,

awkward, and bendable loads, complicating manual handling

activities. The difficulties associated with handling such a large

flexible load are apparently exacerbated by restrictions in vertical

space, commonly encountered in underground coal mines. Prior
work has shown that increasingly restrictive vertical workspaces

result in a progressive increase in peak L5–S1 extensor moments on

the low back (Gallagher et al., 2001). The current study, which

imposed a relatively modest restriction in vertical space compared

to the previous study, demonstrated a similar result in terms of

both peak and average L5–S1 extensor moments. The average

extensor moment was at least partially due to a higher minimum

moment, as subjects began the task in a partial stoop, rather than

standing upright. As vertical workspace diminishes, of course, the

trunk is forced to bend forward. Flexion of the trunk will cause the

center of mass of the upper body to move anterior with respect to

L5–S1, increasing the forward bending moment about this joint.

This effect would be present even without any load in the hands,

and adding a load will only magnify the extensor moment.

The effects of restricted space on spinal loading do not appear

confined to a single plane. Lateral bending moments were also

significantly affected bylower working heights. Specifically, subjects

in this study exhibited higher right lateral bending moments when

exposed to reduced headroom. When considered in conjunction

with the increased extensormoment described above, the combined

loading effect associated with restricted space appears to be

a combination that generates particularly high stress concentrations

in the lateral and posterior annulus fibrosus (Adams et al., 2006).

These stress concentrations are thought to be linked with the disc’s

susceptibility to prolapse under the combined influence of bending

and compression (McNally et al., 1993). It is conceivable that

repetitive handling of loads that produce such stress concentrations

(as would be experienced in installing WMS) might increase the

chances of disc prolapse as a consequence of fatigue failure of tissues

in the posteriolateral aspect of the disc (Adams et al., 2006).

Lifting full-size WMS is challenging for several reasons. These

include the fact that a two-person lift is required, the load is quite

flexible, and the load is an awkward size and shape. It is notable,

for instance, that though the mass handled by a subject when

lifting full-size WMS would be slightly greater compared to the

half-size WMS (1.25 kg on average); the L5–S1 peak extensor

moment was approximately 26 Nm higher, and the right lateral

bending was also significantly higher. There may be several

possible explanations for this result. For example, it is understandable that in a two-person lift, it is quite possible for one

person to assume a greater portion of the load at certain times

since it is practically impossible to coordinate such a lift equally

(Dennis and Barrett, 2003). This would be true with a rigid object;

however, the disparity in loading may be magnified when lifting

an object that is flexible and which is likely to sag and experience

oscillations along its length. In this regard, it should be noted that

it would be difficult (if not impossible) to control the load

distribution of such a large and flexible material in a two-person

lift. While we always had the same person lift the side of the

screen not lifted by the subject, differences in stature may have

led to some differences in loading between subjects. It should be

noted, however, that contrast effects were determined on

a within-subjects basis which should minimize the impact of any

such between-subjects difference. The two-person lift of the fullsize WMS was generally observed to take increased time to

accomplish than the single-person lift of the half-size WMS.

In contrast to the combined loading associated with restricted

space, lifting from the floor was found to influence only the

extensor moment about L5–S1. On the other hand, lifting the screen

from the floor produced the highest observed extensor moments of

any contrasts studied. The fact that extensor moments averaged

45 Nm lower when WMS is oriented vertically suggests that storage

of WMS on the floor is not recommended, unless subsequent moves

are made using mechanical assistance. If manual lifts will be performed, WMS should be stored against the rib (as shown in Fig. 1B).

The results of this study can be compared with other research on

biomechanical demands of manual lifting involving awkwardly

shaped or atypical loads in mining. A study by Plamondon et al.

(2006) involved lifting large drill rods for in-the-hole drilling tasks.

Results of this lifting tasks indicated peak resultant moments

ranging from approximately 185 Nm to 275 Nm depending on the

foot position and height of the drill rod at the beginning of the lift.

Peak asymmetrical moments for this task averaged about 75 Nm.

Studies involving lifts of heavy electrical cable have estimated L5–

S1 resultant moments ranging from 200 to 315 Nm depending on

the posture and seam height when performing the lift, with

a stooping posture generally associated with higher extensor

moments (Gallagher et al., 2001, 2002). In the current study, the

peak extensor moments about L5–S1 ranged from 188 Nm to

315 Nm, and peak lateral bending moments ranged from 170 Nm to

270 Nm. Thus, lifting WMS resulted in spinal loads that were of

a similar range in terms of extensor moments compared to the

above mining tasks; however, the asymmetric moments were

higher in the current study. This may be the result of the fact that

the WMS was often brought to a side-carry position (which would

increase the lateral bending moment), and lifting the screen overhead (as in Fig. 2A) also created significant lateral bending

moments about L5–S1. Overall, the spine moments observed in this

study were in a range similar to previously studied high-risk

mining tasks (Gallagher et al., 2001; Plamondon et al., 2006).

In the study by Gallagher et al. (2002), an extensor moment of

315 Nm was associated with an L5–S1 compressive force of

approximately 6 kN, while an extensor moment of 200 Nm was

associated with a compressive force averaging approximately

3.4 kN. These values are close to the NIOSH Maximum Permissible

Limit (6.4 kN) and Action Limit (3.4 kN), respectively (Waters et al.,

1993). These compressive loads are also very near the reported

range of 6.1–10.2 kN average ultimate strength of vertebral endplates for males aged 20–50 years (Adams et al., 2006). Assuming

that similar moments lead to similar compressive loads, this finding

would suggest that some of the more difficult lifting conditions in

the current study might result in compressive loads that could

cause fractures in the endplates of those performing such a lift.

When considering the fact that this lifting task is repetitive in

nature, it is very likely that fatigue failure of vertebral endplates

would occur with a sufficient number of loading cycles at such

a load magnitude (Brinckmann et al., 1988; Gallagher et al., 2007).

The number of loading cycles necessary would be a function of both

the magnitude of the applied load and the strength of an individual’s lumbar spine. In cadaver fatigue failure studies, it has been

found that 73% of lumbar motion segments survived 5000 loading

cycles when loaded at 30–40% of a spine’s predicted ultimate stress.

When loaded at 60–70% of the ultimate stress, almost two-thirds of

lumbar motion segments tested lasted less than 100 loading cycles

(Brinckmann et al., 1988). Thus, it is important to consider methods

that can reduce the spinal load when handling WMS so that the risk

of injury is reduced. Such methods may include use of mechanical

handling devices such as winches, reducing the magnitude of the

load on the spine, and reducing the frequency of lifting.

The data from this study can be used as a starting point for recommending practices that might reduce low back loading and

musculoskeletal risk during manual handling of WMS. Additionally,

it may be helpful to consider additional methods of storing and

handling screen that might be useful in reducing low back injury

risk. Currently, the most common method of delivering roof screen

to the area being roof bolted is to drag them using a chain to the

active section of the mine, then lift the screens up, one or two at

a time, and either lean them against the rib or leave them lying on

the mine floor. Data from this study clearly show that leaning the

screens against the rib is preferable. Putting the screens in an
upright orientation greatly reduces the biomechanical load on the

low back during lifting, and has the additional benefit of keeping the

screens out of a potentially muddy floor and reduces the chances

that the screens will present a tripping hazard. Broken, bent and

muddy screen is much more difficult to handle and install. As discussed earlier, handling screen in mines with restricted vertical

space resulted in significantly increased low back loading in the

current study. This suggests the need to find methods to increase

mechanical handling of screenwhen restricted headroom is present.

One supply method that may be helpful in both low- and highseam mines is the use of racks or rails (Fig. 7) installed on the

bolting machine.

Fig. 7. Roof bolter with rails for storage of roof screen (Compton et al., 2007).

These racks are capable of holding enough sheets

of WMS to complete the bolting of roof exposed from a newly

mined section. Keeping WMS on the roof bolter is both efficient

(reducing the need to walk back to retrieve sheets of screen) and

can greatly reduce the lifting demands on the worker. Some mines

have found that they can use a winch to load the roof screen on the

rails, eliminating the need to manually handle screens until they

need to be moved just prior to the bolting process. Furthermore,

a more extensive rail system has been tested that facilitates sliding

full sheets of WMS using rails all the way up to the front of the

bolter, eliminating the lifting requirement altogether (Compton

et al., 2007). Development of methods to improve efficiency and

reduce musculoskeletal demands are important considerations

that may help roof screen achieve its potential to prevent serious

injuries caused by the fall of small rocks in underground mines.

As with all studies, limitations associated with the current work

must be addressed. One limitation that must be noted is that the

subjects in this experiment were not experienced with lifting roof

screen, and this limitation may affect the external validity of the

current analysis. However, the performance of the lifting tasks was

considered realistic by a former roof bolter who was part of the

research team. In addition, the sample size was also somewhat

small; however, the finding of numerous significant results

suggests that statistical power was more than adequate for the

measures studied.

5. Conclusions

The following conclusions are drawn from the present study:

1. Restricted vertical space increases both L5–S1 extensor

moments and increased lateral bending moments when

handling wire mesh screen.

2. In contrast to the effects of vertical space restriction, lifting

screen from flat on the floor (as opposed to a vertical orientation) significantly affected only the extensor moments for

L5–S1.

3. Lifting of full-size WMS (a two-person lift) resulted in a 26 Nm

increase in the L5–S1 extensor moment compared to lifting the

half-size WMS (one-person lift), though the average difference

in the load being lifted was approximately 1 kg.

4. Methods of storing, lifting, and carrying WMS can have

a significant effect on the biomechanical stresses experienced by

workers handling this material, and results suggest that simple

steps (i.e., storing WMS upright against the mine ‘‘rib’’) can

reduce the loading experienced by miners handling screens.

Acknowledgements

The authors gratefully acknowledge the assistance of Mr. Bill

Porter, Ms. Kelly Baron, Mr. Al Cook, and Ms. Mary Ellen Nelson in

the performance of this study.

Disclaimer: The findings and conclusions in this report are those

of the author(s) and do not necessarily represent the views of the

National Institute for Occupational Safety and Health.

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