Electromyography of the thigh muscles during lifting tasks in kneeling and squatting postures

Sean Gallagher*,

*Corresponding author. Email: sgallagher@cdc.gov

Jonisha Pollard and William L. Porter

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, PO Box 18070, Pittsburgh,

PA 15236–0070, USA

Underground coal miners who work in low-seam mines frequently handle materials in kneeling or squatting

postures. To assess quadriceps and hamstring muscle demands in these postures, nine participants performed lateral

load transfers in kneeling and squatting postures, during which electromyographic (EMG) data were collected.

EMG activity was obtained at five points throughout the transfer for three quadriceps muscles and two hamstring

muscles from each thigh. ANOVA results indicated that EMG data for nine of 10 thigh muscles were affected by an

interaction between posture and angular position of the load lifted (p5 0.001). Muscles of the right thigh were most

active during the lifting portion of the task (lifting a block from the participant’s right) and activity decreased as the

block was transferred to the left. Left thigh muscles showed the opposite pattern. EMG activity for the majority of

thigh muscles was affected by the size of the base of support provided by different postures, with lower EMG activity

observed with a larger base of support and increased activity in postures where base of support was reduced

(p50.05). Thigh EMG activity was lowest in postures with fully flexed knees, which may explain worker preference

for this posture. However, such postures are also associated with increased risk of meniscal damage.

Statement of Relevance: Kneeling and squatting postures are sometimes used for manual lifting activities, but are

associated with increased knee injury risk. This paper examines the EMG responses of knee extensors/flexors to lifting

in these postures, discusses the impact of posture and kneepads on muscle recruitment and explores the implications for

work in such postures.

Introduction

Workplace characteristics and/or task demands sometimes obligate workers to assume a kneeling or

squatting posture in the performance of their work. In

some occupational settings, these postures may be

transient working positions, interspersed with periods

of standing, sitting or other postures. In others, such as

working in underground ‘low-seam’ coal mines (mines

with less than 122 cm vertical space), workers may use

such postures for almost the entire work shift. Workers

who kneel or squat for large portions of the workday

have demonstrated increased risk of several types of

knee disorders, including osteoarthritis (Anderson and

Felson 1988, Felson et al. 1991, Cooper et al. 1994,

Coggon et al. 2000), chronic prepatellar bursitis

(Sharrard 1964, Tanaka et al. 1982, Kivimaki et al.

1992, 1994) and meniscal injury (Sharrard and Liddell

1962, Baker et al. 2002, 2003). Owing to the increased

knee disorder risk associated with prolonged work in

these postures, the National Institute for Occupational

Safety and Health (NIOSH) is performing research to

better understand the demands of work in kneeling and

squatting postures, so that better interventions and

recommendations may be developed and implemented.

One area of interest is the manner in which the

muscles crossing the knee joint are utilised during work

in kneeling and squatting postures. It would seem

evident that the musculature of both knee flexors and

knee extensors would experience different sets of

demands in restricted postures compared to work in

typical standing or seated postures. In kneeling

postures (of which there are several varieties), the leg

muscles may generate forces to support activities such

as manual materials handling (MMH) and may also be

called upon to stabilise the lower limbs and pelvis in

what may be relatively unstable positions. The

squatting posture is also unstable, but is sometimes

used to perform MMH activities as well. Examination

of the electromyographic (EMG) activity of the major

flexors and extensors of the thigh can provide insight

into the physical demands experienced in different

postures and the trade-offs that may occur as different

postures are adopted.

As vertical work space is reduced, so is the menu of

postures available to the worker. In a 1.2 m vertical
work space, it is possible for most workers to adopt a

stooping posture for brief periods of time; however,

this posture is uncomfortable and fatiguing and is

typically used for locomotion, but not for prolonged

work activities (Bedford and Warner 1955). Instead,

prolonged work is preferentially done in one of several

varieties of kneeling posture. A recent observational

study in a low-seam mining operation indicated that

kneeling with both knees in full flexion was adopted

for over 83% of the observation time for roof bolters

and continuous miner operators, 5% in a near 908

included angle kneeling posture and 10% of time

kneeling on one knee, with almost no squatting (J.

Pollard, personal communication on unpublished data,

2009. Data available upon request from J. Pollard,

NIOSH, Pittsburgh Research Laboratory, Pittsburgh,

PA. 2009). In slightly higher seam coal mines

(approximately 1.5 m), squatting was used

approximately 10% of the time (J. Pollard, personal

communication 2009).

Space restrictions not only change the postures that

can be adopted, but can also change the nature of the

work tasks that are being performed. This is evident in

terms of MMH activities in vertically restricted space.

Due to the geometry of the workspace, lateral load

transfers are more prevalent and workers tend to be

forced into asymmetric motions when lifting. Furthermore, unlike the standing posture, it is often difficult to

reposition the body when lifting in a kneeling posture

or a squat. Thus, lifting in these postures is characterised by a twisting motion of the trunk, where the

load is shifted from one side of the body to the other.

Previous research on lifting in restricted postures

has shown that lifting capacity and muscular strength

are reduced in both kneeling and squatting postures

compared to a standing position (Ayoub et al. 1985a,b,

Gibbons 1989, Gallagher and Hamrick 1992, Haslegrave et al. 1997). In addition, physiological costs of

lifting in a kneeling posture have been shown to be

higher than in a stooping posture during lateral

transfers of load (Gallagher et al. 1988). However,

previous research has not examined the role of the

knee flexors and extensors during lifting activities in

squatting and kneeling postures. Accordingly, the

present exploratory investigation was performed to

examine the EMG activity of knee flexors and

extensors during a lateral lifting task in five kneeling

postures and a squatting posture. EMG activity, an

indirect measure of muscle activation levels, can help

with understanding the physical demands of work in

kneeling and squatting postures. In addition, the effect

of three kneepad conditions was examined: no

kneepads; an articulated kneepad; a non-articulated

kneepad. The kneepads chosen for study are commonly used in US low-seam coal mines and were

studied to evaluate whether EMG responses may be

affected by the potential for increased resistance,

changes in patellar motion with vs. without kneepads,

possible antalgic posture responses and the potential

for the kneepad straps to impact EMG activity,

particularly the knee flexors.

Methods

Participants

Nine participants (6 male, 3 female) took part in this

study. The average age was 35+17 years (mean+SD),

while the average body mass and height were

69.7+10.6 kg (mean+SD) and 168.0+7.6 cm

(mean+SD), respectively. Prior to enrolling in the

study, each participant read and signed an informed

consent form, which was approved by the NIOSH

Human Subjects Review Board. Since participants

were required to repeatedly adopt kneeling postures,

each participant was asked a series of questions to

determine if they had ever had any significant injury to

the knee. None of the participants had ever had knee

surgery. One subject had been diagnosed with bursitis,

which did not require any intervention and a second

participant had slight nerve damage due to a

motorcycle accident. However, neither participant was

symptomatic at the time of testing.

Experimental design

Independent variables in this study consisted of

kneepad condition (three levels), posture (six levels)

and angular block position (ABP; five levels). The

block lifted was the size and mass of a cinderblock

commonly used in US underground coal mines

(dimensions: 19 cm (height) 6 39 cm (width) 6 15 cm

(depth); mass: 11.3 kg). Kneepad states included no

kneepad, an articulated kneepad with a hard

contoured outer shell (AIS Knee Armor; American

Industrial Supply, Inc., Knoxville, TN, USA) and a

non-articulated kneepad with a soft outer shell

(Barwalt UltralightTM Knee Pad: KN-1; Barwalt Tool

Co., Post Falls, ID, USA). Figure 1 illustrates the

kneepads used in this study.

Postures included lifting on both knees with the

knees in full flexion (with vertical space restrictions of

97 cm and 122 cm), lifting on one knee (right knee

down) with the left knee up and left leg supported by

the foot (with vertical space restrictions of 97 cm and

122 cm), lifting on both knees with knees near a 908

angle (122 cm vertical space restriction only) and a

squat (122 cm vertical space restriction only). Prior to

the start of the load transfer, the subject was shown a

graphic depicting the posture to be adopted during the

trial and was asked to adhere as closely as possible to
the posture depicted in the graphic. Figure 2 provides

examples of lifting tasks in each of the main postures

examined. It should be noted that task performance

required some deviation from the idealised postures

depicted in the graphics. For example, to accomplish

the load transfer while kneeling in full flexion, some

knee extension was required to both reach and place

the load; however, subjects were asked to minimise the

degree to which the buttocks came off the heels.

Similarly, subjects in the ‘near 90’ posture needed to

flex the knees to maintain balance during the load

transfer, but were asked to keep the buttocks as high as

possible during the transfer process.

Figure 1. Kneepads used in the study: (a) articulated kneepad with hard contoured outer shell; (b) non-articulated

kneepad with soft outer shell.

ABPs were derived from motion analysis markers

placed on the block and were defined in relation to the

initial block position. Specifically, an ABP of 08

represented the position of the block at the start of

the lift (23 cm directly lateral to the participant’s right

knee), an ABP of 908 was the position of the block

when it was directly in front of the participant and an

ABP of 1808 represented the position of the block at

the end of the lift (23 cm directly lateral to the

participant’s left knee). Data were also obtained at

ABPs of 45 and 1358 (intermediate between the

positions described above).

Vertical space restrictions were achieved using an

adjustable ceiling raised and lowered via a system of

pulleys. The adjustable ceiling was not rigid in nature

but was suspended by ropes from the ceiling and

consisted of a frame constructed of polyvinyl chloride

pipe, to which wide-mesh netting was affixed. The

netting allowed motion analysis cameras to be

deployed above the level of the ceiling and still obtain

data on marker positions. The levels of vertical space

restriction (97 and 122 cm) were selected to represent

examples of working heights present in low-seam coal

mines, which would limit working postures to varying

degrees.

Dependent variables consisted of normalised EMG

data of ten thigh muscles. These muscles were the left

and right pairs of the vastus lateralis, rectus femoris,

vastus medialis, biceps femoris and semitendinosis.

The presentation orders for the three kneepad

states were randomised. Within each kneepad state, a

restricted randomisation determined the order of the

six postures (i.e. a separate random order for postures

was generated within each kneepad state) and the effect

of block position was evaluated within each lifting

trial. Based on this randomisation scheme, a

split-split-split plot design (with no whole-plot factor)

was employed to evaluate EMG responses. As an

exploratory analysis, a comparison-wise type I error

rate alpha level of 0.05 was employed (Bender and

Lange 2001). The statistical model used is presented in

Equation (1) below:

Yijkl 1⁄4 m þ p þ aj þ ðapÞiðjÞ þ bk þ ðabÞjk þ ðbpÞkiðjÞ

þ gl þ ðagÞjl þ ðbgÞkl þ ðabgÞjkl þ ðgpÞilðjkÞ þ eijkl

ð1Þ
Figure 2. Examples of the postures and lifting tasks used in this study: (a) kneeling in full flexion; (b) kneeling on one knee;

(c) squat; (d) kneeling with both knees near 908.

Electromyographic preparation

The longer of the two kneepads was the articulated

pair and they were placed on the right and left knees of

the participant to mark the superior-most aspect of the

kneepad. Preferred locations of the electrodes for each

of the thigh muscles were derived from Ericson et al.

(1985) and are provided in Table 1. The location of the

placement for each EMG electrode was marked on the

participant’s thigh using an ink pen. Disposable selfadhesive Ag/AgCl dual snap surface electrodes

(Noraxon USA Inc., Scottsdale, AZ, USA) with

electrode spacing of 2 cm centre to centre were

employed. Each electrode site was shaved (if necessary)

and cleaned using a skin preparation pad consisting of

70% isopropyl alcohol and pumice (Dynarex Corp.,

Rochester, NY, USA). Electrodes were placed over the

bellies of the muscles, distal to the motor point regions

(Ericson et al. 1985). In some cases, it was not possible

to place the electrodes in the ideal location due to the

length of the articulated kneepad. In such cases, the
electrode was placed as closely as possible to its ideal

location, but always distal to the motor point. Two

reference electrodes were required (one for each

wireless transmitter) and these were placed at remote

sites.

Isometric maximum voluntary contractions

(MVCs) were obtained for the thigh muscles of both

right and left legs (LeVeau and Andersson 1992). The

participant was instructed to lie in a supine position in

a Biodex System 2 accessory chair (Biodex Medical

Systems, Inc.; Shirley, NY, USA) with the knee angle

at approximately 908 with the hips (also at approximately 908) and ankle secured via Velcro straps. The

participant was then instructed to attempt knee

extension with maximal effort for at least 5 s while a

researcher provided verbal encouragement. The participant was then instructed to flex the knee with

maximal effort for at least 5 s, again while being

verbally encouraged by a researcher. Each leg was

tested separately. The number of MVCs performed by

the participants ranged from 1 to 4 and was based on

assessment of effort and inspection of the EMG signal.

EMG activity normalised to single-angle MVCs in the

lower extremity have been found to provide similar

results to angle-specific normalisation techniques in

dynamic lower extremity activities (Yang and Winter

1984, Knudson and Johnson 1993, Burden et al. 2003).

Furthermore, single-angle MVCs have been recommended as the most appropriate normalisation technique for patterns of EMG activity in dynamic lower

extremity tasks (Knudson and Johnson 1993).

All EMG measurements were made using a

Noraxon Telemyo 2400R – World Wide Telemetry

system with 16 channels (Noraxon USA Inc.). The

gain was set to 10 for all channels. Several hardware

filters were in place: first order high-pass filters set to

10 Hz +10% cut-off; eighth order Butterworth/Bessels

low pass anti-alias filters set to 500 Hz + 2% cut-off.

The common mode rejection was 4100 dB. The

sampling rate for all EMG data was set at 1020 Hz.

Table 1. Preferred electrode positions for thigh muscles

(from Ericson et al. 1985).

Muscle Location

Rectus femoris Midpoint of the distance between the

anterior superior iliac spine (ASIS)

and patellar apex

Vastus lateralis 80% of the distance from the ASIS to the

medial knee joint space

Vastus medialis 75% of the distance from the ASIS to the

lateral knee joint space

Semitendinosis Midpoint of the distance from the ischial

tuberosity to the medial knee joint

space

Biceps femoris Midpoint of the distance from the ischial

tuberosity to the lateral knee joint

space

Motion analysis

A motion capture system (Eagle; Motion Analysis

Corporation, Santa Rosa, CA, USA) was utilised to

determine the base of support of the participants in the

postures studied. Retro-reflective markers placed on

the foot (heel and toe) and the shank (used to calculate

position of the knees) were used to estimate the base of

support. Specifically, the base of support was estimated

by quantifying the surface area bounded by the parts

of the body making contact with the ground when

kneeling and squatting. Different methodologies were

used for the differing postures. For the bilateral

kneeling postures (near full and near 908), this area was

assumed to be the region bounded by the knees

(represented by anterior thigh markers) and toes

(represented by the lateral toe markers). For one knee,

this area was assumed to be the region bounded by the

left foot (represented by the left lateral toe and left heel

markers) and the right lower leg from right knee

(represented by the anterior thigh marker) to right toe

(represented by the lateral toe marker). For squat, this

area was assumed to be the region bounded by the left

and right feet with only the balls of the feet making

contact with the ground. Therefore, the horizontal

distance between the left and right feet was determined

from the ankle markers and the length of the ball of the

foot was determined from the literature using the

subject’s anthropometry. In addition, markers located

at several locations on the box allowed researchers to

track the movement of the box throughout the lifting

task.

Procedure for lifting trials

Calibration of the motion analysis system was

performed prior to the initiation of the lifting trials.

The participant donned the randomly assigned

kneepad (if necessary) and then performed the lifting

tasks in the six postures described earlier (randomly

assigned within each kneepad state). For each lifting

task, the participant adopted the posture of interest

and the EMG data collection system was activated.

The participant was given a verbal cue to begin the

lifting task. Participants accomplished the load

transfer using their own natural lifting style and pace

and the lift was performed using both hands. When the

lift was completed the participant returned to a neutral

position, facing straight ahead in the posture of

interest. A brief period of rest (but at least 1 min) was
provided between trials. During rest periods,

participants were free to get up out of the kneeling/

squatting positions.

Data conditioning and analysis

EMG data were low-pass filtered to 500 Hz and highpass filtered to 20 Hz using a fourth order Butterworth

filter (MATLAB1; The MathWorks, Inc., Natick,

MA, USA). The signal was then rectified by taking the

absolute value. Data from all tests were normalised by

dividing by the MVC for each muscle. Mean amplitude

values of the normalised values were then calculated by

determining the running mean of every 102 samples or

10% of the sampling rate (LeVeau and Andersson

1992). Trials containing evidence of artefacts (brief

spikes of activity with magnitude greater than 150%

MVC and uncharacteristic of surrounding EMG

activity) were eliminated from the analysis.

Data were analysed using the Statistix software

package (Analytical Software, Tallahassee, FL, USA).

Residual analysis of the non-transformed EMG data

indicated a fan-shaped pattern of residuals in all 10

muscles. Thus, the EMG data were transformed by

taking the natural log, which resulted in a near normal

distribution of residuals for all muscles, based on

visual inspection of the residual plots. All ANOVA

results reported below are based on the log

transformed data. As repeated measures were obtained

for ABP, lower bound adjustments were computed to

check for violations of compound symmetry. Linear

regression was used to evaluate the relationship

between the base of support provided by a posture to

the magnitude of normalised EMG activity of the knee

flexors and extensors.

Results

Estimates of the base of support for each posture are

provided in Figure 3.

Figure 3. Estimated base of support for each posture

(n 1⁄4 9). Error bars represent the standard deviation.

A summary of the split-split-split

plot ANOVA for EMG activity of each muscle is

provided in Table 2.

Table 2. Summary of significant main effects and interactions for normalised activity of all muscles

Kneepad Posture Block location

(A) (B) A 6 B (C) A 6 C B 6 C A 6 B 6 C

Left vastus lateralis *** ***

Left rectus femoris *** *** ***

Left vastus medialis *** *** ***

Left biceps femoris *** * **

Left semitendinosis ** *** ***

Right vastus lateralis *** *** ***

Right rectus femoris *** *** ***

Right vastus medialis *** *** ***

Right biceps femoris *** *** ***

Right semitendinosis *** *** ***

*p 5 0.05; **p 5 0.01; ***p 5 0.001.

As can be seen in this table, nine out

of the 10 muscles investigated were affected by an

interaction between the posture adopted and the

location of the block throughout the lift. The sole

exception was the left vastus lateralis, which was affected

by the main effects of posture and block location, but not

their interaction. The kneepad main effect was not found

to significantly influence EMG activity for any of the

muscles studied. However, a significant interaction

between kneepad condition and block location was

found to affect activity of the left biceps femoris

(p 5 0.05). Results of lower-bound adjustments for ABP

indicated no violations of the assumption of compound

symmetry involving this factor.

Figure 4 provides a summary of EMG activity at

different time points during the lift in each posture and

is thus an overall representation of the interaction of

posture and block location. This figure provides

summaries of EMG data at the start and end positions

(as the block was picked up and set down, respectively)
and at three intermediate points during the lift

(ABPs 1⁄4 45, 90 and 1358).

Inspection of Figure 4 provides several insights into

the activation of the thigh muscles in different postures

and at different phases during the lift.

Figure 4. Summary of electromyographic activity by posture and object location. For each posture and object location

there are two groups of bars representing the activity of the left and right thigh muscles. Note that the bars representing the left

and right thigh muscles mirror their arrangement in the body if looked at from the superior aspect.

One consistent

pattern that can be observed with all postures is high

activation of the right thigh muscles at the beginning of

the lift and the high activation of the left thigh muscles

as the load is transferred to the left side. Peak activity

of the left thigh muscles tended to occur when the

block was at the 135 or 1808 positions. In most cases,

the lowest overall EMG activity was observed when

the block was at the 45 or 908 position.

Several differences in muscle activity can be noted

between postures in Figure 4. The squat posture

generally exhibited the highest muscle activity levels

throughout the entire lift and this was particularly true

for the knee extensors. Kneeling in full flexion (in

either 97 cm or 122 cm heights) typically exhibited

the lowest muscle activity of any posture. Kneeling

at 908 exhibited very similar activation patterns to

the full flexion posture, but tended to have slightly

higher EMG activity. Kneeling on one knee (in both

heights), however, resulted in higher activity than the

other kneeling postures, with a notable increase in

right and left side extensor EMG at the end of the lift
compared with the other kneeling postures. The left

vastus lateralis was the only muscle not to exhibit a

significant interaction between posture and block

position, although the p-value for this interaction was

less than 0.10.

The presence or absence of kneepads had virtually

no effect on EMG activity in this study. The one

exception is that the left biceps femoris was affected by

an interaction of kneepad*block location, shown in

Figure 5.

Figure 5. Interaction of kneepad state and block location

on electromyographic activity of the left biceps femoris.

MVC 1⁄4 maximum voluntary contraction.

Compared with the kneepad states, the no

kneepad state resulted in lower EMG activity of the

left biceps femoris in all block locations other than the

08 ABP.

Table 3 contains results of regression analyses

examining the relationship between the base of support

provided in the various postures and the peak normalised EMG observed during the lifting tasks for all

muscles.

Table 3. Results of regression analyses examining the relationship between the normalised peak electromyographic

activity observed during the lifting tasks for all muscles and the base of support in mm2 (n 1⁄4 9).

LBF LST LVL LRF LVM RBF RST RVL RRF RVM

Constant 122.56 50.98 68.90 91.95 129.67 100.30 30.68 29.97 96.50 124.44

Base of support

R2 70.017*0.73

70.005

0.14

70.027*

0.78

70.019*

0.69

70.026*

0.75

70.006

0.05

0.012*

0.82

70.012*

0.95

70.027*

0.65

70.026

0.31

L 1⁄4 left; BF 1⁄4 biceps femoris; ST 1⁄4 semitendinosis; VL 1⁄4 vastus lateralis; RF 1⁄4 rectus femoris; VM1⁄4 vastus medialis; R 1⁄4 right.

*Indicates p 5 0.05.

Note: A negative coefficient for base of support indicates a reduction in peak electromyographic activity with a larger base of support.

Results of this analysis indicate that the peak

EMG activity for the majority of the thigh muscles

studied was inversely related to the base of support

provided by the posture.

Discussion

Restricted vertical workspace not only requires

adoption of awkward postures (such as kneeling and

squatting), it also encourages asymmetric motions

when performing lifting tasks. The present study

investigated a lateral load transfer in various kneeling

and squatting postures – in this case, the load was

transferred from the right side of the body to the left.

Not surprisingly, it was found that EMG of the right

thigh muscles was greatest at the initiation of the lift

and EMG of the left thigh muscles was greatest toward

the end of the lift (typically peaking at the 135 or 1808

ABPs). This shift in muscle activity clearly reflects the

change in force demands on the thigh muscles as the

block was lifted from right to the left. Lowest EMG

values were typically observed at the 45 and 908 ABPs.

The reason for the decrease in muscle activity at this

portion of the lift may be due to a decreased load

moment as the load is brought closer to the body in

this region and/or the fact that the middle of the lift

may involve less acceleration/deceleration of the load,

which may reduce the forces required of the muscles.

Additionally, centring the load distributes the weight

of the block between the two legs, thus requiring less

muscle activity.

It was apparent that the base of support provided

by the different postures had an influence on thigh

muscle activity. Three basic postural groups can be

identified that had similar bases of support and similar

EMG responses to the lifting tasks. Postures in which

both knees were on the ground (kneeling near full

flexion and the near 908 flexion) had the largest base of

support and generally elicited lower magnitudes of

EMG activity. In full flexion, both the knees and pelvis

were well supported. The resulting postural stability

appears to have reduced the force demands of the thigh

muscles to stabilise the hip and knee joints, particularly

when compared with the other postures studied.

Slightly greater EMG activity was observed in the near

908 posture than in full flexion, when both knees were

supported by the floor but the pelvis was no longer

supported by the lower leg. However, these differences
were not large and the similarities in muscle activity in

these postures were striking.

The second postural grouping was that involving

kneeling on the right knee with the left leg supported

by the foot. These postures were associated with a base

of support almost half that of the two knee postures

and demonstrated an increased magnitude of muscle

activity compared with postures where both knees were

down. This was particularly true of the left thigh at the

end of the lift (where significant quadriceps activity

was observed). It appears that this configuration of the

body required additional muscle activity to maintain

postural stability, which previous research has shown

to be less than that when kneeling on two knees

(Bhattacharya et al. 2009). In addition, it should be

noted that the raised left knee constituted a barrier,

around which the load had to travel. The increased

load moment associated with circumvention of this

barrier may have also contributed to the higher thigh

muscle activity observed in these postures.

The highest EMG activity of the thigh muscles was

recorded in the squat posture, which was clearly the

most unstable of the postures studied. The base of

support in this posture consisted solely of the balls of

the feet and was markedly smaller than that in the

other postures studied. In this posture, both the knee

and pelvis are in an unsupported position, which

apparently leads to an increased need to activate the

thigh muscles to maintain adequate support and

stability throughout the lift.

EMG activity differences in the different postures

are a function of the fact that several of the thigh

muscles (rectus femoris, biceps femoris and semitendinosis) cross both the knee and hip joints and thus act

on both joints. Previous studies have conclusively

shown that there is no ability to selectively contract

fibres of a particular region of a two-joint muscle so

that only one joint is acted upon (Basmajian and de

Luca 1985). On the contrary, the entire muscle will

contract regardless of the joint (or joints) moved.

Which joint is moved depends upon the degree to

which each joint is immobilised or supported (Fujiwara and Basmajian 1975).

The results obtained in this study and the

interpretation above may help to explain some of the

postural preferences demonstrated by miners working

in low-seam coal mines. Field observations of postures

adopted by roof bolters and continuous miner

operators indicate an overwhelming preference for

kneeling with both knees in full flexion. As mentioned

previously, this posture was used for 86% of the time

that these workers were under observation in a lowseam coal mine (J. Pollard, personal communication

2009). One reason that miners may favour this position

is due to the postural stability afforded by this body

configuration (Bhattacharya et al. 2009). Another

motivation may be a by-product of the enhanced

stability; the fact that, in a stable posture with a large

base of support, there is a greatly reduced need for

active contraction of the thigh muscles. The lower the

force demands on the muscles, the lower the body’s

requirement for oxygen to replenish adenosine

triphosphate stores in the muscle. Therefore, kneeling

with both knees in full flexion reduces the need to

employ active contraction of the thigh muscles for

postural control, reducing energy cost (at least in the

lower extremity) and may well spare the cardiorespiratory system. Lower metabolic demands

associated with this posture can also be a strong

behavioural motivator. Workers like to conserve

energy, especially in performance of occupational tasks

(Cavanagh and Kram 1985, Trafimow et al. 1993,

Minetti et al. 1995, Straker 2003). The other postures

provide less stability and would be expected to increase

local metabolic cost and may be less preferred as a

result.

However, while the full flexion posture provides a

stable and energy efficient posture from which to work

in restricted postures, the knee joint itself may incur a

significant burden resulting from the choice to use this

posture. Specifically, the fully flexed knee is known to

be associated with meniscal tears (Baker et al. 2002,

2003) and the development of knee osteoarthritis

(Cooper et al. 1994, Coggon et al. 2000, Manninen

et al. 2002). Previous research has suggested that one

possible pathway to cumulative meniscal lesions or

tears may be the result when the medial or lateral

meniscus is repeatedly (or continually) caught between

the condyles of the femur and the tibia during full knee

flexion (Sharrard and Liddell 1962). An alternative

proposal has suggested that increased laxity of the

anterior cruciate ligament due to prolonged or

repeated kneeling may lead to instability in the knee

and that sudden movement or internal/external

rotation of the joint may then result in meniscal

damage (Atkins 1957, Sharrard and Liddell 1962).

Knee osteoarthritis involves degenerative dissolution

of normal cartilage behaviour and function. The

breakdown of a joint’s cartilage results in a loss of

shock absorption in weight bearing and may

ultimately result in bone to bone contact during joint

movement, leading to severe pain and disability.

Miners have been identified specifically as an

occupational group associated with development of

knee osteoarthritis (Atkins 1957, MacMillan and

Nichols 2005).

It is an unfortunate fact (in the current context)

that feedback regarding metabolic load is almost

immediately perceptible to an individual, while the

wear and tear on the tissues of a joint may be the result
of a relatively long-term process, in which feedback

may be deferred. This is not to suggest that postures

requiring additional energy expenditure will necessarily

confer immunity from tissue damage. However, if

muscle recruitment can be reduced (and energy saved)

in the short term by an individual via sinking into

deeper knee flexion and the resulting meniscal wear

and tear does not become immediately apparent, it

would certainly seem a reasonable choice (in the short

term) to use the posture that is both more stable and

that saves energy. However, the long-term effects of

such a choice may ultimately be very detrimental. It

may be noted that the reduced energy expenditure of

the fully flexed kneeling posture also makes it a

challenge to recommend, as an alternative, a posture

that increases energy demands. Any such

recommendation is likely to be quickly discarded by

workers as soon as the increased physical demand

becomes evident.

It should be noted that wearing kneepads of the

sort used in this study resulted in virtually no

influence on thigh muscle activity. Thus, none of the

potential influences related to kneepad use detailed

in the Introduction appears to have impacted EMG

activity required for the task studied. A companion

study demonstrated that these kneepads were effective at decreasing peak pressures on the bony

structures of the knee by distributing forces across

more surface area of the superior portion of the tibia

(Porter et al. 2010). Results of these two studies

suggest a positive benefit regarding the use of such

kneepads, without a trade-off in terms of increased

muscle activity.

Several limitations of this study should be noted.

One limitation is the relatively small sample size.

Further, subjects in this study were not underground

miners and it is possible that EMG activity may differ

between novices and experienced coal miners in

performing these load transfers. Some of the postures

employed (specifically kneeling in full flexion and

squatting) may have introduced the opportunity for

EMG artefacts of the hamstrings via tissue compression, although the artefact checking performed during

analysis should have helped to minimise this effect. In

addition, as subjects used their own natural lifting

cadence, it is possible that differences in lifting

dynamics might have influenced the results; however,

normalisation over the task should have helped to

control such an effect. As with any study examining

anisometric contractions, the interpretation of the

relationship between EMG amplitude and force

requires great caution due to the numerous factors

that can modify this relationship. While there was not

a great deal of dynamic motion of the legs during these

lateral load transfers, and while fixed epochs were

assessed during the load transfers as recommended

during anisometric contractions (de Luca 1997), the

differences in knee flexion between certain of the

postures studied would likely involve relative

movement of the electrodes with regard to active

muscle fibres. It should be recognised that the

amplitudes of the EMG signals may not provide a

straightforward indication of mechanical muscle

outputs in such cases (Basmajian and de Luca 1985).

In summary, the results of the present study have

several implications worth noting for those interested

in reducing knee morbidity due to work in kneeling

or squatting postures. One important issue

highlighted is that a kneeling posture that would be

desirable to workers from an energy conservation

standpoint (e.g. kneeling in full flexion) is known to

generate damage to the menisci due to pressures

generated by the femoral condyles in the flexed knee

posture. Researchers wishing to intervene to reduce

knee morbidity due to use of full flexion kneeling

will need to contend with the powerful motivating

influence of the energy conservation associated with

this posture, as well as the improved stability

afforded in this position. Furthermore, the data

suggest that the muscles of the lower extremity have

to compensate for smaller bases of support via

increased muscle activation during materials handling

activities in kneeling and squatting postures. Finally,

data from the present study suggest that kneepads

(at least those studied here) do not significantly

affect EMG activity and, thus, they seem to engender

no downside in terms of increased muscle activity

and related energy cost.

Conclusions

Based on the results of the present investigation, the

following conclusions are drawn:

(1) In a lifting task involving a right to left transfer

of a load, the muscles of the right thigh are

most active at the initiation of the lift and the

muscles of the left thigh are most active near

the end of the lift. Lowest overall EMG activity

was observed during the middle portion of the

lift.

(2) Results of the analysis indicate that the peak

EMG activity for the majority of the thigh

muscles studied was inversely related to the

base of support provided by the posture.

(3) Wearing kneepads was not found to influence

EMG activity of the thigh muscles in this study.

(4) Kneeling on both knees was associated with the

lowest EMG activity, while kneeling on one

knee was associated with increased activity and
squatting resulted in the highest amount of

EMG activity of any posture.

(5) The reduced EMG activity observed when

kneeling in full flexion suggests lower metabolic

demands, which may be a reason that this

posture is preferred by miners working in lowseam coal mines.

(6) Despite the reduced thigh muscle EMG activity

in the fully flexed kneeling posture, this posture

has been associated with development of

meniscal tears and development of knee

osteoarthritis.

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