Task-specific postures in low-seam underground coal mining

Susan M. Moore*,

* Corresponding author. 626 Cochrans Mill Road, PO Box 18070, Pittsburgh, PA

15236, United States. Tel.: þ1 412 386 6613. 

E-mail address: SMMoore@cdc.gov (S.M. Moore). 

Jonisha P. Pollard, Mary Ellen Nelson

Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research, Human Factors Branch, United States

abstract

The objective of this study was to determine low-seam mine worker exposure to various postures as they

pertain to job classifications and job tasks. Sixty-four mine workers from four low-seam coal mines

participated. The mine workers reported the tasks they were required to complete and the two postures

they used most frequently to perform them. They were provided with a schematic of postures from

which to select. The two postures reported most frequently were identified for each task along with the

job classification of the workers performing the tasks. Of the 18 tasks reported, over two thirds were

performed by at least two different job classifications and over one third were performed by four or more

job classifications. Across tasks, the postures used appeared to vary greatly. However, when grouped by

job classification, the most frequently reported posture across all job classifications was kneeling near

full flexion. Operating the continuous miner was associated with frequent squatting and was likely used

because it affords great mobility, allowing operators to move quickly to avoid hazards. However, for

environments with a restricted vertical height such as low-seam mining, the authors recommend

squatting be avoided as data demonstrates that large amounts of femoral rollback and high muscle

activity for the extensors when performing lateral lifts in this posture. Kneeling near full flexion was

reported as the most frequently used posture by all job classifications and was likely due to the fact that

it requires the least amount of muscle activity to maintain and has reduced pressures at the knee.

However, the authors recommend this posture be avoided when performing lateral lifting tasks. Like

squatting, kneeling near full flexion results in increased femoral rollback and may increase the stresses

applied to the meniscus. Unlike lateral lifting, maintaining a static posture results in knee loading and

muscle activity such that the mine worker should consider kneeling near full flexion and sitting on their

heels. Although kneeling near full flexion is associated with injuries, there are benefits to this posture

that are realized when statically kneeling (minimal muscle activity, allows worker to maintain an upright

torso in low heights, and decreased loading at the knee). However, cartilage is avascular and nourished

by synovial fluid. Therefore, one should frequently rotate between postures, assuming a more upright

kneeling posture when possible and frequently fully flexing and extending the knee allowing nutrients to

the cartilage.

Relevance to industry: In 2009, over one fourth of underground coal mines that produced coal in the

United States were considered low seam with an average working height of <109.2 cm (MSHA, 2009)

restricting workers to their knees. Data exists regarding the biomechanical implications of kneeling

postures and demonstrates the possibility of detrimental consequences to varying degrees for each

posture. With each posture posing a different level of exposure to musculoskeletal disorder risk factors, it

is essential to determine the postures mine workers use to perform their job tasks and how their postural

options are restricted by the low-seam underground mining environment.

1. Introduction

In 2009, 532 underground coal mines produced a total of 330.8

million tons of coal (MSHA, 2009). Of these mines, 148 (28%) were

considered low seam with a seam height of less than 109.2 cm and

produced 19.1 million tons of coal. For economic reasons, the seam

height of the coal corresponds with the working height (from mine

floor to mine roof) of the mine. Thus, at least 6% of underground

coal produced in the United States in 2009 came from a mine where

workers were restricted.

Individuals who work in underground coal mines face a variety

of challenges. Many studies have demonstrated that low-seam
mine workers suffer multiple forms of injury to the knee such as

meniscal tears, osteoarthritis, ligament tears, and bursitis, or beat

knee (Hodgson, 1975; McMillan and Nichols, 2005; Roantree, 1957;

Sharrard and Liddell, 1962, 1963, 1965; Watkins et al., 1958). These

injuries are likely attributed to the low working heights confining

workers to kneeling and squatting postures which have both been

associated with knee injuries (Baker et al., 2003; Coggon et al.,

2000; Sharrard and Liddell, 1962, 1963, 1965).

Occupational knee injuries plague many industries and are not

limited to mining. Many occupational exposures have been linked

to the long term development of knee injuries. Worker position

and motion causes the knee to be the most frequently injured

lower extremity in all industries (US Bureau of Labor Statistics,

2009). Tibiofemoral osteoarthritis has been associated with the

frequency of knee bending required at work, frequent squatting,

side-knee bending, and sitting crossed legged, and frequent heavy

lifting (Anderson and Felson, 1988; Tangtrakulwanich et al., 2006;

Cooper et al., 1994). An association between kneeling and

squatting has also been found in the prevalence of surgically

treated meniscal injuries (Baker et al., 2003). The likelihood for

developing patellofemoral osteoarthritis has been found to

increase with kneeling, squatting and heavy lifting (Amin et al.,

2008). Additionally, physically strenuous work and daily lifting

of loads was found to increase the risk of knee pain in the

working Finnish population (Miranda et al., 2002). In any setting,

knee injuries can be extremely debilitating and effect one’s

quality of life. In fact, knee pain is quite persistent with 66% of

people that report severe pain still suffering from this pain one

year later (Miranda et al., 2002).

Recently, the National Institute for Occupational Safety and

Health (NIOSH) investigated the muscle activity of the knee flexors

and extensors, pressure applied to the knee, forces and moments at

the knee, and the joint angles of the knee while subjects assumed

a series of different kneeling and squatting postures (Gallagher

et al., 2011; Mayton et al., 2010; NIOSH, 2008; Moore et al., 2009,

2010, 2010; NIOSH, 2011; Pollard, 2008, 2009; Pollard et al., 2011,

2010). For each parameter investigated, certain postures appeared

to be more or less detrimental than others. However, the postures

likely to be detrimental were not entirely consistent across the

parameters investigated. Rather, a complex interaction was

observed which suggests that postural rotation strategies are an

important intervention to consider as a means for minimizing the

negative impact of the various postures while maximizing their

possible benefits.

Mine workers employ a variety of postures to complete their

job duties. Each of these postures place very different demands

on a workers’ knee exposing them to different types of risk and

exposure levels of these risk factors. In mining, the frequencies

with which various restricted postures are used to perform work

are unknown. Moreover, the postures selected are likely to

depend on the task being completed. Thus, certain job classifications may be inclined to use only a select number of postures.

Recent research conducted by NIOSH identified significant

differences in the biomechanical demands of kneeling and

squatting postures (Gallagher et al., 2011; Mayton et al., 2010;

NIOSH, 2008; Moore et al., 2009, 2010; NIOSH, 2011; Pollard,

2008, 2009; Pollard et al., 2011, 2010). All of these postures

would pose different levels of exposure to musculoskeletal

disorder risk factors. To determine the risks for mine workers, it

is essential to determine the postures they use to perform their

job tasks and how their postural options are restricted by the

low-seam underground mining environment. The objective of

this study was to determine the postures routinely used by lowseam mine workers as they pertain to job classifications and

job tasks.

2. Methods

Mine workers at four low-seam coal mines agreed to participate

in this study. The working height at these mines at the time of data

collection was 91.4 cm,109.2 cm,121.9 cm, and 137.2 cm (NOTE: The

seam height, and thus the corresponding working height, of a mine

fluctuates. All mines in the study were considered low-seam based

on their average seam height. The working height of these mines

during data collection restricted the mine workers to their knees

throughout their entire shifts). All four mines were room-and-pillar

mines with a continuous miner machine and one or two dual-boom

roof-bolter machines.

A total of 64 mine workers were included in the study: beltman

(n 1⁄4 2), continuous miner operator (n 1⁄4 5), section foreman (n 1⁄4 5),

mechanic (n 1⁄4 6), mobile bridge operator (n 1⁄4 10), shuttle car

operator (n 1⁄4 6), scoop operator (n 1⁄4 6), roof-bolter operator

(n 1⁄4 14), and maintenance shift worker (n 1⁄4 10). The mine workers

(all male) self-reported their height (175 ± 9 cm), weight

(83.1 ± 14.8 kg), age (37.3 ± 13.1 years), time in their job classification (8.0 ± 11.3 years), and time working in low-seam mines

(8.3 ± 10.3 years). Mine workers wear numerous items on their

mine belts (e.g., self-rescuers, cap lamp battery, tools). As these

items contribute to the weight on the knees, the mine belts and

items on them were weighed using a dial scale (5.1 ± 1.8 kg).

The mine workers were then asked to identify the various tasks

they perform. A total of 18 tasks were identified (Table 1): hanging

curtain, hanging cable, loading and unloading supplies, rock dusting, building stoppings, shoveling, mechanic duties, repairing

equipment, advancing power load center, greasing equipment,

changing continuous miner bits, moving or advancing belt,

scooping faces, operating the roof bolter, operating the mobile

bridge, operating the shuttle car, operating the continuous miner,

and moving about the mine.

To aide with the usage of mine terminology the following

definitions are provided:

• Rib e walls on the sides of an underground coal mine

• Roof e ceiling of an underground coal mine

• Return e a hallway in an underground coal mine

• Face e working wall where the coal is being cut to advance the

mine (i.e., mine workers are cutting through this area to make

the mine bigger and gather more coal)

Descriptions of the identified tasks are also provided:

• Hanging curtain. The mine worker will use a “spad gun” to

drive a nail into the mine roof. The spad gun is a hand held type

of hammer. The mine worker holds onto the shaft of the gun,

and then pulls the middle portion outward. The worker then

pushes or hits the middle back in, thereby driving a flat nail

into the roof. Typically, the mine worker will then poke a hole

through the curtain itself to hold it in place.

• Hanging cable. The power cables of many underground mining

machines (e.g., continuous miner, roof bolter, etc.) are suspended from the mine roof to reduce their risk of being

damaged by mobile equipment. The mine worker often hangs

a hook from a roof bolt plate and then lifts the cable and hangs

it on this hook. The roof bolter cable is lighter and easier to

handle than the continuous miner cable. While it may only be

necessary to hang a roof bolter cable when in a cross section

where the shuttle car is operating, the continuous miner cable

will be hung whenever it is in close proximity to the shuttle car.

Shuttle cars are continually transporting coal from the mine

face to the belt, so ensuring the continuous miner cable is off

the ground is essential.
Table 1

This table shows the number of workers who reported performing each task, their job classifications, and the primary and secondary postures they use to perform each task

(with the number of responders for each posture listed above the representative schematic).
• Loading and unloading supplies. Supplies are brought in on the

rail haulage to each working area. From the end of the rail,

supplies are delivered to differing areas within the mine using

a piece of mobile equipment known as the scoop. Supplies are

loaded into the bucket of the scoop and delivered to locations

in the working area. It takes approximately 1 h for the scoop to

be loaded with supplies and an additional hour for the supplies

to be unloaded at their respective destinations. Typical supplies

include: bags of rock dust (22.7 kg); buckets of continuous

miner cutting head bits (w13.6 kg); crib blocks (w11.3 kg);

packs of wedges (w9.1 kg); replacement parts, tools, or tool

bags; and supplemental roof supports (e.g., straps, pie pans).

• Rock dusting. Pulverized limestone (rock dust) is used in

underground coal mines to control explosive coal dust. Rock

dusting may either be done by hand or with a machine. For

machine dusting (e.g., trickle duster, swinger duster), the mine

worker will empty bags of rock dust into a hopper and a fan

blows dust down the return. For hand dusting, the mine worker

typically cuts the bag of rock dust into two halves using a razor

knife. The mine worker will then carry half of the bag (11.3 kg)

in one arm while throwing the rock dust about the roof, ribs,

and mine floor with the other hand or a hand shovel.

• Building stoppings. Stoppings are walls used to form air courses

that facilitate removal of dust and methane gas from the mine.

Similar to laying bricks, stoppings are built using large blocks

that are bonded together using block bond. Block bond is

a powder that is mixed with water in a bucket or lightweight,

metal, square container. The bags of block bond weigh

approximately 22.7 kg. While the blocks and block bond may

be delivered by the scoop operator, many of the smaller

supplies must be gathered by the mine worker as they are

typically left at the last location a stopping was built. The mine

worker must then dig a trench in the roof, floor, and into both

ribs. Next, the mine worker places a plank of wood down in the

floor trench and begins placing the blocks in the same staggered pattern used by brick layers. Half blocks are used to fill in

the empty spaces by the rib and are created by breaking a block

into two pieces with a hammer and chisel.

• Shoveling. The mine worker will typically have 2e3 different

styles of shovel that they may use depending on the location of

the coal buildup. The shovels are typically one with a long

handle, one with a wide head and handle (w1 m in length)

with a hand grip at the end, or a very narrow shovel for small

locations. Some areas that commonly require shoveling are:

under feeders, at and along the beltlines, along the ribs,

frequently traveled areas such as main roadways, and on the

continuous miner after a roof fall.

• Mechanic duties and repairing equipment. For mechanic duties

or repairing equipment, carrying a large number of tools is

necessary. This is accomplished by wearing tools on the belt

itself or by using a tool pouch (w13.6 kg) which may be worn

across the body, dragged, or carried. Replacement parts (e.g.,

motor, pulley) are typically delivered by other workers from

the outside. However, these parts must still be transported to

the work location which may be many feet from the delivery

location. Furthermore, in order to access the parts being

repaired/replaced, the mine worker may be reaching into the

machine, next to the machine, on top of the machine, or under

the machine. This mine worker may have assistance from

a mechanic’s helper or the operator of the machine that is

down, or they may be working alone.

• Advancing power load center. The power load center provides

electricity to power mining equipment and must be moved

with the mobile equipment as the mine advances. The mine

workers performing this task must shut off the power. The

power load center must then be loaded onto the bucket of the

scoop. Because the power load center is typically on wheels,

this may be done by attaching the power load center to the

scoop ram (mechanical arm in the bucket) which will then pull

the power load center into the bucket of the scoop. The cables

are often attached to the scoop and dragged to the new location. However, the mine workers drag, pull, and manipulate the

cables often.

• Greasing equipment. When greasing equipment, the mine

worker will carry the grease gun (i.e., mechanics would typically have it near their tool box). In order to have easier access

to grease fittings, mines may attach tubes to the fitting and run

the tubes to the surface of the machine. The mine worker will

then pump grease into the tube. However, the mine worker

may be next to, on top of, or under the machine. This task is

typically performed during regularly scheduled maintenance.

• Changing continuous miner bits. The bits on the cutting head of

the continuous miner become worn during the mining process

and must be replaced. Continuous miners vary in number of

bits per square meter of the drum (cutting head) and may

range as much as 0.06e0.08 bits per square meter. This job is

performed by one or more persons. The scoop operator

delivers the bucket of bits and the mine worker will change

only those bits that are dull. The drum of the continuous

miner has holes into which the base of the bit slides. The bit

is then held in place by a pin. This pin is removed or put in

place using pliers. Thus, to remove a dull bit, the mine

worker uses a pair of pliers to remove the pin, slides the dull

bit out of place, slides the new bit into place, and then uses

pliers to insert the pin again. Frequently, these bits get

“stuck” and a hammer is needed to remove the pin or bit

from the drum and place a new bit in place.

• Moving or advancing belt. Coal is transported out of the mine

using belt conveyors. As the mine advances, additional belt will

need to be erected. Advancing the belt requires more belt

structure to be added (e.g., frames with rollers, structures to

which the frame with rollers are attached) to the previously

existing belt structure. When more belt structure must be

erected, the scoop will deliver these materials to the general

area of construction. These materials are then manually

transported from the delivery location to the point of

construction. Lengthening the belt structure requires the

existing belt to be spliced to add an additional piece of belt.

Thus, mine workers must also drag, lift, or carry belting

material. Due to the heavy nature of the belting material, mines

frequently advance belt at pre-determined lengths (e.g., every

two cross cuts) and cut the belt for this length. Then, when the

mine workers need to pull more on, they have a set location

(normally at the head of the section) where the belting was

loaded off from a car into a belt loader. The belt is then added

and pulled from there. Mine workers will also have racks that

they put the belt on, and then load these racks onto a scoop if

needed.

• Scooping faces. The mine worker will use the scoop to pick up

loose rock and coal and either push it towards the face for the

continuous miner to retrieve or transport it to the belt drive

hopper where it is dumped into the hopper that leads to the

belt.

• Operating the roof bolter. A roof bolter machine is used to

reinforce the mine roof by drilling and installing bolts through

the roof. The mine worker positions himself next to the roof

bolter where he can reach the control levers. Drill steels, bits,

bolts, resin, plates, and any other supplemental roof supports

(e.g., straps) are typically stored on top of the roof bolter. The

mine worker will retrieve these supplies and throw them
forward near the boom arm. The drill steel and bit are mounted

on the boom arm and a hole is drilled into the mine roof. Once

the hole is complete these components are then removed, resin

is pushed into the hole in the mine roof, and the bolt and plate

are mounted to the boom arm. The bolt is then spun into the

hole containing the resin, torqued, and held until the resin sets.

Once the bolts in a given row are installed, the mine worker

will move to the back of the machine and use controls to move

the roof bolter forward to the location where the next row of

bolts will be installed.

• Operating the mobile bridge. The mobile bridge, like the

shuttle car (see description for operating the shuttle car

below), is one option for an underground haulage vehicle used

to transport raw coal from the continuous miner to the belt

conveyor. The mine worker is positioned next to, and facing,

the machine and its controls. The mine worker will then move

the mobile bridge forward or backward while looking to either

side to avoid collisions with other equipment, the mine rib, or

other mine workers.

• Operating the shuttle car. The shuttle car, like the mobile

bridge, is one option for an underground haulage vehicle used

to transport coal from the continuous miner to the belt

conveyor. The mine worker typically rides in the operator’s

compartment which is affixed to the side of the machine. Most

compartments are designed with two seats, one facing forward

and the other backward. The mine worker will drive the shuttle

car to the continuous miner where it is loaded with coal. The

mine worker then moves to the opposite seat and drives the

shuttle car to the belt hopper. As the mine worker operates the

shuttle car, the worker constantly inspects the area visually to

avoid collisions with other equipment, the mine rib, trailing

cables, or other mine workers.

• Operating the continuous miner. The continuous miner is used

to rip coal from the face and push this coal to the onboard

conveyor. The mine worker is positioned near the machine and

uses a large remote control to advance the continuous miner as

it cuts coal from the face. The mine worker then uses the

conveyor system on the machine to transfer the coal onto the

haulage vehicle (e.g., mobile bridge, shuttle car). While oper

ating the machine, the mine worker is constantly observing the

­

cutting drum, the body of the machine, and the haulage vehicle

to ensure that they are not in a position where the machinery

could cause a striking or pinning accident.

• Moving about the mine. The mine worker will adopt a posture

depending upon the current working height, shape and weight

of materials/supplies they may be transporting, and the total

distance to be traveled.

After identifying the tasks they perform, the mine workers then

looked at a schematic of postures (Fig. 1) and selected the two

postures they use the most to perform the task.

Fig. 1. Schematic of postures to which mine workers referred when describing the postures they most commonly used to perform various mining tasks.

The mine workers

were offered the opportunity to describe a posture that did not

appear in the schematic if their most commonly used postures

were not shown. The mine workers then identified the most

physically demanding and the least physically demanding tasks.

The researchers verified that the mine workers’ self-reported

postures compared well with how they actually perform their

duties through qualitative observations of the tasks as they were

performed in the mine.

3. Results

Of the 64 mine workers involved in the study, 41 indicated

a specific task that they felt was the least demanding and 37 indicated a task that they felt was the most demanding. Of the 41 mine

workers who reported the least demanding task, 14% identified

tramming, 17% identified riding the mantrip in and out of the mine,

and 20% identified running the scoop. Of the 37 mine workers who

reported a most demanding task, handling the continuous miner

cable and shoveling was reported by 24% and 35% of the respondents, respectively.

Table 1 describes the number of workers who reported performing each task, their job classifications, and the primary and

secondary postures they used to perform the task.

The primary posture was determined to be the posture that was

reported by the largest number of workers and the secondary

posture was the posture used by the next largest number of

workers.

Many tasks were performed by a variety of job classifications. Of

the 18 tasks, 12 (67%) were performed by at least two different job

classifications; 6 tasks (33%) were performed by four or more job

classifications and included: hanging curtain, advancing the power

load center, building stoppings, rock dusting, operating the roof

bolter, and shoveling. While advancing the power load center is not

a daily task, the other 5 tasks are performed at a high frequency in

underground coal mines. Operating the roof bolter typically

requires a higher skill level than the other 5 tasks in this group. Of

the 18 tasks, 32% had a primary or secondary posture that involved

crawling (two-point or four-point crawling). Of these tasks, those

that are typically performed on a daily basis include: mechanic

duties, hanging cable, moving about the mine, and scooping faces.

The other 15 tasks required kneeling near full flexion, kneeling near

90° knee flexion, and kneeling on one knee. Squatting was only

reported for operating the continuous miner.

Two unique postures not included in Fig. 1 were identified by

the mine workers: sitting on buttocks with lower legs folded

towards the body and crossed at the ankle (sitting crossed legged)

and sitting back on a seat in a reclined position. The mine workers

that reported sitting crossed legged were mobile bridge operators

who worked for one particular mine. At this mine, a rubber mat was
affixed to the side of the mobile bridge near the operator controls.

The workers would sit on the mat and be dragged along the mine

floor next to the machine. The authors are not aware of any other

mine that follows this practice. The workers who reported sitting in

a reclined position were shuttle car operators who sit in the operator’s compartment affixed to the machine.

Table 2 illustrates the first and second most frequently reported

postures by job classification.

Table 2

This table shows the most frequently reported and second most frequently reported postures by job classification.

Interestingly, kneeling near full

flexion was the most frequently reported posture for all seven job

classifications. The second most frequently reported posture varied

across the job classifications. Scoop operators, continuous miner

operators, and foremen all indicated the need to frequently crawl.

Maintenance shift workers indicated that they duck walk (i.e.,

stooped walking) routinely. Contrary to this, the roof bolter operators, mechanics, and beltmen indicated that they kneel near 90° of

knee flexion on a regular basis.

4. Discussion

In this study, postures used by low-seam coal mine workers

were evaluated based upon job classification and tasks specific to

underground coal mining. While a variety of postures were used for

the various tasks, mine workers in all job classifications reported

kneeling near full flexion to be the most predominant posture used

across all tasks they performed at the mine. However, several

differences were noted between job classifications when evaluating

the second most predominant posture. Interestingly, over one third

of the job tasks investigated were performed by at least four

different job classifications. The most demanding tasks were

handling the continuous miner cable and shoveling as reported by

24% and 35% of the respondents, respectively. The researchers’

observations in the field correlated well with the questionnaire

data suggesting that the mine workers were able to appropriately

identify the postures they used for the various tasks. Based on the

postures identified in this study and the findings from other published works, the authors have drawn three main conclusions

which will be discussed in more detail.

First, for environments with a restricted vertical height such as

low-seam mining, squatting should be avoided. Previous data

demonstrated that squatting while performing a lateral lift with

a block resulted in the knee extensors exhibiting more than twice

the amount of activity as the kneeling postures tested. For example,

these muscles exhibited 30% maximum voluntary contraction

(MVC) during a squat and only 15% MVC when kneeling near 90° of

knee flexion, the most upright posture with the second lowest

muscle activity (Gallagher et al., 2011; Moore et al., 2010; NIOSH,

2011). The additional muscle activity required to maintain a squatting posture relative to other postures may lead to an earlier onset

of fatigue. Moreover, squatting creates a significant change in

internal joint structure orientations which occur in high flexion.

Femoral “rollback” causes the lateral compartment of the femur to

move 30 mm posterior and the medial compartment to move 9 mm

when in a deep squat (Williams and Logan, 2004). This rollback

increases the varus moments at the knee when squatting (Pollard

et al., 2011). Additionally, in high flexion the meniscus is essential

in distributing tibiofemoral compressive force and sustains significant forces (Li et al., 2004; Yao et al., 2008).

Operating the continuous miner was a task associated with

frequent squatting (See Table 1). This posture is likely used because

it affords great mobility, allowing operators to move quickly to

avoid hazards. However, this enhanced mobility comes with a cost

of decreased stability, increased meniscal loading, and increased

risks for the long term development of osteoarthritis and meniscal

injuries (Amin et al., 2008; Baker et al., 2003; Coggon et al., 2000;

Pollard and Porter, 2011; Yao et al., 2008). The exposure to the risk

factors associated with squatting factors are likely to be high in

continuous miner operators as they perform this task for the

duration of their shift. Consideration should be given to design

tools or strategies to minimize the operators’ need or preference to

squat for sustained periods.

Second, when performing lateral lifting tasks kneeling near full

flexion should be avoided. Kneeling near full flexion was reported

as the most frequently used posture by all job classifications (see

Table 2). This is likely due to the fact that it requires the least

amount of muscle activity to maintain (when compared to other
kneeling and squatting postures), creates a large base of support,

and has reduced pressures at the knee and internal stresses

(Gallagher et al., 2011; Pollard, 2009; Porter et al., 2010; Pollard and

Porter, 2011). However, like squatting, kneeling near full flexion

results in increased femoral rollback and may increase the stresses

applied to the meniscus (Pollard, 2008; Pollard et al., 2011). Additionally, when sitting on the heels, the orientation of the ankle may

cause large rotational moments at the knee which could have

detrimental effects on the meniscus, ligaments, and cartilage.

Therefore, foot posture is an important consideration when

kneeling near full flexion. The added effect of a lateral lifting task,

while kneeling may increase the twisting at the knee thereby

compounding the effects increased varus/valgus and internal/

external rotational moments. For these reasons, the combination of

lateral lifting while kneeling near full flexion is not ideal. Rather,

kneeling near 90° of knee flexion should be the worker’s first choice

as it requires less muscle activity than the other postures available

(kneeling on one knee, squatting) and does not expose the knee to

large moments. When kneeling near 90° the upper body is positioned close to the knee, thereby reducing the flexion moment at

the knee. Moreover, kneeling near 90° of knee flexion does not

expose the knee to the large shear forces, which could be up to 20%

body weight, observed when kneeling on one knee (Pollard et al.,

2011).

Practical reasons such as the height of the mine may limit

a worker’s ability to select kneeling near 90° of knee flexion. In this

case, kneeling on one knee will become necessary. However, the

worker should consciously alternate the knee in contact with the

ground to minimize the impact on any one knee. Lateral lifts

frequently occur in the underground coal mining environment. This

recommendation would impact how workers perform lateral lifts

when completing tasks such as: hanging cable, hanging curtain,

building stoppings, loading or unloading supplies, shoveling, rock

dusting, advancing power load center, and moving or advancing

belt.

Third, unlike the lateral lifting scenario discussed above, maintaining a static posture results in knee loading and muscle activity

such that the mine worker should consider kneeling near full

flexion and sitting on their heels. Although kneeling near full

flexion is associated with injuries, there are many benefits to this

posture that are realized when statically kneeling. It requires

minimal muscle activity, it allows the worker to maintain an

upright torso in low heights, and it is associated with decreased

loading at the knee. Additionally the risk of skin effects is reduced

in this postures, as is has a 9.7 psi lower pressure when compared

to kneeling on one knee (Porter et al., 2010). It is important to note

that prolonged use of kneeling near full flexion without deviation is

not recommended. Cartilage is avascular and is nourished by

synovial fluid within the joint capsule. Joint motion allows this fluid

to flow throughout the joint space providing nutrition (O’Hara

et al., 1990). Static kneeling or squatting does not allow cartilage

to receive the necessary nutrition. Therefore, when performing

static kneeling in any posture, one should frequently rotate

between postures, assuming a more upright kneeling posture when

possible. Frequent breaks where the knee is flexed and extended

through its full range of motion allowing nutrients to the cartilage

should also be considered. Furthermore, to reduce the risk of

damage to the meniscus, workers should attempt to keep their

ankles straight allowing their heels to stay in line with their torso.

This will reduce the rotational moments at the knee and increase

the force transmitted through the heels, thereby reducing the

normal and shear loading at the knee (Pollard et al., 2011; Teichtahl

et al., 2006; Lynn et al., 2008). Reducing the loading at the knee is

vital in this posture and can be accomplished by increasing the

loading through the foot when sitting on the heels or through the

use of a body-weight-assist device. A device worn at the ankles or

on the boot will provide body weight support by reducing the

flexion angle of the knee and supporting some upper body weight

(Fig. 2).

Fig. 2. A body weight support device worn at the ankle that reduces the flexion angle

at the knee and increases the distribution of upper body weight through the heels.

Static kneeling postures are frequently used in underground coal mining regardless of job type (see Table 2). In fact, the

only job tasks that did not list static kneeling near full flexion as

a primary or secondary posture were advancing the power load

center, hanging cable, scooping faces, and operating the shuttle car

(see Table 1).

There are several limitations to this study. First, only four mines

were evaluated all of which were located in Pennsylvania.

However, the environmental conditions (i.e., wet, dry, undulating

mine roof) and the equipment used (i.e., shuttle car, mobile

bridge) were different across these mines. Therefore, the findings

in this study should be representative of the low-seam mining

community. During observation periods, the researchers observed

the postures used by the mine workers to perform various tasks

which correlated well with what the mine workers had selfreported.

The previously reported biomechanical data demonstrated that

the pressure on the knee varied somewhat depending upon the

selected posture and the existence of a lateral lift (Mayton et al.,

2010; Moore et al., 2009; Porter et al., 2010). However, it is

important to note that the magnitude of pressure applied to the

knee in all postures is of concern. Therefore, while minimizing the

time spent in those detrimental postures is an important component of reducing the risk of knee injuries, it cannot be the only

component. Future investigations should consider evaluating the

effect of these postures on the entire body (e.g., knee, ankle, back)

while subjects simulate the tasks described in this paper. With this

information, a postural rotation strategy may be developed for each

task that maximizes the overall benefit of each posture for the

worker while minimizing any detrimental effects. This information

could be incorporated into various ergonomics processes or software programs used by safety and health professionals in the

mining industry to disseminate this information (Burgess-Limerick

et al., 2007; Torma-Krajewski et al., 2007).
5. Conclusion

Low-seam coal mine workers perform numerous tasks

throughout their work day. While there are always multiple ways to

perform one task, the restricted vertical workspaces and design of

commonly used mining equipment severely limit the postures

available for these workers. In effect, mine workers are forced to

perform work while kneeling and squatting. This study found that

low-seam coal mine workers predominantly use the kneeling near

full flexion posture to perform work which has been associated

with the development of knee osteoarthritis. Given the restricted

environment, it is imperative for miners to modify their posture

through postural rotation strategies or through the use of protective equipment designed to reduce the loading at their knees.

Therefore, NIOSH has partnered with a kneepad manufacturer and

industrial designers to develop a novel kneepad and evaluate body

weight assist devices that will address the high pressures at the

knee and the desire to adopt kneeling near full flexion for many

tasks. Evaluation of these products will incorporate biomechanical

data and mine worker feedback.

6. 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.

Acknowledgments

The authors would like to acknowledge Alan Mayton, William

Porter, Susan Bealko (formerly of NIOSH), Jeffrey Welsh, and Eric

Rainis (formerly of NIOSH) for their assistance in data collection.

Rox Coal, Inc. and TJS Mining, Inc. are acknowledged for providing

researchers from NIOSH with the opportunity to collect data at

their mine sites. The authors would also like to thank Greg Green

for providing details regarding how various mining tasks are

performed.

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