THE EFFECTS OF SCALING HEIGHT AND SCALING 

BAR DESIGN ON APPLIED FORCES AND BILATERAL MUSCLE ACTIVITY 

OF THE BACK AND SHOULDERS 

William Porter, Sean Gallagher, Carrie Reinholtz, Janet Torma-Krajewski

National Institute for Occupational Safety and Health 

Pittsburgh, PA 

Hand scaling is a physically demanding job and is responsible for numerous overexertion injuries in 

mining. This experiment studied rib scaling from an elevated bucket to examine force generation 

capabilities and electromyographic responses to a prying subtask. Subjects exerted force using two bars 

(steel and fiberglass) at five target heights. Work height significantly affected peak prying force during 

scaling activities with highest force capacity at the lowest level (p = 0.0188). Bar type did not affect force 

generation (p = 0.7843): However, use of the fiberglass bar required significantly more muscle activity to 

achieve the same force (p < 0.05). It was concluded that miners should scale points on the rock face that 

are below their knees, and reposition the bucket as often as necessary to do so. Additional research is 

needed to fully understand the impact of bar type on the physical demands of an entire scaling task. 

INTRODUCTION 

 The roof and ribs of underground mines are 

examined and scaled on a regular basis to remove loose 

surface rock. Hand scaling is a physically demanding task, 

that is associated with overexertion injuries. Most 

overexertion injuries involve the back and shoulders with the 

major injury being a pulled back muscle (MSHA, 2000-2004). 

The average lost days per scaling overexertion injury was 65 

days compared to 28 days for all types of scaling related 

injuries. 

 In high seam mines, scaling requires miners to work 

from an elevated bucket, positioned a few feet from the rock 

face. Once in position, miners tend to start scaling at shoulder 

level and then work downward. The scaling process starts 

with a visual inspection of the rock face to locate cracks or 

other signs that a rock must be removed. The miner then uses 

a scaling bar to remove loose rock from the face. This 

normally involves a hard jabbing of the bar into a crack 

between the rock and the face and then prying to pull the rock 

away. The bucket is usually not moved until the miners have 

scaled everything they can reach from their current position. 

 The goal of this study was to examine the force 

generation capabilities and electromyographic responses to 

simulated hand scaling exertions in a laboratory setting. The 

focus of this study was the prying action on the rock face 

since the maximum effort occurs during this subtask. It was 

thought that knowledge of the physical demands of this 

subtask could lead to recommendations for appropriate work 

height, bar type and bucket position. 

METHODS 

Subjects 

 Ten male subjects aged 32-53 and in good health 

participated in this study. Two subjects had previous 

experience with hand scaling in a mining environment. The 

subjects were volunteers and operated under terms of 

informed consent. The mean height of the subjects was 180.3 

cm (SD= 3.8 cm) and the mean weight was 90.5 kg (SD= 

10.28 kg). Prospective subjects were excluded if they 

indicated they had not completely recovered from any prior 

musculoskeletal injury, or if any medical conditions 

contraindicated their participation. 

Experimental Design 

 Two independent variables, the height of the target 

on which the force was generated (five levels) and the scaling 

bar type (2 levels), were investigated during this study. 

Subjects were treated as a blocking variable. Dependent 

variables consisted of the force (in Newtons) generated by the 

subject against a dynamometer at five specified target heights, 

and normalized surface electromyographic (EMG) activity 

from four muscles (Keppel, 1991; Kirk, 1982). 

 A split-split plot experimental design was used in this 

study. Subjects were treated as whole plots, while the target 

height was treated as split plot, and bar type constituted the 

split-split plot. Two randomizations were performed within 

each subject – the first was a randomization of the target 

heights and within each level of this variable was restricted 

randomization of the bar type. Three replications of each 

combination of target height and bar type were performed 

(Keppel, 1991; Kirk, 1982). 

Procedure 

 Four EMG electrodes were placed on the skin’s 

surface above the left and right deltoids (medial division) and 

left and right erectores spinae muscle groups (L3 lumbar 

level). The signal was collected at 1000 Hz and amplified 

using a low noise instrumentation amplifier with a commonmode rejection ratio of 110 dB. All EMG signals were 

processed using a high-pass filter to remove potential motion 

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artifacts in the form of frequencies below 20 Hz and a lowpass filter to remove unwanted frequencies above 450 Hz. 

 The EMG signal for each muscle was normalized to 

signals obtained during maximal voluntary contractions 

(MVC). For the back muscles, the MVC consisted of an 

isometric trunk extension against resistance from a flexed 

posture. For the deltoids, the MVC consisted of shoulder 

abduction against resistance. 

Figure 1. Experimental Setup. 

 A force gauge was mounted on a wall at one of five 

locations, Figure 1. The five vertical locations of the force 

gauge (in cm) were 0, 56, 107, 144, and 176. The five heights 

chosen represented the 50th percentile male’s anatomical 

positions of the floor, knee, elbow, shoulder, and stature. To 

ensure correct placement of the bar, the tip was placed on the 

dynamometer by researchers before each test. Subjects were 

asked to apply a downward maximum voluntary isometric 

muscular force in accordance with standard isometric strength 

testing procedures (Caldwell et al., 1974). 

 Within each vertical target height, subjects were 

randomly assigned to first use either a steel or a fiberglass 

scaling bar. Both bars were 250 cm long and approximately 3 

cm in diameter; the steel bar weighed 6.4 kg, while the 

fiberglass bar weighed 4.0 kg. Two minutes of recovery time 

were given after each trial (Caldwell et al., 1974). 

Data Treatment 

 Data analysis consisted of analysis of variance 

(ANOVA). Significant effects on target height were 

followed-up with Bonferroni-corrected post hoc procedures to 

determine which of the heights were significantly different 

with respect to the exerted force or EMG activity. 

RESULTS 

Prying Force 

 From the data analysis, it was found that peak prying 

force was significantly affected by target height 

(F(4,36)=3.39, p=0.019). The type of bar was found to have 

no significant effect on either the peak (F(1,245)=0.08, 

p=0.784) or mean (F(1,245)=0.50, p=0.480) force generated. 

The relative differences in prying forces (peak and mean) as a 

function of work height are shown in Figure 2. Results of the 

Tukey post hoc tests indicated that the force generation at the 

floor target height was significantly greater than the force 

generated at the waist and shoulder heights. 

60

65

70

75

80

85

90

95

100

105

110

Floor Knee Waist Shoulder Stature

Work Height

Force (N)

Peak

Mean

( bars represent standard error of the mean )

Figure 2. Prying force as a function of work height. 

Muscle Activity 

 The type of bar used during the prying task was 

found to significantly affect muscle activity (as detailed 

below), but the height at which the task was done was not 

significant (p>0.05). For this reason, muscle activation data 

were aggregated over the heights and analyzed with respect to 

bar type. Separate analyses were performed on the mean and 

peak normalized EMG data. 

0

10

20

30

40

50

60

70

Left Deltoid Right Deltoid Left ES Right ES

Muscle Group

Normalized EMG (% MVC)

Fiberglass

Steel

** significant difference at p<.001

 * significant difference at p<.01

*

**

*

Figure 3. Mean muscle activity as a function of bar type. 

 The effect of bar type on mean muscle activity is 

shown in Figure 3. Subjects demonstrated 10% greater mean 

muscle activity in the left deltoid (F(1,245)=9.41, p=0.002) 

and the right erector spinae muscle (F(1,245)=14.86, 

p=0.0001) with the fiberglass bar than with the steel bar. 

Figure 4 illustrates a significant interaction between bar type 

and work height for mean muscle activity of the right deltoid 

(F(4,245)=2.45, p=0.047). The right deltoid was less

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activated at knee and stature levels when using the steel bar 

(stature contrast 6.15; F(1,245)=7.26, p<0.01; knee contrast 

7.05; F(1,245)=5.89, p<0.05). This interaction may be due to 

the divergence of the data at knee and stature levels, postural 

changes at these two levels, or differences in bar properties. 

15

20

25

30

35

40

Floor Knee Waist Shoulder Stature

Work Height

Normalized Mean EMG (% MVC)

Fiberglass

Steel

 ( bars represent standard error of the mean )

Figure 4. Interaction between bar type and work height 

for the mean muscle activity of the right deltoid. 

 Figure 5 illustrates the effect of bar type on peak 

muscle activity. Peak muscle activity data, like mean data, 

demonstrated subjects experienced significantly greater 

muscle activity for both left (F(1,245)=7.21, p=0.008) and 

right deltoids (F(1,245)=12.06, p=0.0006) when using the 

fiberglass bar rather than the steel bar. 

0

10

20

30

40

50

60

70

80

Left Deltoid Right Deltoid Left ES Right ES

Muscle Group

Normalized EMG (% MVC)

Fiberglass

Steel

*

**

** significant difference at p<.001

 * significant difference at p<.01

Figure 5. Peak muscle activity as a function of bar type. 

DISCUSSION 

 The objective of this study was to examine the force 

generation capabilities and electromyographic responses to 

simulated hand scaling activities. Results of this study 

showed force generation capability was a function of work 

height and muscle activity was primarily a function of bar 

type. 

Effect of Work Height 

 Work height was found to significantly affect the 

maximum amount of prying force a subject could generate 

during this simulated subtask. This study indicated a reduced 

capacity in prying force around the middle of the body, and 

higher force generation at head level and below knee level. 

The higher forces at lower heights may be attributed to using 

body weight to increase force generated, while higher forces 

at stature level may be attributed to “hanging” on the bar to 

increase the amount of force exerted. Around the waist area a 

subject cannot easily use their body weight to generate 

additional force, which may result in reduced capacity. The 

amount of muscle activity stayed relatively constant at each 

target height. Thus, force differences observed in this study 

were not due to changes in muscle activity but were most 

likely due to mechanical advantages such as body weight. 

These results lead to the conclusion that miners should scale 

points on the rock face that are below their knees and 

reposition the bucket as necessary to accommodate that work 

height region. While higher force capacity was observed at 

stature level the act of scaling at that level requires a miner to 

manipulate the bar substantially more then at the knee level 

which could introduce additional joint forces and awkward 

postures. It is for this reason that stature level was not 

included in the recommendation. It must be recognized that 

two workers will often be in the same bucket so coordination 

of the bucket movement is a critical aspect of this 

recommendation. 

Effect of Bar Type 

 The type of bar used was not found to affect the 

force generated at any heights tested. However, increased 

muscle activity was required to generate the same amount of 

force with the lighter fiberglass bar as opposed to the heavier 

steel bar. A heavier bar may necessitate more muscle activity 

to hold, lift, and control the bar for the entire task. However, 

for the subtask studied the heavier bar required significantly 

less muscle activity in some muscles studied. For this 

subtask, results suggest that if a person needs to utilize a tool 

to exert a force, a lighter tool may actually require more 

muscle activity to reach the same force level generated by a 

heavier tool. However, it may be necessary to use both types 

of bars or even several different lengths of bar to reduce the 

stress placed on the body during all subtasks of hand scaling. 

More research on scaling tasks and the effect of using 

different bar types must be completed to offer 

recommendations on bar type. 

 Previous research on scaling bars by Marras and 

Lavendar (1991) examined overhead roof scaling using 

different weights and types of scaling bars. Their study based 

recommendations on predicted spine compression forces and 

striking force at the tip of the scaling bar. The authors found 

predicted spine forces were lower for the lighter bar and gave 

slightly lower average striking force, but the difference in 

striking force was not found to be statistically significant. 

The results of the current study indicated that it was more 

difficult to get the same prying force when using the lighter 

bar. Subjects had to work harder as indicated by the increased 

muscle activity to accomplish the same task when using the

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9
lighter bar. Differences in the two studies are likely due to the 

different subtasks examined. The question remains which 

type of tool is best for which task. A holistic study of scaling 

is needed to make full recommendations on bar type. 

Limitations 

 The one subtask (prying) studied may not be 

representative of the overall demands of scaling. The findings 

of this study should be used to develop an experimental 

protocol to study other subtasks so that work practice 

guidance could be developed with the goal of reducing the 

injury risk to miners. Two other possible limitations are that 

most subjects were untrained in hand scaling and that work 

heights were calculated from the 50th percentile male’s 

anthropometric data instead of using subject specific work 

heights. These two limitations are felt to have little impact on 

the validity of the results since the subject sample closely 

conformed to the 50th percentile for stature (180.3 cm [+ 3.8] 

as opposed to the 50th percentile of 177.8 cm) and the two 

workers who had experience in hand scaling displayed the 

same data trends as the untrained subjects. 

Conclusion 

Results of this study support the following conclusions: 

• Work height affected the peak and mean force 

generation capability during simulated scaling activities 

with the highest force generated at the lowest level. 

• The type of bar utilized did not significantly influence 

the force applied in scaling. 

• The lighter fiberglass bar was generally associated with 

higher muscle activity for the prying subtask studied 

when compared to the heavier steel bar. 

 An important recommendation for the mining 

industry is to reposition the bucket to allow for scaling of rock 

below knee level. In addition the data will provide a better 

understanding of the physical requirements associated with 

hand scaling. 

ACKNOWLEDGMENTS 

 The authors would like to acknowledge the 

contributions of the following individuals in the conduct of 

this study: Mary Ellen Nelson, Al Cook, George Fischer, Bill 

Rossi, and Leigh McClure. 

DISCLAIMER 

 The findings and conclusions in this report are those 

of the authors and do not necessarily represent the views of 

the National Institute for Occupational Safety and Health 

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*This work is not subject to U.S. copyright restrictions