Pressure distribution on the anatomic landmarks of the knee and the effect

of kneepads

William L. Porter*,

*	 Corresponding author. Tel.: þ1 412 386 5222; fax: þ1 412 386 6710.
 

E-mail address: wlporter@cdc.gov (W.L. Porter).
 

Alan G. Mayton, Susan M. Moore

Office of Mine Safety and Health Research, National Institute for Occupational Safety and Health, 626 Cochrans Mill Road, P.O. Box 18070, Pittsburgh, PA 15236, United States

abstract

This study examines stress transmitted to anatomic landmarks of the knee (patella, combined patella

tendon and tibial tubercle) while in static kneeling postures without kneepads and while wearing two

kneepads commonly worn in the mining industry. Ten subjects (7 male, 3 female) simulated postures

utilized in low-seam mines: kneeling in full flexion; kneeling at 90° of knee flexion; and kneeling on one

knee while in one of three kneepad states (no kneepads, non-articulated kneepads, and articulated

kneepads). For each posture, peak and mean pressure on the anatomic landmarks of the knee were

obtained. The majority of the pressure was found to be transmitted to the knee via the combined patellar

tendon and tibial tubercle rather than through the patella. While the kneepads tested decreased the

maximum pressure experienced at the combined patellar tendon and tibial tubercle, peak pressures of

greater than 25 psi were still experienced over structures commonly injured in mining (e.g. bursa sac e

bursitis/Miner’s Knee). The major conclusion of this study is that novel kneepad designs that redistribute

the stresses at the knee across a greater surface area and to other regions of the leg away from key

structures of the knee are needed.

1. Introduction

According to the 2007 Mine Safety and Health Administration

(MSHA) database of reported cases of accident/injury/illness, 227

knee injuries occurred in underground coal. Furthermore, the

associated total number of days lost in 2007 was 13,681 and the

incidence rate was 0.5 per 100 full-time equivalents. In 2007, the

median days lost due to a knee injury in coal operations was 41

days. This was nearly twice that which was observed for the back

(22 days).

In underground coal mining, the working height of the mine

typically coincides with the height of the coal seam. Low-seam

mines are those mines with a seam height of no more than 4200. At

the National Institute for Occupational Safety and Health (NIOSH),

an analysis of musculoskeletal injury data from eight low-seam coal

mines was conducted (Gallagher et al., 2009). These data indicated

that the highest frequency of injury was to the knee. In fact, when

compared to the low back, the frequency of knee injury was 1.7

times greater. Additionally, the average cost was $13,121 per knee

injury which was quite similar to that for the low back ($14,378).

Using this average cost per knee injury and the 227 knee injuries

reported to MSHA in 2007 for all underground coal mining, it can

be estimated that, in 2007, the financial burden of knee injuries in

underground mining was nearly three million dollars.

Many other 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’ (McMillan and Nichols, 2005; Roantree, 1957; Sharrard and

Liddell, 1962; Sharrard, 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., 2002, 2003; Coggon

et al., 2000; Cooper et al., 1994; Felson et al., 1991; Sharrard and

Liddell, 1962; Tanaka et al., 1985).

Sharrard (1963, 1965) recorded the stresses at the knee when

a mine worker was shoveling coal while starting from a kneeling at

full-flexion posture. When compared to the mine worker’s initial

full-flexion posture, the stresses at the knee varied a large amount

during the activity. Furthermore, Sharrard (1963) reported that the

greatest proportion of miners suffering from ‘Beat Knee’ were those

that knelt in one place for extended periods of time (i.e. static

postures).

Mine workers use kneepads of varying types to help redistribute

and diminish the effects of the stresses applied to the knee while

kneeling. However, the effectiveness of the kneepads is unknown.

Despite the fact that nearly all low-seam coal mine workers wear
kneepads, knee injuries continue to occur and are relatively severe

as was discussed earlier. Thus, a detailed understanding of the

stresses at the knee while in postures associated with low-seam

mining both with and without kneepads may provide insight into

the injury mechanism. Pilot data indicated that nearly all the stress

at the knee is transmitted from the ground via the patella, patellar

tendon, and tibial tubercle while kneeling. Therefore, the objective

of this study was to determine stress transmitted to the knee

through these landmarks while in static postures associated with

low-seam mining without kneepads and while wearing two

commonly used kneepads (one articulated and one nonarticulated).

2. Methods

2.1. Subjects

Ten subjects (7 male, 3 female) participated in this study. The

average age was 34 years (SD 1⁄4 17) with an age range of 19e60

years. The average weight and height were 683 N (SD 1⁄4 98) and

169 cm (SD 1⁄4 8) (154 pounds (SD 1⁄4 22) and 66 in (SD 1⁄4 3)),

respectively. Prior to participation in the study, each subject was

asked a series of questions to determine if they had ever had any

serious injury to the knee; none of the subjects had ever had

surgery on their knees. One subject was diagnosed with bursitis

which did not require any intervention and a second subject had

slight nerve damage due to a motorcycle accident. Additionally,

some subjects reported that their knee would “click.” However,

when asked if they ever experienced a “catching” or “locking”

feeling (a possible indicator of a meniscal injury), they responded

that they had not. Prior to participating in the study, each subject

read and signed an informed consent form approved by the NIOSH

Human Subjects Review Board.

2.2. Experimental design

NIOSH researchers interviewed over 60 low-seam mine workers

whereby the mine workers identified the posture they utilized to

perform various mine tasks (e.g. building stoppings, hanging

curtain). As a result of these data, the postures utilized in this study

were selected (Fig. 1).

Fig. 1. Schematic showing each posture tested during the experiment. Kneeling at 90°

knee flexion (Near 90), Kneeling on right knee (One Knee), Kneeling at full flexion

(Near Full).

Two working heights were investigated

97 cm (3800) and 122 cm (4800). Since low-seam mines are generally

those of working heights of 4200 or less, these heights represented

a typical working height classified as a low-seam and a working

height that was higher than a low-seam but still required the mine

worker to adopt kneeling and crawling postures to perform their

tasks. The 1000 total difference in the two working heights selected

allowed researchers to determine if differences in seam heights in

and around the low-seam level would significantly affect pressure

at the knee. It should be noted that all postures were not performed

at both seam heights, only those postures that were reasonable for

a seam height were investigated. For example, a mine worker

would not kneel at 90° of knee flexion in a 3800 mine as this would

be extremely difficult at such a low working height.

Subjects performed each of the above postures for three kneepad conditions: no kneepads, articulated kneepads, and nonarticulated kneepads. The articulated and non-articulated kneepads

were selected based on their widespread use in the mining

industry. Several distributors of kneepads to the mining industry

were contacted in 2007 and asked to provide the most frequently

ordered kneepads for the previous year. From these data, the most

commonly requested articulated and non-articulated kneepads

were selected. The articulated kneepads consisted of a hard outer

shell with hard rubber padding on the inside. The straps were also

rubber and crossed a few inches above and a few inches below the

crease of the knee. The non-articulated kneepads consisted of a soft

outer rubber shell and soft inner foam padding. Again, the straps

crossed a few inches above and below the crease of the knee.

2.3. Subject preparation

A custom-built pressure sensor was used to measure pressures

(TactArray T2000; Pressure Profile Systems; Los Angeles, CA). The

sensor used capacitive sensor technology and was pre-shaped to

conform to the knee when flexed at 90°. The sensor consisted of 196

individual pressure sensing units that varied in sizes ranging from

1.101 cm2 to 1.464 2 2 2 cm (0.1707 in to 0.227 in ) and was 3.2 mm

(0.13 in) thick. Due to the pre-shaped nature of the sensor, the

distance (dead space) between the sensing units was not constant

across the sensor. With the sensor affixed to the knee, the distance

between sensing units in the medial-to-lateral direction was

constant at 1.7 mm (0.065 in). However, in the superior-to-inferior

direction, the distance between sensing units varied from 3.1 to

4.7 mm (0.121e0.187 in) in the region where the patellar tendon

and tibial tubercle rested and from 3.1 to 8.2 mm (0.121e0.322 in)

in the region where the Patella rested (Fig. 2). Additionally, there

was extra fabric on either end of the sensor so that a piece of Velcro

could be sewn on to this fabric. To affix the sensor to the leg when

the knee ® was at 90° of flexion, an Ace bandage was first wrapped

around the thigh so that half of the bandage was unwrapped and

the bottom edge of the bandage was two inches from the crease of

the knee. A rectangular piece of Velcro was then adhered such that

half attached to the skin and the other half to the bandage. Eight

pieces of hypoallergenic athletic tape were then applied in an

asterisk pattern such that half of the tape was adhered to the Velcro

on the thigh and the other half was adhered to the skin of the thigh.

The corresponding piece of Velcro on the sensor was then adhered.

The remainder of the Ace® bandage was then wrapped around the

thigh, and metal fasteners were used to attach it to itself. In

a similar manner, the inferior end of the sensor was adhered to the
lower leg. This method of fixation demonstrated that the sensor did

not move while the subject performed the postures described

above.

It was next necessary to determine which sensing units corresponded to various anatomic landmarks (patella, patellar tendon,

and tibial tubercle). Using a wooden dowel, a researcher palpated

the perimeter of these landmarks and the sensing units corresponding to each landmark were identified (Fig. 2).

Fig. 2. Sensor pad layout with sensing units identified during palpation as the Patella (A) and the PTT (B) for a representative subject with the shaded cells identifying the pressure

distribution during kneeling in full flexion (Near Full) and kneeling on one knee (One Knee) for the same subject.

The same

researcher palpated the anatomic landmarks for all subjects.

Preliminary tests were conducted to determine the researcher’s

repeatability for palpating the landmarks. Based on this information, it was decided that the patellar tendon and tibial tubercle

(PTT) would be grouped together since their small size resulted in

unacceptable intra-observer repeatability. Palpating the patella and

the combined PTT were both repeatable to within 6.7% of the total

area. These anatomic landmarks were also palpated at the

conclusion of testing to ensure that the sensor had not moved.

Palpations indicated that the pressure sensor did not significantly

shift during testing.

2.4. Testing procedure

The order in which the three kneepad states were tested (no

kneepad, articulated kneepad, non-articulated kneepad) was

randomized. Within each kneepad state, the order in which

postures were tested was then randomized as well.

The subject first donned the required kneepad, if necessary,

according to the randomized order. Subjects were then instructed

to place their knee in a reference position in order to zero the

pressure sensor. This position was 90° of knee flexion for postures

that were initiated with the knee in this posture. The reference

position was a squat for postures that were initiated with the knee

in a fully flexed position (e.g. full flexion). A researcher directed

subjects to view a stick-figure chart of test postures as the principal

way to inform them of the posture they were to assume. The

subjects were also instructed to keep their hands central to their

body. Some subjects chose to hold their hands in the air around

their chest while others let their hands rest on their thighs or at

their sides. In some instances, subjects inquired as to whether or

not they were in the correct posture. In these cases, a general verbal

response was given to the subject. Subjects were given leeway to

assume the posture in a way that maximized their comfort unless

the subject assumed a posture that was considerably different from

that which was requested. For example, a subject was instructed to

enter a posture of 90° of knee flexion, but actually positioned

themselves in a posture near full flexion. In such cases, the

researchers instructed the subjects to adjust their posture.

Once the subject was in the posture of interest, the data

collection system was initiated and a researcher instructed the

subject that data collection had begun. Data were collected for a

10-s interval and the subject was told to stop. Immediately afterward, the data were saved and reviewed for acceptability by

viewing a color map display of the pressure distribution. Between

tests, the subjects were instructed to sit in a chair and place their

knee near 90° of flexion making sure not to apply a load to the

sensor. The sensor was then allowed to recover until the maximum
and average pressures across the sensor were one pound per square

inch or less. If an error occurred during data collection, the sensor

was allowed to recover and then the trial was repeated.

2.5. Data processing/analysis

Pressure data were obtained for every sensing unit at each time

point for the 10 s of data collected at a variable sampling rate of

approximately 5 Hz. The equipment used was unable to provide

a consistent sampling rate, variations of up to 0.5 Hz were

observed. The ratio of pressure between the patella and the

combined PTT was determined first. The patella and combined PTT

both consisted of several sensing units. In order to arrive at

a pressure ratio calculation, the pressure across these sensing units

was first summed for both the patella and the combined PTT. This

was done for all time points. At each time point, the sum of the

patellar pressure was then divided by the sum of the combined PTT

pressure and the sum of the patellar pressure, creating a ratio for

each point in time. These ratios were then summed together across

all time points and then divided by the total number of time points.

This yielded the mean pressure ratio between the patella and the

combined PTT (see Appendix for detailed equations).

Next, it was necessary to characterize the magnitude of pressures across the patella and combined PTT. The mean pressure for

both structures was determined for every time point. This value

was then summed and divided by the total number of data points.

This yielded the mean of the mean pressures on the patella and

combined PTT across all time points.

The mean maximum pressure was determined by summing the

maximum pressure at each time point and dividing by the total

number of time points. Finally, some measurement of the distribution of stress across the structures was necessary. To do this,

a measure of variance was calculated. The variances were calculated for the patella and combined PTT at each time point. These

variances were then summed and divided by the total number of

time points yielding the mean variance across each structure.

Statistical analyses were performed using Statistix 8.0 for

Windows. Analyses performed included a split-plot analysis of

variance (ANOVA) and a priori orthogonal contrasts. Contrasts for

kneepad states included: 1) comparisons of the no-kneepad state

versus wearing kneepads, and 2) comparing the non-articulated

versus articulated kneepads. Contrasts for posture states included:

1) comparison of kneeling with both knees in full flexion to

kneeling on the right knee only (across both heights), and 2)

comparing the 3800 work height to the 4800 work height. All

contrasts were tested using a T statistic with an alpha 1⁄4 0.05. As an

exploratory analysis, multiplicity corrections were not applied in

the data analyses (Bender and Lange, 2001). Specifically,

a comparison-wise Type I error rate alpha level of 0.05 was

employed.

3. Results

For all postures tested, the majority (>60%) of the pressure was

placed on the combined PTT region (Fig. 3).

Fig. 3. Mean pressure ratio at the PTT region across postures (*p < 0.05).

A significant difference

(p < .05) was observed for kneepad conditions, posture, and subject

for the mean pressure ratio. On further analysis it was found that no

significant difference existed between the no-kneepad state and

the two kneepad states, while a significant difference existed

within the two kneepad states with the articulated kneepad

exhibiting a greater mean pressure ratio for the PTT region

(p < .0001). On further examination of the posture significance, the

kneeling at full-flexion (near full) condition showed a significantly

greater mean pressure ratio for the PTT region (p < .0001) when

compared to the kneeling on right knee (one knee) conditions. No

significant difference between the 3800 and 4800 working heights

was observed for any dependent measures.

The mean of the mean pressure at the patella region showed

that only modest amounts of pressure (<10 psi) were applied to the

patella for all postures (Fig. 4).

Fig. 4. Mean of the mean pressure at the patella region for the various postures

(*p < 0.05).

A significant difference (p < .05) was

observed for kneepad conditions, posture, and subject. As with the

pressure ratio, it was found that no significant difference existed

between the no-kneepad state and the two kneepads states, but

a significant difference existed between the two kneepad states

with the non-articulated kneepad exhibiting a greater mean of the

mean pressure for the patella region (p < .0001). On further

examination into the significance of the posture condition, it was

determined that the one knee condition showed significantly

greater mean of the mean pressure for the patella region

(p < .0001) when compared to the near full conditions.

The mean of the mean pressure at the combined PTT region

showed a considerably higher level of applied pressure (>15 psi)

than that which was observed for the patella region (Fig. 5). As with

the mean of the mean pressure for the patella region, a significant

difference (p < .05) was observed for posture and subject. In

contrast to the patella region, the PTT region did not show
significant difference due to kneepads. As compared with the

patella region, the reverse trend was found for the PTT region when

looking at the one knee versus near full condition with the one knee

showing significantly greater mean of the mean pressure

(p < .0001) when compared to the near full conditions.

Fig. 5. Mean of the mean pressure at the PTT region for the various postures

(*p < 0.05).

The mean of the maximum pressure at the patella region

showed that highly variable (1.3 ±1.1 to 27.1 ±17.2 psi) maximum

pressure was applied to the patella for the different postures

(Fig. 6).

Fig. 6. Mean of the maximum pressure at the patella region for the various postures

(*p < 0.05).

As with the mean of the mean pressure, significant differences (p < .05) were observed for kneepad conditions, posture, and

subject for the mean of the max pressure. Again, it was found that

no significant difference existed between the no-kneepad state and

the two kneepad states, but that a significant difference did exist

between the two kneepad states with the non-articulated kneepad

exhibiting a greater mean of the maximum pressure for the patella

region (p 1⁄4 .0006). Additionally, the one knee condition showed

significantly greater mean of the maximum pressure for the patella

region (p < .0001) when compared to the near full conditions.

The mean of the maximum pressure at the PTT region showed

a considerably higher level of pressure (>25 psi) than that which

was observed for the patella region for all postures regardless of the

kneepad condition (Fig. 7).

Fig. 7. Mean of the maximum pressure at the PTT region for the various postures

(*p < 0.05).

A significant difference (p < .05) was

observed for kneepad conditions and posture, but not between

subjects for the PTT region. No significant difference existed

between the two kneepad states, but a significant difference existed between the no kneepad and the two kneepad states with the

no-kneepad state exhibiting a greater mean of the maximum

pressure for the PTT region (p < .0001). Looking at significant

differences within posture, the only significant difference was

found between the near full compared to the one knee conditions,

with the one knee postures exhibiting a greater mean of the

maximum pressure (p < .0001).

The mean of the variance at the patella region showed considerably different magnitudes 2 of variance across postures (w0 psi

for near full postures to w60 psi2 for all other postures). Similarly,

significant differences (p < .05) were observed for posture and

subject, but not for the different kneepad conditions. The one knee

condition showed a significantly greater mean of the pressure

variance for the patella region (p < .0001) when compared to the

near full conditions.

The mean of the variance at the PTT region showed relatively

consistent levels of variance across postures (w175 psi2 for nokneepad conditions and w75 psi2 for both kneepad conditions).

Significant differences (p < .05) were observed for posture and

kneepad condition but not for subject. The no-kneepad condition

was found to have a significantly greater mean of the variance for

the PTT region when compared to the two kneepad states

(p < .0001). No significant difference was found between the two

kneepad states. The one knee condition showed significantly

greater mean of the variance for the PTT region (p < .0001) when

compared to the near full conditions.

4. Discussion

In this study, the stress transmitted to the knee through the

patella, patellar tendon, and tibial tubercle were determined while

in static postures associated with low-seam mining without

kneepads and while wearing two kneepads commonly used in the

industry (one articulated and one non-articulated). The results

yielded several important pieces of information. The majority of the

pressure was found to be transmitted to the knee via the combined

patellar tendon and tibial tubercle. The kneepads better distributed

the stresses at the combined patellar tendon and tibial tubercle
region across a larger surface area decreasing the maximum

stresses experienced by these structures. However, peak pressures

of greater than 25 psi were still experienced.

The mean pressure ratio calculation was designed to identify if

the kneepad design, in regards to both construction and padding,

affected how the pressure was applied to the knee during kneeling

postures. The mean pressure ratio showed that the majority (over

60%) of the pressure applied to the knee during kneeling is applied

to the PTT region. While the kneepads tested did not drastically

affect how this pressure was distributed, the two kneepads did

show limited ability (w10%) to transfer loading to/away from the

PTT region. The non-articulated kneepad transferred slightly more

pressure from the PTT to the patella as compared to the no-kneepad

condition, while the articulated kneepad exhibited the opposite

trend. The overall impact on the stress distribution amongst knee

structures was minimal. While the articulated and non-articulated

kneepads use different materials, they both employ an outer shell

and an inner padding that rested directly against the knee region.

These data suggest that alternative design ideas be investigated so

that the stresses applied to the key anatomic structures of the knee

may be distributed to other, perhaps less harmful, regions. For

example, a novel kneepad design that redistributes the forces at the

knee across the larger surface area of the shin could drastically

reduce the magnitude of stress on the knee.

To understand how the loading of the stress across the patella

and the combined PTT regions reacted to the postures and kneepads tested, the mean of the mean pressure and the mean of

maximum pressure was calculated. The difference observed in the

average pressure applied to the patella region, while significant,

had little effect on the total pressure levels. Additionally, while the

expected drop in the mean maximum pressure from no kneepad to

kneepad conditions was observed, no significant difference was

observed between the kneepads.

These findings demonstrate that the kneepads tested had little

affect on the overall application of stress to the knee. Without the

use of kneepads, high variation was observed due to “hot spots”

developing over small areas while nearby areas were nearly

unloaded. As expected, both kneepads drastically reduced the

variation (60% reduction) for the PTT region, indicating that kneepads are effective for distributing pressure across the knee. Unexpectedly, the two kneepads tested did not respond differently with

respect to variation of pressure application for both regions, indicating that the differences in the material of the kneepads had little

effect at distributing the stress over the patella and the PTT.

The reduction in point-loading and decreased variation can help

explain the necessary comfort factor for working in kneeling

postures and why some form of kneepad is preferred to none at all.

The results show that although kneepads have a slight effect on the

peak pressure levels and the variance observed during kneeling

postures, the current design/materials used have a relatively

negligible change on mean pressure. In other words, although the

overall pressure was not reduced significantly, the kneepads tested

redistributed the pressure more evenly across the knee structures.

Thus, same or similar materials may be useful for future novel

kneepad designs that attempt to redistribute the forces at the knee

over larger surface areas.

The pressure measurements indicate what happens while

kneeling in full flexion. During full-flexion postures, low pressure

(<3 psi mean, <10 psi peak) is applied to the patella; this is a third

or less of what was seen in other postures. Additionally, it was

found that kneeling in a full-flexion posture exhibited almost 50%

less pressure applied to the patella and the PTT when looked at as

a whole. Although far from the ideal, working in a full-flexion

posture appears to provide the most direct and immediate stress

relief to the surface of the knee and therefore comfort to the knee

for mine workers that must work frequently in a kneeling posture.

Prolonged use of the posture, however, may lead to less obvious

deterioration and weakening of other important knee components

of the knee joint, e.g., tissue damage to the meniscus, stretching of

ligaments and tendons.

One hypothesis of this study was that seam height would have

a significant effect on the pressure measured at the knee. The

analysis showed that the two seam heights tested (3800 and 4800)

had no significant effect on any of the dependent measures. One

explanation is, that while the postural difference on a whole body

scale would be different between the two heights, the knee

posture/included angle did not drastically change and therefore did

not affect the pressures observed.

An early qualitative clinical investigation of bursitis (Watkins

et al., 1958) suggested that the majority of force found during

kneeling postures is transferred through the tibial tubercle. Similar

to the results of the current study, Watkins et al. also found that

kneepads might redistribute this load to neighboring anatomical

features of the knee altering the types of injuries experienced by

the worker. As with the current study, these results also suggest

that simply changing the type of material used in the kneepad may

not be sufficient. Rather, a novel design of kneepads may be

required that deliberately alters the loading of the knee such that

key anatomic structures are not vulnerable to excessive loads.

Other published findings that evaluated the external pressure

applied to the structures of the knee were sought to compare with

the results of this study, but none were found. Nevertheless, some

studies have evaluated the pressure on the cartilage of the knee

while in various postures such as deep knee flexion (Li et al., 2005;

Hefzy et al., 1998). Future studies should consider evaluating the

effect that postures associated with low-seam mining have on the

internal structures of the knee. Specifically, the forces in

the primary ligaments and meniscus as well as the pressure on the

cartilage will yield important information for kneepad designs

aimed at reducing injury risk.

For example, the forces in the knee structures have been

extensively studied using robotic technology (Rudy et al., 1996;

Song et al., 2004). In some of these studies, an anterior or posterior load was applied to simulate a clinical exam. During this safe

application of load (134 N), the measured forces found in posterior

cruciate ligament, the primary ligamentous stabilizer to posterior

loads at the knee, have been shown to be as high as 129 N (Harner

et al., 2000). Additionally, load-to-failure studies have been performed to determine the point of failure of the posterior cruciate

ligament (Kennedy et al., 1976). Determining the forces in the

posterior cruciate ligament during postures associated with lowseam mining would provide insight into the relative risk to that

structure during these postures. Other factors such as the internal

and external rotation of the tibia during these postures may pose

additional risk to other structures of the knee and have also been

investigated previously (Sharrard and Liddell, 1962; Pollard, 2008).

4.1. Limitations

The results of this study should be interpreted in consideration

of the limitations. The pressure sensor design and construction

needed to accommodate the shape of the knee in flexed positions

and was of a cupped design. The largest gap areas were in the

central portion of the curved section of the sensor pad, or where the

superior region of the patella resided. Since the inferior border of

the patella was the only portion experiencing loading, the impact of

these larger gaps is likely minimal. However, “hot spots” may have

been missed in the PTT region. Therefore, the results of this study

likely underestimated the peak and mean stresses in this region.
Only two seam heights and three kneepad conditions were

tested in a limited number of postures for this study. While these

conditions are not representative of all situations for underground

low-coal mining, it was decided to limit the study to these conditions due to concerns about the length of subject testing. Information received through in-mine observations and feedback from

miners indicated that the conditions tested would represent the

majority of circumstances a low-coal miner would find themselves

using. For example, the two kneepads used in the study were found

to be the most widely used in the mining industry. Moreover, while

only static postures were evaluated, dynamic postures will be

addressed in future efforts to determine if such postures and

motions are different from the findings of this study. During the nokneepad condition, subjects did experience minimal discomfort at

the knee while simulating the various postures. However, subjects

indicated that the presence of the pressure sensor (3.2 mm thick)

provided sufficient cushioning. Thus, it is unlikely that this

discomfort greatly altered their posture.

5. Conclusion

The goal of this study was to investigate the impact of posture

and kneepads on the application and distribution of pressure across

the bony structures of the knee. In particular, this study sought to

determine if the designs of the current kneepads used in the lowseam mining industry responded differently to the tests performed,

and if different postures resulted in different levels of stress to the

knee. From the results of this study, it can be concluded that

kneepads currently used in low-seam mining are an effective tool

for decreasing the peak pressure on the bony structures of the knee

by distributing the forces across more surface area of the superior

portion of the tibia. However, a substantial magnitude of stress still

exists and is located near key anatomical landmarks of the knee

(e.g. bursa sac). Therefore, novel kneepad designs are needed to

significantly reduce the magnitude of pressure applied to the

structures of the knee. The new design should focus on redistributing the pressure away from the PTT region of the knee to

other parts of the lower leg such as the shin.

Future work will focus on the design and fabrication of a kneepad that demonstrates durability for use in the harsh mining

environment and a reduction in the stresses at the knee for

postures associated with low-coal mining. The advantages and

increased effectiveness of the new kneepad design compared to

traditional kneepads will be evaluated and validated with laboratory and field testing.

Acknowledgments

The authors would like to acknowledge the contributions of the

following individuals in the conduct of this study: Sean Gallagher,

Jonisha Pollard, Eric Rainis, Mark Redfern, and Kurt Beschorner.

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.

Appendix

Aij represents a matrix of the cells that make up the Patella (P)

region (see Fig. 2) with i 1⁄4 sample number and j 1⁄4 cell number.

Bij represents a matrix of the cells that make up the Patellar

Tendon & Tibial Tubercle (PTT) region (see Fig. 2) with i 1⁄4 sample

number and j 1⁄4 cell number (Equations for the patellar tendon &

tibial tubercle region are shown, patella equations are similar).

Mean pressure ratio: the average over time of the ratio of the sum

of a region over the sum over both regions

Xm

Bij

j 1⁄4 1

q m

Xn

i 1⁄4 1X X Aij þ Bij

j 1⁄4 1 j 1⁄4 1

n

n 1⁄4 total number of samples, m 1⁄4 total number of cells in B, q 1⁄4 total

number of cells in A.

Pressure mean of mean: the average of a region averaged over

time

Xm

Bij n j 1⁄4 1

m

X

i 1⁄4 1

n

n 1⁄4 total number of samples, m 1⁄4 total number of cells in B.

Pressure mean of max: the maximum of a region averaged over

time

Xn

maxðBiÞ

i 1⁄4 1

n

n 1⁄4 total number of samples.

Pressure mean of variance: the variance of a region averaged over

time

n

ðstdevðBiÞÞ X 2

i 1⁄4 1

n

n 1⁄4 total number of samples.

References

Baker, P., Coggon, D., Reading, I., Barrett, D., McLaren, M., 2002. Sports injury,

occupational physical activity, joint laxity, and meniscal damage. Journal of

Rheumatology 29 (3), 557e563.

Baker, P., Reading, I., Cooper, C., Coggon, D., 2003. Knee disorders in the general

population and their relation to occupation. Occupational and Environmental

Medicine 60, 794e797.

Bender, R., Lange, S., 2001. Adjusting for multiple testing e when and how? Journal

of Clinical Epidemiology 54, 343e349.

Coggon, D., Croft, P., Kellingray, S., Barrett, D., McLaren, M., Cooper, C., 2000.

Occupational physical activities and osteoarthritis of the knee. Arthritis &

Rheumatism 43 (7), 1443e1449.

Cooper, C., McAlindon, T., Corlett, E.N., Egger, P., Dieppe, P., 1994. Occupational

activity and osteoarthritis of the knee. Annals of Rheumatic Diseases 53, 90e93.

Felson, D.T., Hannan, M., Naimark, A., Berkeley, J., Gordon, G., Wilson, P.W.,

Anderson, J.J., 1991. Occupational physical demands, knee bending, and knee

osteoarthritis: results from the Framingham study. Journal of Rheumatology 18

(10), 1587e1592.

Gallagher, S., Moore, S., Dempsey, P.G., 2009. An analysis of injury claims from lowseam coal mines. Journal of Safety Research 40, 233e237.

Harner, C.D., Vogrin, T.M., Hoher, J., Nenjamin, C., Woo, S.L.-Y., 2000. Biomechanical

analysis of a posterior cruciate ligament reconstruction. American Journal of

Sports Medicine 28 (1), 32e39.

Hefzy, M.S., Kelly, B.P., Cooke, D.V., 1998. Kinematics of the knee joint in deep

flexion: a radiographic assessment. Medical Engineering & Physics 20, 302e307.

Kennedy, J.C., Hawkine, R.J., Willis, R.B., Danylohuk, K.D., 1976. Tension studies of

human knee ligaments. Yield point, ultimate failure, and disruption of the

cruciate and tibial collateral ligaments. Journal of Bone and Joint Surgery 58,

350e355.
Li, G., DeFrate, L.E., Park, S.E., Gill, T.J., Rubash, H.E., 2005. In vivo articular cartilage

contact kinematics of the knee. American Journal of Sports Medicine 33 (1),

102e107.

McMillan, G., Nichols, L., 2005. Osteoarthritis and meniscus disorders of the knee as

occupational diseases of miners. Occupational and Environmental Medicine 62,

567e575.

Pollard, J.P., 2008. Development of a computational model to determine tibiofemoral forces and moments during kneeling. Master’s thesis, University of

Pittsburgh.

Roantree, W.B., 1957. A review of 102 cases of beat conditions of the knee. British

Journal of Industrial Medicine 14, 253e257.

Rudy, T.W., Livesay, G.A., Woo, S.L.Y., Fu, F.H., 1996. A combined robotic/universal

force sensor approach to determine in situ forces of knee ligaments. Journal of

Biomechanics 29 (10), 1357e1360.

Sharrard, W.J.W., Liddell, F.D.K., 1962. Injuries to the semilunar cartilages of the

knee in miners. British Journal of Industrial Medicine 19.

Sharrard, W.J.W., 1963. Aetiology and pathology of beat knee. British Journal of

Industrial Medicine 20, 24e31.

Sharrard, W.J.W., 1965. Pressure effects on the knee in kneeling miners. Annals of

the Royal College of Surgeons of England 36, 309e324.

Song, Y., Debski, R.E., Musahl, V., Thomas, M., Woo, S.L.-Y., 2004. A three-dimensional finite element model of the human anterior cruciate ligament:

a computational analysis with experimental validation. Journal of Biomechanics

37, 383e390.

Tanaka, S., Halperin, W.E., Smith, A.B., Lee, S.T., Luggen, M.E., Hess, E.V., 1985. Skin

effects of occupational kneeling. American Journal of Industrial Medicine 8,

341e349.

Watkins, J.T., Hunt, T.A., Fernandez, R.H.P., Edmonds, O.P., 1958. A clinical study of

beat knee. British Journal of Industrial Medicine 15, 105.