MUSCULOSKELETAL STRESS ON MINERS PERFORMING 

ROOF SCREENING OPERATIONS 

Susan Kotowski†

, Sean Gallagher*, Kermit Davis, † Kelly Baron*, Craig Compton* 

†

Low Back Biomechanics and Workplace Stress Laboratory, 

University of Cincinnati, Cincinnati, OH 

*National Institute for Occupational Safety and Health, Pittsburgh Research 

Laboratory, Pittsburgh, PA 

Roof screen is often bolted to the mine ceiling to help control hazardous rock falls 

in coal mines. While the screen prevents rock fall injuries, its installation may 

expose the miner to musculoskeletal stress. The purpose of this study was to 

evaluate methods of handling roof screen. Subjects performed installation tasks 

under a normal and intervention condition while trunk kinematics and muscle 

activity data were collected. Trunk kinematics were not affected by the 

intervention but were significantly higher in the morning than in the afternoon. 

Muscle activity did not differ significantly with seam height but was significantly 

reduced by the intervention. Overall, this study showed that musculoskeletal 

stressors during screen installation were reduced by the proposed intervention. 

INTRODUCTION

Coal miners work in a hazardous environment 

where they are frequently exposed to poor roof 

conditions that put them at risk of a rock fall injury. 

Currently, very few coal mines in the Eastern U.S. 

install protective roof screens, though most 

Western longwall development sections make 

extensive use of screen. Between 1999 and 2004, 

there were more than 500 reported injuries each 

year resulting from rock falls. A detailed analysis of 

rock fall injuries during a single year determined 

that nearly 100% of them occurred where the 

miners should have been protected by roof support 

(Bauer and Dolinar, 2000). An analysis of injury 

data from two eastern U.S. coal mines showed that 

when roof screen was used, the number of rock fall 

injuries was reduced (Robertson et al., 2003). 

Of all the surface control techniques, roof 

screens (welded wire mesh) provide by far the 

most protection. The reason that roof screen is 

more effective than other supports is simply that 

it covers more surface area of the roof– close to 

100% protection can be achieved. 

However, the handling of roof screen may 

introduce other injury hazards to workers—

specifically musculoskeletal disorders resulting 

from lifting, carrying, and installing the screen. The 

screen is typically lifted by two workers on either 

side at the rear of the bolter, and carried to the 

front where it is placed on temporary roof supports 

and then bolted to the roof. Screen handling can 

be a challenge for the operators because it often 

requires overhead lifts and awkward positioning, 

i.e., lifting, pulling, and twisting movements. In 

addition, the screen can be quite heavy depending 

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

gage of the steel. Typical sheets weigh about 30 

pounds, but one mine in the western U.S. installs 

8-gauge steel sheets that are 20 feet long and 5.5 

feet wide, and weigh over 50 pounds. Evidence 

has shown that high levels of exposure to a 

combination of physical factors such as a repetitive 

lifting of heavy objects in extreme or awkward 

postures conveys risk of musculoskeletal injury 

(Hales and Bernard, 1996). 

There have been very few ergonomic analyses of 

roof screen installation process. As a result, there is 

a need to identify high-risk activities and develop 

work methods to reduce the risk factors associated 

with implementation of the roof screens. The 

purpose of this study was to evaluate methods of 

handling roof screen to assess whether the 

provision of elevated rails to support screen 

reduces musculoskeletal stress of the task. 
METHODS 

Subjects 

Eight male subjects with an average age of 55 

(SD=6) years old participated in the study. All of 

the participants had some experience in mining 

operations. 

Experimental Design 

A blocked, repeated measures design was 

implemented with the blocking factor of seam 

height. The independent variables were seam 

height (high and low), side of bolter (right and left 

side), time (morning and afternoon), and 

intervention (with and without). Within the 

intervention trials there were two methods of 

moving the screen, as described in the following 

section. The dependent variables were trunk 

kinematics and muscle activity of trunk and 

forearm muscles. Two repetitions were performed 

for each trial. For each subject, electromyography 

(EMG) data was collect for 24 trials, and Lumbar 

Motion Monitor (LMM) data was collected for 48 

trials. 

Experimental Conditions 

Two seam heights were tested – 60 inches (low) 

and 84 inches (high), which corresponded to 

common seam heights in mines. Two subjects 

performed each task simultaneously, with one 

subject on the left and the other subject on the 

right side of the bolter. The subjects took turns 

lifting the screen from both the left and right sides 

of the bolter. Due to data collection capabilities 

(only one EMG system), each participant 

completed all conditions twice—once in the 

morning and once in the afternoon where 

kinematics were collected in both sessions while 

muscle activity was collect in one of the sessions 

(half in the morning and half in the afternoon). For 

the intervention type, there were two conditions: 

one where the participants carried the roof 

screens above the bolter and one that had rails 

above the bolter to help the participants slide the 

roof screens above the bolter. Within the trials 

with rails there were two methods: 1) subjects 

were straight across from one another and 2) 

subjects were at an angle to each other while 

sliding the roof screen. The rails were elevated 

metal tubing that ran almost the entire length of 

the bolter and were located on both sides of the 

bolter. The rails allowed the participants to slide 

the screens over the bolter without having to 

negotiate the objects located on the bolter or 

having to lift the roof screen the full length of the 

bolter (Figure 1). 

Tasks 

A 5 foot by 13 foot piece of metal roof screen 

weighing approximately 30 pounds was placed 

on the ground at the back of the bolter. For each 

condition (combination of seam height, 

Figure 1. Test performed in Human 

Performance Research Mine. Note rails used 

to slide roof screen across bolter. Miners are 

under 60” roof. 

side, and intervention), the roof screen was 

transferred over the bolter by the pair of subjects. 

The roof screen was picked up and either lifted 

over the bolter or slid on the intervention rails, 

and finally placed at the front of the bolter. 

Subjects started in an upright or slight flexed 

position (depending on seam height) before 

simultaneously bending to pick up the screen. 

Equipment 

Trunk kinematics were measured using the Lumbar 

Motion Monitor (LMM) which is essentially an 

exoskeleton of the spine worn on the back using a 

shoulder harness and waist belt. The LMM 

measures the instantaneous trunk position, 

velocity, and acceleration in the sagittal, lateral, and 

twist planes. 

EMG was used to measure the muscle activity of 

the right and left pairs of the latissimus dorsi, 

erector spinae, internal obliques, external obliques, 

rectus abdominius, and forearm muscles. A pair of 

electrodes were placed over each muscle belly and 

EMG signals were appropriately filtered and 

processed. All EMG signals were normalized to a 

Roof 

Screen Rails 
Maximum Voluntary Contraction value obtained in 

either flexion or extension for the trunk and 

maximum grip exertion for the forearm muscles. 

Statistical Analyses 

Since LMM data was collected in both the 

morning and the afternoon, the repeats of each 

trial were averaged for the trunk kinematic 

variables before being analyzed. Since EMG data 

was collected in either the morning or the 

afternoon there was no time variable analyzed. In 

addition, preliminary analysis showed no 

significant differences between the two subintervention trials so they were combined for the 

subsequent analyses. A split-plot analysis of 

variance (ANOVA) was then conducted for each 

dependent variable (trunk kinematics and muscle 

activity) using SAS (SAS Institute Inc., Cary, NC) 

to determine whether significant main effect and 

interactions were present. Follow-up multiple 

comparisons using Tukey tests to determine the 

source of the difference (α=0.05)

RESULTS 

Trunk Kinematics 

The study variables task and side were not 

significant for any of the kinematic variables. None 

of the interactions were significant except for the 

time x side interaction for maximum left and right 

twist (p<0.05). The interaction time x seam was 

also significant for average sagittal velocity. 

The time variable (morning versus afternoon) was 

statistically significant for almost all the trunk 

kinematic variables. In all cases, the positions, 

velocities, and accelerations were higher in the 

morning than in the afternoon. Figure 2 shows the 

differences in trunk postures between the morning 

and the afternoon. On average, subjects had 

approximately 6o

 (~26%) more flexed postures in 

the morning, with increases ranging from 4.2o

 to 

8.3o

 (16-48%). 

A morning and afternoon difference was also found 

for maximum lateral and sagittal velocities where 

morning velocities were 9.1o/sec. (22%) and 

18.5o

/sec. (30%) faster than the afternoon 

Positional Differences Between AM & PM

0

10

20

30

40

50

60

70

Max Left

Bend

Max Lateral

Range

Max Flexion Max Sagittal

Range

Max Right

Twist

Degrees

AM

PM

 Figure 2. Trunk Postures in AM & PM tasks. 

motions (lateral AM=50.2 o/sec, PM=41.1 o/sec, 

sagittal AM=80.2o

/sec, PM=61.7 o/sec). Lateral 

and sagittal accelerations followed a similar trend. 

Maximum lateral accelerations were 70.6o

/sec2

(28%) faster in the morning and maximum sagittal 

accelerations 117.4o

/sec2 (40%) faster (lateral 

AM=318.4o

/sec2, PM=247.8o/sec2, sagittal 

AM=412.9o

/sec2, PM=295.5o/sec2). 

In addition, seam height (high versus low) was 

also statistically significant for the majority of the 

trunk kinematic variables. Maximum sagittal range 

was just over 10o

 (22%) greater at the high seam 

condition (Figure 3). Maximum left twist was 

nearly 7o

 (87%) greater in the high seam condition 

and maximum right twist was nearly 5o

 (31%) 

greater than at the low seam height. Maximum 

sagittal range was nearly 11o

 (22%) greater in the 

high seam condition. However, at the low seam 

height maximum extension was nearly 12o

 (107%) 

less and maximum left bend was nearly 3o

 (16%) 

less than at the high seam height. 

 

Positional Differences Between High & Low Seam

-10

0

10

20

30

40

50

60

Max Left

Bend

Max

Extension

Max Sagittal

Range

Max Left

Twist

Max Right

Twist

Degrees

High

Low

 Figure 3. Trunk Postures in High & Low Seams 
Average lateral and twist velocities were also 

significantly higher at the high seam height with 

increases ranging 2.1-5.1o

/second (38-78%) faster 

than at the low seam height (lateral HS=7.7o

/sec, 

LS=5.5 o/sec, twist HS=11.5 o/sec, LS=6.5 o/sec). 

Maximum lateral and sagittal velocities followed a 

similar trend with 5.6o

/sec. (13%) and 6.7o/sec. 

(10%) increases at the high seam height, 

respectively (lateral HS=48.2 o/sec, LS=42.6 o/sec, 

sagittal HS=73.7 o/sec, LS=67.0 o/sec). In addition, 

maximum lateral acceleration was nearly 

48o

/second2 (18.6%) faster while maximum twist 

acceleration was 128.9o

/second2 (49.2%) faster at 

the high seam height compared to the low height 

(lateral HS=304.6o

/sec2, LS=256.9 o/sec2, twist 

HS=391.0 o/sec2

, LS=262.0 o/sec2). 

Muscle Activity 

There was no statistically significant effect of seam 

height on muscle activity. Intervention and side 

were significant along with the interactions seam x 

intervention, seam x side, and intervention x side 

for selected muscles. As can be seen in Figure 4, 

the left erector spinae, and left and right pairs of the 

internal obliques had increased muscle activity 

under the normal condition as compared to the 

intervention condition. Increases in muscle activity 

between the normal condition as compared to the 

intervention ranged from 14.5 to 29% of MVC (or a 

26-97% increase in activity). 

There were significant differences in muscle 

activity when the side of the bolter (left or right) the 

subject was on was analyzed (Figure 5). Muscle 

activity for the right and left pairs of the erector 

spinae, and internal and external obliques was 

significantly greater for the muscles on the side of 

the body contralateral to the side of the bolter on 

which the task was being performed. Although the 

subject was tested on both sides of the bolter, the 

magnitudes of the differences were not equal when 

both side of the muscle pair and side of the bolter 

were considered. 

Task duration was measured for each intervention 

conditions. The straight intervention took the least 

time (16.4 seconds on average), followed by the no 

intervention condition (17.0 seconds) and the 

angled intervention (17.4 seconds). 

DISCUSSION 

There were several interesting results from the 

current study. First, the results indicated that

Difference between Normal and Intervention

0

10

20

30

40

50

60

70

80

LES LIO RIO LEO REO LF RF

Percent (%) MVC

Normal

Intervention

Figure 4. Muscle Activity During Normal and 

Intervention Conditions (LES-left erector spinae, LIOleft internal oblique, RIO-right internal oblique, LEO-left 

external oblique, REO-right external oblique, LF-left forearm, 

RF-right forearm) 

0

10

20

30

40

50

60

70

80

LES RES LIO RIO LEO REO LRA RRA LF RF

Percent (%) MVC

Left

Right

Figure 5. Muscle Activity Between Left and Right 

Sides (LES-left erector spinae, RES-right erector spinae, 

LIO-left internal oblique, RIO-right internal oblique, LEO-left 

external oblique, REO-right external oblique, LRA-left rectus 

abdominis, RRA-right rectus abdominis, LF-left forearm, RFright forearm)

participants lifted with more awkward and 

extreme postures as well as utilized faster 

motions in the morning session as compared to 

the afternoon. This may indicate that miners 

(and workers in general) may be at more risk of 

back injuries in the morning due to the motions 

they adopt. Although not collected in the current 

study, muscle activities could further 

differentiate the response in the morning and 

afternoon and shed more light into the 

biomechanical responses. 

Second, it was not surprising seam height had 

an impact on the way the participants 

transferred the roof screen. The low seam 

required the participants to flex forward while 

transferring the roof screen as well as made it 

more difficult to move due to less clearance 
between roof and bolter. It was interesting that 

muscle activity was not impacted by the seam 

height. This may indicate a muscular 

compensation during the more flexed conditions 

of the low seam. The length/strength 

relationship of the muscles would indicate that 

trunk muscle forces may be different under 

these two conditions (higher forces for low 

seam versus high seam). 

Third, the intervention appeared to reduce the 

biomechanical stresses on the miners. While 

trunk kinematics were not impacted by adding 

the slide rails, muscle activity was dramatically 

decreased for several trunk muscles and the 

forearm muscles. This would indicate that the 

loads on the trunk and arms were smaller for 

the intervention. This is good news with respect 

to reducing the stress on miners because it 

increases the likelihood that the screen will be 

installed. Similar interventions in Australian 

mines reportedly have had some success in 

facilitating installation of screen. 

Fourth, there was also a difference when lifting 

from the right as compared to the left side. 

Marras and Davis (1998) also reported that 

individuals lift differently for left and right 

asymmetries, indicating that adaptations occur 

in the different directions. A similar response 

could be occurring in this task where individuals 

are more adapted to lift in one direction 

compared to the other. This would indicate that 

bolter operators on one side of the task might 

be at more risk of injury than the other. There 

also appears to be compensation between the 

trunk and arms when lifting in one direction than 

the other. The forearms were utilized slightly 

more on the left while the trunk muscles were 

higher on the right side. 

In order to put the results in perspective, a few 

other limitations must be addressed. First, the 

muscle activity was collected on one subject at a 

time and thus, the concurrent muscle responses 

are unknown. While we did observe all subjects 

on both sides of the roof screen, we do not know 

for sure that the coactivity pattern was identical 

under the repeats, especially since one occurred 

in the morning and one in afternoon. Second, 

there were limited subjects (n=8) and these 

subjects may not have been representative of the 

coal mining population. Third, all participants 

were males. Females may exhibit different EMG 

and trunk motion responses. Fourth, although the 

conditions simulated a real mine environment, the 

environmental factors were stable with adequate 

lighting. Other conditions such as poor lighting, 

cold temperatures, and slippery floors may have 

an impact on the responses of miners. 

CONCLUSIONS 

Based on the results of this study, the following 

conclusions were drawn: 

1. The use of a rail to slide the roof screen 

across the bolter appeared to reduce back and 

forearm muscle activity. Additional work is 

needed to reduce demands of the beginning lift of 

the roof screen and final placement on the 

Automated Temporary Roof Support. 

2. Subjects had increased trunk motion in the 

morning and may be more vulnerable to back 

injury at the beginning of a shift as a result. 

3. While the rail system reduced physical 

demands during the middle portion of the screen 

installation, it is clear that additional work is 

necessary to reduce the physical demands of 

screening, particularly during the initial lift onto 

the rail. Future work will examine potential 

methods of reducing these demands and making 

the process more efficient. 

ACKNOWLEDGMENTS 

Mention of specific items or products in this report 

does not constitute endorsement by the National 

Institute for Occupational Safety and Health.

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.

REFERENCES 

Bauer, E.R., and Dolinar, R.J., 2000. NIOSH 

Information Circular 9453, pp. 99-110. 

Hales, T.R., Bernard, B.P., 1996. Orthop Clin North 

Am, 27, 4, 679-709. 

Marras, W.S., and Davis, K.G., 1998. Spine loading 

during asymmetric lifting using one versus two 

hands. Ergonomics 41, 6, 817-834, 1998. 

Robertson, S.B., Molinda, G.M., Dolinar, D.R., 

Pappas, D.M. Best practices and bolting machine 

innovations for roof screening. 2003 SME Annual 

Meeting, Feb 24-26, Cincinnati, Ohio, preprint 03-

158. Littleton, CO: Society for Mining, Metallurgy, 

and Exploration, Inc., Feb; 1-8, 2003.