RI 9681 

REPORT OF INVESTIGATIONS/2011 

Department of Health and Human Services 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 
Report of Investigations 9681 

Demands on the Knee During Kneeling and Squatting Activities 

Common to Low-Seam Mining 

Susan M. Moore, Ph.D., Jonisha P. Pollard, MS, William L. Porter, MS, 

Sean Gallagher, Ph.D., Alan G. Mayton, CMSP, PE 

DEPARTMENT OF HEALTH AND HUMAN SERVICES
 Centers for Disease Control and Prevention
 

National Institute for Occupational Safety and Health
 

Office of Mine Safety and Health Research
 

Pittsburgh, PA • Spokane, WA
 

June 2011 
This document is in the public domain and may be freely 

copied or reprinted. 

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do not constitute NIOSH endorsement of the sponsoring organizations or their programs or 

products. Furthermore, NIOSH is not responsible for the content of these Web sites. 

The findings and conclusions in this report are those of the authors and do not necessarily 

represent the views of the National Institute for Occupational Safety and Health. 

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DHHS (NIOSH) Publication No. 2011–176 

June 2011 

SAFER • HEALTHIER • PEOPLETM 
Contents
 

Executive Summary ................................................................................................................... 1
 

Introduction................................................................................................................................ 2
 

Methods..................................................................................................................4
 

Subjects ............................................................................................................................. 4
 

Experimental Design .......................................................................................................... 4
 

Preliminary Measurements................................................................................................. 6
 

EMG Measurement ............................................................................................................ 7
 

Pressure Measurement ...................................................................................................... 8
 

Motion Analysis Measurement...........................................................................................10
 

Additional Testing Equipment and Syncing .......................................................................12
 

Testing Procedure.............................................................................................................12
 

Data Analysis ....................................................................................................................13
 

Results..................................................................................................................18
 

EMG Data .........................................................................................................................18
 

Pressure Data ...................................................................................................................21
 

Net Forces and Moments and Resulting Joint Kinematics Data.........................................28
 

Discussion of Results................................................................................................................33
 

EMG..................................................................................................................................33
 

Pressure............................................................................................................................33
 

Net Forces and Moments ..................................................................................................34
 

Implications of Results ......................................................................................................34
 

Conclusions ..............................................................................................................................37
 

Acknowledgments.....................................................................................................................38
 

References ...............................................................................................................................39
 

Appendix A ...............................................................................................................................42
 

Appendix B ...............................................................................................................................47
 
Figures
 

Figure 1. Schematic showing each posture tested during the experiment. ................................. 5
 

Figure 2. Photographs of the articulated (A) and nonarticulated 

(B) kneepads used in this study. ................................................................................ 6
 

Figure 3. Subject wearing pressure sensor in the preshaped 90° flexion position. ..................... 9
 

Figure 4. Pressure sensor layout with individual pressure 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 while kneeling near full flexion 

and kneeling on one knee for the same subject. ........................................................ 9
 

Figure 5. A) Anatomical marker set and B) testing marker set of theupper and lower
 

right side of the body. (R=Right, L=Left)....................................................................11
 

Figure 6. Diagram of external shank forces and reaction forces and moments for
 

kneeling near full flexion with respect to the global coordinate system (GCS)...........16
 

Figure 7. Diagram of external shank forces and reaction forces and moments for kneeling 

near full flexion with respect to the anatomical shank coordinate system (ASCS). ....16
 

Figure 8. External force diagrams with respect to the anatomical shank coordinate system
 

(ASCS). ....................................................................................................................17
 

Figure 9. Summary of EMG activity by posture and block position. Each posture and block
 

position has 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. .........................19
 

Figure 10. Interaction of kneepad condition and block position on EMG activity of the 

left biceps femoris across postures. ..........................................................................20
 

Figure 11. Mean pressure ratio at the PTT region across postures. (* indicates significant
 

difference with p < 0.05)............................................................................................21
 

Figure 12. Mean of the mean pressure at the patella region for the various postures.
 

(*indicates significant difference with p < 0.05). ........................................................22
 

Figure 13. Mean of the mean pressure at the PTT region for the various postures.
 

(*indicates significant difference with p < 0.05). ........................................................23
 

Figure 14. Mean of the maximum pressure at the patella region for the various postures.
 

(*indicates significant difference with p < 0.05). ........................................................24
 

Figure 15. Mean of the maximum pressure at the PTT region for the various postures.
 

(*indicates significant difference with p < 0.05). ........................................................25
 

Figure 16. Mean of the variance at the patella region for the various postures
 

(*indicates significant difference with p < 0.05). ........................................................26
 

Figure 17. Mean of the variance at the PTT region for the various postures
 

(*indicates significant difference with p < 0.05). ........................................................27
 

Figure 18. Typical pressure distribution (psi) for squatting (A) and kneeling near
 

full flexion (B)............................................................................................................28
 

Figure 19. Knee angles (degrees) for all postures. Varus and internal rotation angles are
 

positive. ....................................................................................................................29
 
Figure 20. Mean tibial forces normalized to body weight (% BW). Lateral, superior, 

and anterior forces are positive.................................................................................30
 

Figure 21. Mean tibial moments normalized to body weight times height (% BW*Ht).
 

Extension, varus, and internal rotation moments are positive....................................32
 

Figure 22: Pelvis coordinate system highlighting the location of the right hip joint center ..........43
 

Figure 23: Orientation of the ATCS and ASCS..........................................................................44
 

Tables 

Table 1. Summary of significant main effects and interactions for normalized 

activity of all muscles. ...............................................................................................18
 
Acronyms and Abbreviations
 

AJC Ankle joint center 

ASCS Anatomical shank coordinate system 

ASIS Anterior superior iliac spine 

ATCS Anatomical coordinate system of the thigh 

L.ASIS Left anterior superior iliac spine 

R.ASIS Right anterior superior iliac spine 

HJC Hip joint center 

KJC Knee joint center 

MCS Measured coordinate system 

MTCS Measured coordinate system of the thigh 

MSCS Measured coordinate system of the shank 

BW Body weight 

Ht Height 

GCS Global coordinate system 

Ft/c Thigh-calf contact force 

Fh/g Heel-gluteus contact force 

F1 Forces at the foot 

F2 Forces at the knee 

WLL Weight of the lower leg 

One Knee Kneeling on one knee 

Near Full Kneeling near full flexion 

Near 90 Kneeling near 90° of knee flexion 

Squat Squatting 

MSHA Mine Safety and Health Administration 

NIOSH National Institute for Occupational Safety and Health 

EMG Electromyography 

MVC Maximum voluntary contraction 

MAV Mean amplitude value 

PTT Patellar tendon and tibial tubercle 

ANOVA Analysis of variance 
Demands on the Knee During Kneeling and Squatting 

Activities Common to Low-Seam Mining
 

Susan M. Moore, Ph.D., Jonisha P. Pollard, MS, William L. Porter, MS, 

Sean Gallagher, Ph.D., Alan G. Mayton, CMSP, PE
 

Office of Mine Safety and Health Research
 

National Institute for Occupational Safety and Health
 

Executive Summary 

In 2009, the operating height of approximately one fourth of underground coal mines in the 

U.S. restricted mine workers to kneeling, crawling, and/or stooping posture to perform work 

[MSHA 2009]. The large number of knee injuries to these workers is likely attributed to 

exposure to musculoskeletal disorder risk factors (prolonged kneeling, crawling, and twisting on 

one’s knees). Therefore, the National Institute for Occupational Safety and Health has 

investigated three different biomechanical parameters (muscle activity of the knee flexors and 

extensors, pressure at the knee, and the net forces and moments at the knee) as subjects assumed 

postures common to low-seam mining, both with and without kneepads. The postures evaluated 

included: (1) kneeling near full flexion; (2) kneeling near 90° of knee flexion; (3) kneeling on 

one knee; and (4) squatting. The pressure and the net forces and moments at the knee were 

evaluated as subjects statically assumed these postures. However, negligible muscle activity 

existed for these static postures. Therefore, muscle activity of the knee flexors and extensors was 

evaluated for each posture while subjects performed a lateral lift that is common to low-seam 

mining where they lifted a 25-lb block from their right side, transferred it across their body, and 

placed it on the ground on their left side. The results indicated that, relative to the stresses posed 

by other kneeling postures, some postures had may have more detrimental effects than others. 

Considering the potential impact of the three biomechanical parameters, several key 

recommendations were made regarding when it may be most appropriate to use specific postures. 

Additional recommendations were also made regarding the design of kneepads. 
Introduction
 

Underground coal mining creates a unique set of environmental stressors not found in other 

occupations. These stressors include reduced visibility, uneven terrain, falling roof material, and 

other hazards which require mine workers to don numerous forms of personal protective 

equipment including hard hats, steel-toed boots, and cap lamps. However, one of the most 

challenging stressors is often the extreme restriction in vertical working height present in many 

underground coal mines. Although dependent on mining conditions, the working height of a 

mine is typically similar to the height of the coal seam. Seam height can be classified as low (≤ 

43in) and medium (44 to 60 in) with higher seams being greater than 60 in. Low-seam heights 

restrict mine workers’ postures, forcing them to kneel, crawl, and/or stoop to perform work. 

Mine workers in low-seam mines spend their entire shift (8–10 hours) confined to these postures. 

A previous NIOSH evaluation of the 2007 Mine Safety and Health Administration (MSHA) 

injury database indicated that 84 injuries to the knee were reported for seam heights of 30 to 54 

in. These injuries were associated with an average of 47 days away from work for the low seams 

and 62 days for the mid seams. In an analysis of musculoskeletal injury data from eight lowseam coal mines, Gallagher et al. [2009] reported that the cost per knee injury was $13,121 on 

average. Considering the 84 injuries that occurred in mines with a seam height of 30 to 54 in, it 

can be estimated that these injuries cost more than $1,000,000 in 2007. The majority of these 

workers are more senior and male. According to a recent national survey of the mining industry 

conducted by NIOSH, the average age of coal mine workers (surface and underground mines 

combined) is 43.8 years (95% CI: 42.5 to 45.1 years). NIOSH also determined that, within this 

population, only 3.8% are female. 

Due to the fact that mine workers are confined to their knees for the duration of their shift, 

many studies regarding knee injuries in mining have focused on this population of workers. 

These studies have demonstrated that low-seam mine workers suffer from a variety of knee 

injuries, including meniscal tears, osteoarthritis, ligament tears, and bursitis (i.e., beat knee) 

[McMillan and Nichols 2005, Roantree 1957, Sharrard 1963, Sharrard 1964, Sharrard and 

Liddell 1962, Watkins et al. 1958]. These injuries are likely attributable to low working heights, 

which confine workers to kneeling and squatting postures, both of which have been associated 

with knee injuries [Baker et al. 2002, Baker et al. 2003, Coggon et al. 2000, Cooper et al. 1994, 

Felson et al. 1991, Sharrard and Liddell 1962, Tanaka et al. 1985]. 

Although not mandated by the government, many low-seam coal mine workers choose to 

wear kneepads to protect their knees from the uneven mine floor which may contain sharp, 

jagged rocks. The design of these kneepads varies between articulated and nonarticulated styles; 

however, they typically employ an outer shell to protect from the surface conditions and an inner 

padding to provide cushioning to the knee. The types of materials used to form the outer shell 

and inner padding are quite variable. Even though it is clear that these kneepads protect the knees 

from cuts and scrapes, their effect on the muscle activity of the thigh and the forces, pressures, 

and moments at the knee while in postures associated with low-seam mining, was previously 

unknown. The high number of injuries to the knee in low-seam mining suggests that these 

kneepads are only minimally successful in diminishing the negative effects of these parameters. 

The effect that postures assumed by low-seam mine workers and kneepads have on the knee 

must be investigated to improve the design of interventions such as postural rotation strategies, 

2
 
equipment/work station design, and kneepads. Muscle activity, pressure applied to the knee, and 

the net forces and moments at the knee are three biomechanical parameters that researchers have 

previously used to gain insight into the risk of knee injury [Ayoub et al. 1985a, Ayoub et al. 

1985b, Gallagher and Hamrick 1992, Gibbons 1989, Nagura et al. 2002, Perry et al. 1975, 

Sharrard 1964]. In postures associated with low-seam mining, the demands on the flexor and 

extensor muscles of the thigh are expected to be different than that of a normal standing or seated 

work posture. These flexors and extensors may have to generate force to support activities such 

as lifting, and may also be called upon to stabilize the body in what may be relatively unstable 

postures. As such, some postures may result in an earlier onset of fatigue. The pressure at the 

knee has long been considered to be a key parameter leading to the onset of knee injuries. In the 

early 1960s, Sharrard [1964] investigated the effect of a shoveling task on the force and pressure 

at the knee and noted that the greatest portion of mine workers suffering from pre-patellar 

bursitis were those that assumed static kneeling postures. Previous research regarding knee 

forces and moments when kneeling and squatting has shown an increase in these parameters 

compared to walking. Perry et al. [1975] found significantly increased forces on the joint 

surfaces with flexion of greater than 30°. At 30° position, the required force was 210% body 

weight and at 60° the required force was 410% body weight. Nagura et al. [2002] determined the 

forces during squatting activities and found posterior forces increased by 50% over walking and 

stair climbing. 

This study investigated three biomechanical parameters associated with knee loading that are 

potentially related to knee injury risk. In order to determine the effects that commonly used 

postures in low-seam mining and wearing or not wearing kneepads have on these parameters, 

three objectives were met for three kneepad conditions (no kneepad, articulated kneepad, and 

nonarticulated kneepad, both of which were reported by distributors to be very commonly used): 

1.	 Examine the electromyographic (EMG) responses of knee flexors and extensors 

during a lateral lifting task in kneeling and squatting postures associated with lowseam mining (Note: Static trials were excluded from this analysis because only 

minimal activity was observed in these trials). It was hypothesized that changes in 

posture would result in altered muscular demands required to accomplish the lifting 

task, and would be reflected by changes in the magnitudes and patterns of EMG 

activity of the knee flexors and extensors. 

2.	 For static postures associated with low-seam mining, determine the pressures applied 

to the landmarks of the knee identified during pilot testing as being responsible for 

transmitting the majority of load to the knee (patella, patellar tendon, and tibial 

tubercle). It was hypothesized that the pressure and pressure distribution at the knee 

would be significantly affected by wearing a kneepad and by the simulated posture. 

It was further hypothesized that the type of kneepad worn would significantly affect 

the pressure and pressure distribution at the knee. 

3.	 For postures associated with low-seam mining, investigate the net externally applied 

forces and moments at the knee and resulting joint kinematics. It was hypothesized 

that significant differences will be detected in the loading profiles between kneeling 

and squatting, as well as between the low-flexion (kneeling on one knee and kneeling 

near 90° flexion) and higher-flexion (squatting and kneeling near full flexion) 

postures. 

3
 
Methods
 

Subjects 

Ten subjects (seven male, three female) participated in this study. The average age was 

34±17 (mean±SD) years with an age range of 19 to 60 years. The average weight was 154±22 

lbs and the average height was 66±3 in. Prior to participation in the study, each subject was 

asked a series of questions to determine if they had a history of serious injury to the knee. None 

of the subjects reported knee surgery. 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. These subjects’ data compared well with their eight counterparts and were, therefore, 

not excluded from the study. None of the subjects participating in this study were currently 

working in the low-seam mining industry or in occupations that required kneeling, crawling, or 

squatting. Prior to participating in the study, each subject read and signed an informed consent 

form which was approved by the NIOSH Human Subjects Review Board. 

Experimental Design 

In this study, the impact various postures had on the activity of the thigh muscles, pressure 

applied to the knee, and the net forces and moments at the knee were investigated. NIOSH 

researchers interviewed 48 low-seam mine workers, whereby the mine workers identified the 

postures they used to perform various mine tasks (e.g., building stoppings, hanging curtain). The 

postures examined in this study are those indicated by these mine workers and are reflected in 

Figure 1. Two working heights (38 in and 48 in) were investigated in this study. Low-seam 

mines are those with working heights of 43 in or less; therefore, the working heights in this study 

provided a typical working height classified as a low-seam height and a working height that was 

higher than a low-seam height but still required the mine worker to adopt kneeling and squatting 

postures to perform their tasks. The 10 in total difference of the two selected working heights 

allowed researchers to determine if differences in seam heights in and around the low-seam level 

would significantly affect muscle activity, pressure at the knee, and the net forces and moments 

at the knee. 

Not all postures were performed in both seam heights; only postures that were reasonable for 

each seam height were investigated. For example, a mine worker would not kneel near 90° of 

knee flexion in a 38 in mine as this would be extremely difficult at such a low working height. 

4
 
Figure 1. Schematic showing each posture tested during the experiment. 

In addition to investigating the purely static postures depicted in the top two rows of Figure 

1, several dynamic postures were also evaluated. It is quite common for mine workers to lift an 

object (e.g., stopping blocks, roof bolts) across their body while performing their daily duties; 

therefore, each of the static postures was also performed with an added dynamic component. The 

subjects would reach to their right to retrieve a 25-lb block, bring it in front of their body, and 

then set it down on their left side in one smooth motion. The block weight was selected as a 

representative weight considering the weight of stopping blocks (16–55 lbs) and the fact that 

most roof bolter operators move multiple roof bolts at a time (~7 lbs per roof bolt and varies by 

length). In this study, the 25-lb block that was used was positioned 9 in to the right of the lateralmost aspect of the right knee with the midline of the block in line with the front of the right knee. 

A target location where the subjects would set the block was also marked; this location was 9 in 

to the left of the lateral-most aspect of the left knee and marked such that the midline of the 

block, when set down, would be in line with the front of the left knee. This 9-in location was 

selected because it was comfortably within reach for individuals of many different 

anthropometries and did not interfere with other experimental equipment (e.g., force plates). 

Subjects performed each posture for three kneepad conditions: no kneepads, articulated 

kneepads, and nonarticulated kneepads. The articulated and nonarticulated kneepads were 

selected due to popularity in the 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 nonarticulated 

kneepads were selected (Figure 2). 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 nonarticulated kneepads consisted of a soft 

5
 
outer rubber shell and soft inner foam padding. Again, the straps crossed a few inches above and 

below the crease of the knee. 

Figure 2. Photographs of the articulated (A) and nonarticulated (B) kneepads used in 

this study. 

Preliminary Measurements 

Preliminary measurements were taken prior to subject testing, including subject height, 

weight, and thigh-calf and heel-gluteus contact pressures. The contact forces were calculated 

from the contact pressure data. The locations of these forces were then determined with respect 

to the knee. Both the force magnitude and location of the force were necessary inputs to the 

inverse dynamics model described later in this study. Due to the complexity of these 

measurements, measuring the contact pressures during laboratory testing was not possible. Pilot 

testing showed standard repeatability error within 5% body weight for repeated measures of 

contact forces with and without kneepads. Therefore, these contact force measurements were 

collected prior to the start of any other data collection with the subject. These values were 

obtained using a clinical seating pressure sensor system that employs resistive technology 

(Tekscan ClinSeat®, South Boston, MA). For the thigh-calf contact force, the sensor was placed 

on the subject’s lower right leg (from popliteal to the heel), and the subject was instructed to 

squat. The distance from the uppermost sensing unit to the lateral epicondyle of the right femur 

was then measured with a ruler, and pressure data was then collected for five seconds. The 

subject was then instructed to stand and relax. The sensor was then placed on the subject’s lower 

right leg, crossing both the thigh and heel, and the subject was instructed to kneel near full 

flexion. In each case, the distance from the uppermost sensing unit to the lateral epicondyle of 

the right femur was measured. Data was again collected for five seconds. Using the Tekscan 

software, the area that represented the thigh-calf or heel-gluteus contact area was then isolated, 

and the total force and position of the center of pressure were reported with respect to the 

6
 
uppermost sensing unit. The average force magnitude and location of the center of pressure were 

then calculated. 

EMG Measurement 

All EMG measurements were made using a Noraxon TelemyoTM 2400R World Wide 

Telemetry system (Noraxon USA Inc., Scottsdale, AZ) with 16 channels. The gain was set to 10 

for all channels. Several hardware filters were in place: 1st order high-pass filters set to 10 Hz 

±10% cutoff, 8th order Butterworth/Bessels low-pass, anti-alias filters set to 500 Hz or 1,000 

Hz±2% cutoff. The common mode rejection was > 100 dB and the sampling rate for 

electromyography data was set at 1020 Hz. 

The longer of the two kneepad types, the articulated kneepads, were placed on the right and 

left knees of the subject; the superior-most aspect of the kneepads were marked using an ink pen. 

The location of the placement for each EMG electrode (disposable, self-adhesive Ag/AgCl dual 

snap surface electrodes with 2 cm spacing) was then measured and marked again using the ink 

pen. Prior to placement of the electrodes, any hair at the placement sites was shaved and the sites 

were then cleaned and abraded using an electrode skin prep pad. The muscles selected for EMG 

analysis were the right and left: rectus femoris, vastus medialis, vastus lateralis, semitendinosus, 

and biceps femoris. When possible, the electrodes were placed over the belly of the muscles, 

distal to the motor point regions [Ericson 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 those cases, the 

electrode was placed as closely as possible to its ideal location. For the rectus femoris, the 

distance from the anterior superior iliac spine (ASIS) to the apex of the patella was measured and 

the electrodes were placed at the midpoint of this distance. The electrodes were placed along the 

line running from the ASIS to the patellar apex. For the vastus medialis, the distance from the 

ASIS to the medial knee joint space was measured and the electrodes were placed at a point 

which was 80% of this distance. Electrodes were angled at approximately 20° from the midline 

of the body such that they ran along the length of the muscle belly. For the vastus lateralis, the 

distance from the ASIS to the lateral knee joint space was measured and the electrodes were 

placed at a point which was 75% of this distance. Electrodes were angled at approximately 15° 

from the midline of the body such that they ran along the length of the muscle belly. For the 

semitendinosus, the distance between the ischial tuberosity and the medial joint space was 

measured and the electrodes were placed slightly lateral to the midpoint of this distance along the 

long axis of the thigh. Lastly, for the biceps femoris, the distance between the ischial tuberosity 

and the lateral knee joint space was measured and electrodes were placed at the midpoint of this 

distance along the long axis of the thigh. A reference electrode (disposable, self-adhesive 

Ag/AgCl snap surface electrode) was also placed at a remote site above the greater trochanter. 

Maximum voluntary contractions (MVCs) were then determined for the right and left thigh 

muscles. The subject was instructed to lay in a supine position in a BiodexTM chair (Biodex 

Medical Systems, Inc., Shirley, NY) with hip and knee angles at 90°. The subject was then 

instructed to extend his/her knee with maximal effort for at least 5 seconds while a researcher 

provided verbal encouragement. The subject was allowed to rest and was then instructed to flex 

the knee with maximal effort for at least 5 seconds again while verbally encouraged by a 

researcher. 

7
 
Pressure Measurement 

A custom-built pressure sensor (TactArray T2000; Pressure Profile Systems, Los Angeles, 

CA) was used to measure pressures at the knee. This sensor used capacitive sensor technology 

and was preshaped to conform to the knee when flexed at 90°. The sensor was 0.13-in thick and 

consisted of 196 individual pressure-sensing units that varied in size ranging from 0.17 in2 to 

0.23 in2. Due to the preshaped nature of the sensor, the distance between the sensing units (dead 

space) 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 0.07 in. However, in the 

superior-to-inferior direction, the distance between sensing units varied from 0.12 to 0.19 in 

within the region where the patellar tendon and tibial tubercle (PTT) rested and from 0.12 to 0.32 

in within the region where the patella rested. Additionally, there was Velcro® sewn in to extra 

fabric on either end of the sensor to aide with fixation. To affix the sensor to the leg, the knee 

was held at 90° of flexion and 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 2 in 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 aspect of the sensor was 

adhered to the lower leg. Once the sensor was affixed to the subject’s leg, a sheer stocking was 

placed on the leg covering the sensor. This helped protect the sensor from tears while it was 

dragged along the laboratory floor when the subject crawled into the posture of interest. An 

iterative and meticulous testing process was used to determine final details of the fixation 

method, wherein the sensor did not move while the subject performed the activities of interest. 

The next task was to identify 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; the sensing units corresponding to each landmark 

were identified (Figures 3 and 4). 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 PTT would be 

grouped together because their small size resulted in unacceptable repeatability when palpated 

individually. Palpating the patella and the PTT were both repeatable to within 6.7% of their 

respective total areas. These anatomic landmarks were also palpated at the conclusion of testing 

to ensure that the sensor had not moved. The sampling rate for pressure data was approximately 

5 Hz. 

8
 
Figure 3. Subject wearing pressure sensor in the preshaped 90° flexion position. 

 

Figure 4. Pressure sensor layout with individual pressure 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 while kneeling near full flexion and kneeling on 

one knee for the same subject. 

9
 
Motion Analysis Measurement 

An important part of this study was to quantitatively determine the position of the body and 

synchronize this data with a variety of other measurements (e.g., muscle activity, pressure at the 

knee, external forces). Therefore, a motion capture system (Eagle Digital RealTime System; 

Motion Analysis Corporation, Santa Rosa, CA) was used to quantitatively determine the location 

of all the body segments as the subject performed the various postures. 

It was first necessary to establish a marker set that was ideal for the needs of this particular 

experiment, making it possible to determine joint center locations throughout testing. One unique 

problem with this study was that, with the application of the pressure sensor and kneepads, it was 

not possible to track the location of the knee joint center during the experiment. Instead, an initial 

anatomical marker set was developed that included markers on the medial and lateral aspects of 

the knee; a second marker set, where several markers were removed from the subject, was used 

during testing. The anatomical marker set included 41 markers; 8 markers were removed for the 

testing marker set (n=33). The markers comprising the anatomical marker set and testing marker 

set are shown in Figure 5. The testing marker set is a modified version of the Cleveland Clinic 

marker set [Kirtley 2006]. 

10
 
 

Figure 5. A) Anatomical marker set and B) testing marker set of the upper and lower 

right side of the body. (R=Right, L=Left) 

Instrumented with the anatomical marker set, the subject was instructed to stand with their 

arms out to either side of their body making a “T”. This is known as a “T-pose” and is used to 

determine the position of the anatomic landmarks (medial and lateral femoral epicondyles, 

medial malleolus, and right and left anterior superior iliac spine) with respect to the markers that 

were used during testing. After the T-pose data were collected, the eight markers were removed 

yielding the testing marker set. The sampling rate for motion capture data was 60 Hz. 

11
 
Additional Testing Equipment and Syncing 

In addition to the pressure sensor system, motion capture system, and EMG system, two 

force plates were also used during testing (OR6-5, Advanced Mechanical Technology, Inc., 

Watertown, MA). These force plates captured the ground-reaction forces applied to the foot and 

knee which would later be used as inputs to a computational model that solved for the net forces 

and moments at the knee (described later). Only the right knee was investigated in this study. 

The force plate and EMG data were collected (1020 Hz) using the EvaRT 5.0.4 software 

(Motion Analysis Corporation, Santa Rosa, CA) and were read through an analog-to-digital 

board (PCI-6071E, National Instruments, Austin, TX). The pressure sensor system produced a 

sync signal that was output once data collection was initiated. This signal was also read through 

the analog-to-digital board; therefore, pressure, force plate, EMG, and motion capture data were 

all synched. 

Testing Procedure 

A randomized order was used for testing the three kneepad conditions (no kneepads, 

articulated kneepads, and nonarticulated kneepads). Within each kneepad condition, the order in 

which postures were tested was also randomized. 

If the testing condition included a kneepad, the subject first donned the required kneepad. 

Then, for all tests, the subject was instructed to place their knee in a reference position in order to 

set the pressure sensor at zero. In this position the subject was seated with his/her knee at 90° for 

postures with knee angles near 90° (i.e., kneeling near 90° knee flexion and kneeling on one 

knee; see Figure 1). This reference position was a squat for postures with knees in high flexion 

(e.g., kneeling near full flexion). The subject was then shown a schematic (similar to Figure 1) 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 the subject inquired as to the 

whether or not they were in the correct posture. Minor verbal feedback was then provided to the 

subject, and the subject was allowed to assume his/her interpretation of the posture. However, in 

some cases the subject was assuming a posture that was grossly different from the requested 

posture. For example, in some cases the subject was instructed to assume a posture near 90° of 

knee flexion but actually positioned themselves in a posture near full flexion. In these cases, the 

researchers instructed the subject to adjust their posture. 

Once the subject was in the posture of interest, the motion capture, force plate, and EMG 

data collection systems were initiated. The pressure system was then started and a researcher 

informed the subject and other researchers that data collection had begun. The researcher 

operating the pressure system collected data for 10 seconds, then stopped data collection and 

announced cessation to the subject and other researchers. Each researcher then reviewed their 

data for acceptability; the subject was instructed to be seated and place their knee near 90° 

making sure not to apply a load to the sensor. The sensor was then allowed to recover until the 

maximum and average pressure across the sensor was one pound per square inch or less. If an 

error occurred during data collection, the sensor was allowed to recover and the trial was 

repeated. 

12
 
Data Analysis 

EMG Data 

Electromyography data was low-pass filtered to 500 Hz and high-pass filtered to 20 Hz using 

a 4th order Butterworth filter (MATLAB®; The MathWorks, Inc., Natick, MA). Recall, hardware 

filters were in place to high-pass filter to 10 Hz and low-pass filter to 500 Hz. The signal was 

then rectified by taking the absolute value. Data from all tests were normalized by dividing by 

the maximum voluntary contraction for each muscle. Mean amplitude values (MAVs) of the 

normalized values were then calculated by determining the running mean of every 102 samples 

which was 10% of the sampling rate [NIOSH 1992]. Trials containing evidence of artifacts were 

eliminated from the analysis. The lifting portion of the dynamic trials lasted approximately 2 

seconds; however, time was normalized as a percentage of the lift for EMG analysis. 

Data were analyzed using the Statistix 8.0 for Windows (Analytical Software, Tallahassee, 

FL). Residual analysis of the nontransformed EMG data indicated a fan-shaped pattern of 

residuals, thus the EMG data were transformed by taking the natural log, which resulted in a near 

normal distribution of residuals for all muscles. All analysis of variance (ANOVA) results 

reported below use the log-transformed data. Alpha levels were set at 0.05. 

Pressure Data 

Because the pressures applied to the knee sensor during the dynamic trials (i.e., trials where 

the subject twisted at the waist) exceeded the measurable range rendering these measurements 

inaccurate, only the static trials were evaluated for the pressure data. Additionally, squatting 

postures were not examined because the knee was not in contact with the ground during 

squatting. Therefore, pressure data from the static kneeling-near-90°-knee-flexion, kneeling-onone-knee, and kneeling-near-full-flexion postures were evaluated. A cursory view of the data 

clearly indicated that the majority of pressure was transmitted to the knee via the patella and 

PTT, and. the total pressure being transmitted via these two structures was determined. It was 

then necessary to determine the ratio of the pressure that was being transmitted by the patella to 

that which was being transmitted by the PTT (please see Appendix B for detailed equations). For 

the static trials, all 10 seconds of data were evaluated. 

Next, the magnitude of the pressures across the patella and PTT was determined as well as 

the mean pressure of both structures for every sample. This value was then summed and divided 

by the total number of samples. This yielded the mean value of the average pressure on the 

patella and PTT across all samples, or the mean of the mean pressure. The mean maximum 

pressure was also determined by summing the maximum pressures at each sample and dividing 

by the total number of samples. 

Finally, some measurement of the distribution of pressure across the structures was 

necessary; therefore, a measure of variance was taken. The variance was calculated for the 

patella and PTT for each sample. These variances were then summed and divided by the total 

number of samples yielding the mean variance across each structure. 

Statistical analyses were performed using Statistix 8.0 for Windows. Analyses performed 

included a split-plot ANOVA and a priori orthogonal contrasts. Contrasts for kneepad conditions 

included: 1) comparing the no-kneepad and kneepad-present conditions, and 2) comparing 

nonarticulated and articulated kneepads. Contrasts for posture states included: 1) comparison of 

13
 
kneeling with both knees in full flexion to kneeling on the right knee only (across both heights), 

and 2) comparing the 38 in work height to the 48 in work height. All contrasts were tested using 

a t statistic with an alpha level of 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. 

Net Forces and Moments and Resulting Joint Kinematics 

To determine the right knee angles and net externally applied forces and moments at the right 

knee, a computational model was developed in the MATLAB® software (The Mathworks Inc., 

Natick, MA) on a personal computer. This model is based on inverse dynamics [Winter 1990] 

which uses measured ground reaction forces and anthropometric measurements to estimate 

reaction forces and moments. In this linear model, the upper leg (femur) and the lower leg (tibia, 

fibula, and foot) are modeled as rigid bodies attached via a pin joint with three rotational degrees 

of freedom. Using the anatomical marker set (described previously), an anatomical shank 

coordinate system (ASCS) was constructed. This coordinate system was oriented such that when 

the subject was standing in standard anatomical position, the x-axis was in the medial-to-lateral 

direction, the y-axis was in the posterior-to-anterior direction, and the z-axis was in the inferiorto-superior direction. The origin of this system was at the right knee joint center. Similarly, an 

anatomical coordinate system was created for the thigh, with the origin at the right hip joint 

center. These coordinate systems were established to move with the subjects as they assumed the 

various postures, thereby relating all postures to clinical measurements. A positive moment about 

the x axis represents extension rotation, the y axis represents varus rotation, and the z axis 

represents internal rotation. 

The developed computational model is based on the following assumptions. 

•	 The knee is assumed to be a frictionless pin joint. This allows all forces to pass 

directly through the joint center. 

•	 Segments are assumed to be rigid with mass concentrated at the center of mass 

locations. This allows one center of mass to represent the weight of the segment. 

•	 The linear relationship between external forces and moments is applied to the knee. 

This allows a 3-D model to be used to determine the external forces and moments 

applied to the knee. 

•	 The relative movement of pelvic bones is negligible. This allows approximation of the 

hip joint center from palpable pelvic landmarks. 

•	 Thickness of subcutaneous tissue between bone and skin is minimal. This allows the 

assumption that markers placed on palpable landmarks are directly located on the 

landmark. 

•	 The measured thigh-calf and heel-gluteus contact forces are concentrated at the 

measured center of pressure location. This allows the contact forces to be 

represented as a single resultant force, opposed to a pressure distribution. 

•	 The effect of patellar tendon and tibial tubercle on forces externally applied to the 

tibia is negligible. This assumption allows ground contact forces measured at the 

14
 
ground-knee or ground-kneepad interface to be assumed to act at some distance away 

from the knee joint center and not be affected by the patellar tendon or tibial tubercle. 

NOTE: Please see Appendix A for detailed information regarding the development of 

coordinate systems used in the above described model. 

Joint angles were determined for each posture using Euler angle decomposition. The largest 

joint rotations occurred about the x-axis (extension/flexion), followed by the y-axis 

(varus/valgus), and the z-axis (internal/external rotation), yielding an Euler order of Xy’z’’. The 

rotation matrix from the anatomical thigh to the anatomical shank coordinates was created to 

determine the Euler angles. Therefore, motion of the thigh was with respect to the shank. The 

included angle was defined as the angle between the thigh and shank along the flexion/extension 

axis. Varus rotation was positive along the y axis, and internal rotation was positive along the z 

axis. 

Ground reaction forces, segment weight, thigh-calf contact force (Ft/c), and heel-gluteus 

contact force (Fh/g) were inputs into the computational model. External force diagrams for 

kneeling near full flexion with respect to the global coordinate system (GCS), coordinate system 

of the laboratory constructed by the motion capture system, and the ASCS are shown in Figure 6 

and 7. The center of mass location and weight of the shank+foot were determined using 

equations from Hinrichs [1990] which were adjusted to use the knee joint center and ankle joint 

center in this model. The reaction forces and moments were assumed to act in the positive 

directions. 

Squatting and kneeling create a contact force between the thigh and the calf (Ft/c). Kneeling 

near full flexion also creates this contact, and in some subjects there is additional contact 

between the heel and the gluteal muscles (Fh/g). Both Ft/c and Fh/g were modeled as resultant 

forces whose lines of action were in the anterior direction of the shank with centers of pressure 

some distance (i.e., measured by the pressure sensor) along the long axis of the shank, thereby 

contributing only to extension moments and anterior forces. Forces at the foot (F1), forces at the 

knee (F2), and the weight of the lower leg (WLL) were measured with respect to the GCS and 

transformed into the ASCS (Figure 6). Thigh-calf and heel-gluteus contact forces were measured 

with respect to the ASCS. 

Figure 7 shows the orientation of the forces and moments as presented in this research, with 

respect to the ASCS. External force diagrams for all postures are shown in Figure 8. 

15
 
 

 

Figure 6. Diagram of external shank forces and reaction forces and moments for 

kneeling near full flexion with respect to the global coordinate system (GCS). 

 

 

Figure 7. Diagram of external shank forces and reaction forces and moments for 

kneeling near full flexion with respect to the anatomical shank coordinate system 

(ASCS). 

16
 
Figure 8. External force diagrams with respect to the anatomical shank coordinate 

system (ASCS). 

Joint equilibrium was assumed; therefore external forces in the x, y, and z directions were all 

summed to zero to determine the reaction force, R, necessary to stabilize the knee due to the 

application of these external forces. The sum of the external moments at the knee joint in the x, 

y, and, z directions were also summed to equal zero, and subsequently the reaction knee moment 

was then determined. The data presented in this study represents the net externally applied forces 

and moments. The force necessary to stabilize the knee from the application of these forces and 

moments will be equal in magnitude and opposite in direction to the forces and moments 

presented in this study. 

Statistical analyses were performed using Statistix 8.0 for Windows. A two-way 

(3 kneepads × 4 postures) split-plot ANOVA was performed to determine if significant 

differences existed in knee angles, forces, and moments between postures and kneepad 

conditions. A priori orthogonal contrasts included comparisons of the no-kneepad to kneepadpresent (articulated and nonarticulated) conditions, high-flexion (squatting and kneeling near full 

flexion) and low-flexion (kneeling near 90° knee flexion) postures, and squatting versus kneeling 

(kneeling near 90° knee flexion, kneeling on one knee, and kneeling near full flexion) postures. 

All orthogonal contrasts were tested using a t statistic with an alpha value of 0.05. A priori 

nonorthogonal contrasts were tested to determine if significant differences existed between the 

kneepad conditions using Scheffe F-test (p < 0.05). 

17
 
Results
 

EMG Data 

Recall that static trials were excluded from this analysis since only minimal muscle activity 

was observed. A summary of ANOVA results for each muscle is provided in Table 1. 

Table 1. Summary of significant main effects and interactions for normalized activity of all 

muscles. 

Kneepad 

(A) 

Posture 

(B) 

A*B Block 

position (C) 

A*C B*C A*B*C 

Left vastus lateralis *** *** 

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

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

Left biceps femoris *** * ** 

Left semitendinosus ** *** *** 

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

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

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

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

Right semitendinosus *** *** *** 

* indicates significant difference with *p<0.05, **p<0.01, ***p<0.001 

As can be 

seen in this table, 9 out of 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 position, but not 

their interaction. The kneepad main effect was not found to influence EMG activity for any of 

the muscles studied; however, a significant interaction between kneepad condition and block 

position was found to influence activity of the left biceps femoris. 

The location of the block was measured throughout the lifting trials. The angular position of 

the block with respect to its starting position was determined. The subject lifted the block which 

was located directly to his/her right (0°) and transferred it to a location directly to his/her left 

(180°). Figure 9 provides summaries of EMG data at these two positions (as the block was 

picked up and then set down) and at three intermediate points during the lift (45°, 90°, and 135°), 

providing an overall representation of the interaction of posture and block location. 

18
 
Inspection of Figure 9 provides several insights into the activation of the thigh muscles in 

different postures and at different phases during the lift. 

 

Figure 9. Summary of EMG activity by posture and block position. Each posture and 

block position has 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 the high activation of 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° (front left) or 

180° (left) positions. In most cases, the lowest overall EMG activity was observed when the 

block was at the 45° (front right) or 90° (front) position. 

19
 
Several differences in muscle activity can be noted between postures in Figure 9. Squatting 

generally exhibited the highest muscle activity levels throughout the entire lift, and this was 

particularly true in the knee extensors. At the start of the lift (0° condition), the right knee 

extensors exhibited more than twice the activity when squatting compared to any other posture 

(30% MVC compared to 15% MVC in kneeling near 90° knee flexion, the next highest posture). 

Kneeling near full flexion (in either 38-in or 48-in seam heights) typically exhibited the lowest 

muscle activity of any posture. Kneeling near 90° knee flexion exhibited very similar activation 

patterns to the kneeling-near-full-flexion condition, but tended to have slightly higher EMG 

activity. Kneeling on one knee (either seam height), 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 to the other kneeling postures. 

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 and 

block position, shown in Figure 10. 

Figure 10. Interaction of kneepad condition and block position on EMG activity of the 

left biceps femoris across postures. 

Compared to the two kneepad-present conditions, the nokneepad condition resulted in lower EMG activity of the left biceps femoris in all block positions 

with the exception of the initial position (0°). 

20
 
Pressure Data 

For all postures tested, the majority (>60%) of the pressure was placed on the PTT region 

(Figure 11). 

Figure 11. Mean pressure ratio at the PTT region across postures. (* indicates 

significant difference with p < 0.05). 

No significant difference existed between the no-kneepad condition and the two 

kneepad-present conditions; however, a significant difference existed within the two kneepadpresent conditions with the articulated kneepad exhibiting a greater mean pressure ratio for the 

PTT region (p < 0.0001; 86% versus 74%). When evaluating the effect of the postures, the 

kneeling-near-full-flexion postures showed a significantly greater mean pressure ratio for the 

PTT region (p < 0.0001; 88% versus 74%) when compared to the kneeling-on-one-knee 

postures. No significant difference between the 38 in and 48 in working heights was observed for 

any dependent measures. 

21
 
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 (Figure 12). 

Figure 12. Mean of the mean pressure at the patella region for the various postures. 

(*indicates significant difference with p < 0.05). 

As with the pressure 

ratio, it was found that no significant difference existed between the no-kneepad condition and 

the two kneepad-present conditions, but a significant difference existed between the two 

kneepad-present conditions with the nonarticulated kneepad exhibiting a greater mean of the 

mean pressure for the patella region (p < 0.0001; 4.3 versus 2.4 psi). When evaluating the effect 

of the postures, it was determined that the kneeling-on-one-knee postures showed significantly 

greater mean of the mean pressure for the patella region (p < 0.0001; 5.1 versus 0.9 psi) when 

compared to the kneeling-near-full-flexion postures. 

22
 
 

The mean of the mean pressure at the PTT region showed a considerably higher level of 

applied pressure (>15 psi) than that which was observed for the patella region (Figure 13). 

23
 

Figure 13. Mean of the mean pressure at the PTT region for the various postures. 

(*indicates significant difference with p < 0.05). 

As 

with the mean of the mean pressure for the patella region, a significant difference (p < 0.05) was 

observed for posture and subject. In contrast to the patella region, the PTT region did not show 

significant differences due to the kneepad-present condition. As with the patella region, the 

kneeling-on-one-knee postures showed significantly greater mean of the mean pressure 

(p < 0.0001; 17.9 versus 8.2 psi) when compared to the kneeling-near-full-flexion postures. 
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 pressures were applied to the patella for the different 

postures (Figure 14). 

Figure 14. Mean of the maximum pressure at the patella region for the various 

postures. (*indicates significant difference with p < 0.05). 

Again, no significant difference existed between the no-kneepad condition 

and the two kneepad-present conditions, but a significant difference did exist between the two 

kneepad-present conditions with the nonarticulated kneepad exhibiting a greater mean of the 

maximum pressure for the patella region (p = 0.0006; 15.2 versus 9 psi). Additionally, the 

kneeling-on-one-knee postures showed significantly greater mean of the maximum pressures for 

the patella region (p < 0.0001; 18.4 versus 3.1 psi) when compared to the kneeling-near-fullflexion postures. 

24
 
The mean of the maximum pressure at the PTT region showed a considerably higher level of 

pressure (>25 psi) than that observed for the patella region for all postures regardless of the 

kneepad condition (Figure 15).

Figure 15. Mean of the maximum pressure at the PTT region for the various postures. 

(*indicates significant difference with p < 0.05). 

 No significant difference existed between the two kneepadpresent conditions, but a significant difference existed between the no-kneepad and the two 

kneepad-present conditions with the no-kneepad condition exhibiting a greater mean of the 

maximum pressure for the PTT region (p < 0.0001; 41.9 versus 28.5 psi). Looking at significant 

differences within posture, the only significant difference was found between the kneeling-nearfull-flexion and kneeling-on-one-knee postures, with the kneeling-on-one-knee postures 

exhibiting a greater mean of the maximum pressure (p < 0.0001; 37.1 versus 28.7 psi). 

25
 
The mean of the variance at the patella region varied considerably across postures (Figure 

16). 

Figure 16. Mean of the variance at the patella region for the various postures 

(*indicates significant difference with p < 0.05). 

The kneeling-on-one-knee postures showed significantly greater means of the pressure 

variances for the patella region (p < 0.0001; 39.3 versus 1.1 psi) when compared to the kneelingnear-full-flexion postures. 

26
 
 

The mean of the variance at the PTT region showed relatively consistent levels of variance 

across postures (Figure 17). 

Figure 17. Mean of the variance at the PTT region for the various postures (*indicates 

significant difference with p < 0.05). 

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-present conditions 

(p < 0.0001; 173.3 versus 67.6 psi). No significant difference was found between the two 

kneepad-present conditions. The kneeling-on-one-knee postures showed significantly greater 

means of the variances for the PTT region (p < 0.0001; 124.2 versus 85.8 psi) when compared to 

the kneeling-near-full-flexion postures. 

27
 
Net Forces and Moments and Resulting Joint Kinematics Data 

All subjects had thigh-calf contact forces greater than 20% of body weight (BW) for 

squatting with a mean of 39 ±14 % BW. Mean thigh-calf contact when kneeling near-full flexion 

was 28 ± 13% BW. In the kneeling-near-full-flexion posture, seven subjects had contact between 

the heel and gluteal muscles, with a mean of 11 ± 6% BW. Typical pressure distribution profiles 

for squatting and kneeling near full flexion are shown in Figure 18. 

 

 

Figure 18. Typical pressure distribution (psi) for squatting (A) and kneeling near full 

flexion (B). 

Significant differences were seen between postures for the included (p < 0.001) and internal 

rotation angles (p < 0.001) (Figure 19). When compared to the kneeling postures, squatting 

showed higher flexion and internal rotation (p < 0.001). A priori orthogonal contrasts showed 

significant differences in knee flexion between the high-flexion (squatting, kneeling near full 

flexion) and low-flexion (kneeling on one knee, kneeling near 90° of knee flexion) postures 

(p < 0.001), and between kneeling near 90° of knee flexion and kneeling on one knee (p < 

0.001). Differences in internal rotation angles were observed between high-flexion and lowflexion postures (p < 0.001), and between squatting and kneeling near full flexion (p < 0.001) 

postures. The no-kneepad condition showed higher knee flexion (included angles = 35 ± 24°) 

than the nonarticulated kneepad-present condition (included angles = 41 ± 26°) (p = 0.033). No 

significant differences were observed between the articulated (40 ± 26°) and nonarticulated 

kneepads. 

28
 
Figure 19. Knee angles (degrees) for all postures. Varus and internal rotation angles 

are positive. 

Posture had a significant effect on tibial forces (Figure 20). When compared to the 

kneeling postures, squatting showed significantly higher superior, lower posterior, and lower 

resultant forces (p < 0.001). Low-flexion postures showed greater posterior, inferior, and 

resultant forces than the high-flexion postures (p < 0.001). Squatting showed significantly higher 

medial (p = 0.002), superior (p <0 .001), and resultant forces (p < 0.001), but lower posterior 

forces than kneeling near full flexion (p < 0.001). Kneeling on one knee showed significantly 

higher medial, posterior, inferior, and resultant force magnitudes than kneeling near 90° knee 

flexion (p <0.001). The use of kneepads affected the medial forces (p = 0.036), and the nokneepad condition (-6.1 ± 8.1 % BW) had higher medial forces than the articulated kneepadpresent condition (-4.3 ± 7.6 % BW). 

29
 
Figure 20. Mean tibial forces normalized to body weight (% BW). Lateral, superior, and 

anterior forces are positive. 

30 
Posture also had a significant affect on tibial moments (Figure 21). Flexion moments had 

the greatest magnitudes for all postures. Of the postures examined, kneeling on one knee had the 

greatest flexion moments, squatting had the greatest varus moments, and kneeling near 90° knee 

flexion had the greatest internal rotation moments. Kneeling on one knee and squatting had the 

highest resultant moments with comparable magnitudes of 5 ± 4% BW*Ht for kneeling on one 

knee and 5 ± 2% BW*Ht for squatting. When compared to the kneeling postures, squatting 

showed significantly lower internal rotation (p = 0.027), increased varus (p < 0.001), and 

increased resultant moments (p = 0.027). High-flexion postures showed significantly higher 

varus (p < 0.001), internal rotation (p = 0.009), and resultant moments compared to the lowflexion postures (p = 0.007). Squatting showed significantly higher varus moments than kneeling 

near full flexion (p < 0.001). Kneeling on one knee showed significantly higher flexion 

(p < 0.001), varus (p = 0.012), and resultant moments, but significantly lower (p < 0.001) 

internal rotation moments than kneeling near 90° knee flexion (p = 0.01). Kneepad use affected 

the internal rotation moments (p < 0.001), with the no-kneepad condition (0.23 ± 0.32 % 

BW*Ht) having significantly smaller internal rotation moments than the nonarticulated (0.36 ± 

0.36 % BW*Ht) (p = 0.049) and articulated kneepad-present conditions (0.36 ± 0.35 % BW*Ht) 

(p = 0.008). 

31
 
Figure 21. Mean tibial moments normalized to body weight times height (% BW*Ht).
 

Extension, varus, and internal rotation moments are positive.
 

32
 
Discussion of Results 

The results of this study include several key findings which may be useful in providing 

recommendations to reduce the prevalence and severity of knee injuries in low-seam coal mine 

workers. The three biomechanical parameters investigated demonstrated that, relative to 

oneanother, the postures in this study had potential beneficial and detrimental effects on the 

worker. The key points of interest for each parameter are as follows: 

EMG 

The flexors and extensors of the knee were affected by the interaction between the 

posture adopted and the location of the block during the lift. The lowest EMG activity was 

typically observed when the block was raised from the ground and in front of the subject. This 

suggests that supplies should be stored on elevated platforms in the mine such that minimal leg 

muscle activity is required to load and unload supplies. The highest muscle activity was noted for 

squatting followed by kneeling on one knee. Thus, both of these postures should be expected to 

lead to an earlier onset of fatigue for the flexors and extensors of the knee than kneeling near full 

flexion and kneeling near 90° of knee flexion. 

Pressure 

The majority of the pressure was found to be transmitted to the knee via the PTT (over 60%). 

Kneepads were not found to have a significant effect on the pressure ratio between the patella 

and the PTT, the mean of the mean pressure, or the mean of the maximum pressure. Therefore, 

contrary to the original hypothesis of this study, kneepads, in their current form, may result in 

increased comfort but have no measurable ability to reduce the risk of pressure-driven knee 

injuries. However, the hypothesis that postures would have a significant effect on pressures at the 

knee was supported by this study. Compared to kneeling on one knee, kneeling near full flexion 

resulted in a significant shift in the distribution of the pressure between the patella and the PTT 

regions with more pressure being applied to the PTT region. This shift was also found to 

correlate with an increase in the mean pressure applied to the PTT region. The mean pressure at 

the PTT region was significantly greater than that for the patellar region. Due to the minimal 

muscle activity required to maintain a kneeling-near-full-flexion posture, the assumption is that 

mine workers would frequently and naturally adopt this posture to reduce the onset of fatigue. 

However, pressure data clearly indicate that kneeling near full flexion places significant pressure 

on the PTT region which may lead to injuries such as prepatellar bursitis. It is imperative that 

manufacturers redesign kneepads such that they effectively reduce the pressure at the knee and, 

specifically, at the PTT region. 

33
 
Net Forces and Moments 

As hypothesized, significant differences were found between squatting and kneeling postures 

as well as between the high-flexion (squatting and kneeling near full flexion) and low-flexion 

(kneeling near 90° knee flexion and kneeling on one knee) postures. Both high-flexion kneeling 

and squatting create a significant change in internal joint structure orientations, such as femoral 

rollback and varying segment contact forces. During increased knee flexion, the tibia internally 

rotates. However, tibial loading during squatting is significantly different during kneeling, with 

mean forces acting in the medial, posterior, and superior directions of the tibia when squatting. In 

all the kneeling postures examined, the forces acting on the tibia occurred in the medial, 

posterior, and inferior directions. When kneeling on one knee, the highest resultant force 

magnitudes were noted and shear loading across the knee joint (i.e., medial forces) occurred. In 

effect, high posterior forces, as much as 60% BW are transmitted to the tibia in addition to 

flexion moments up to 5% BW*Ht. These data suggest that statically kneeling on one knee 

results in osteoarthritis and mensical tears. Furthermore, the high flexion and loading patterns 

present when kneeling near full flexion and squatting may also place a worker at a higher risk for 

developing osteoarthritis of the medial compartment and meniscal tears. 

Implications of Results 

Preliminary investigations demonstrated that the muscle activity of the flexors and extensors 

of the knee was minimal when statically kneeling, which shows that the least amount of activity 

was observed for kneeling near full flexion. Additionally, the mean pressure at the knee was less 

when kneeling near full flexion than in other postures. This suggests that workers would likely 

experience less fatigue and more comfort when kneeling near full flexion. This posture is likely a 

predominant posture used by workers where the vertical working height is restricted. However, 

this posture resulted in increased internal tibial rotation, which could lead to osteoarthritis or 

meniscal tears. As such, prolonged use of this posture is not advised. 

Lateral lifting tasks are very common in the low-seam mining industry (e.g., building 

stoppings, shoveling). For these tasks, the muscle activity of the flexors and extensors of the 

knee are expected to be at their greatest when squatting or kneeling on one knee. This suggests 

that these two postures may result in an earlier onset of fatigue. However, the resulting pressure 

at the knee was not evaluated in this study as the magnitude of these pressures exceeded the 

sensor capacity. NIOSH has recently published a study whereby these pressures were estimated 

for the postures investigated in the current study while performing a lateral lift [Mayton, et al. 

2010]. These data indicated that kneeling near full flexion while performing a lateral lift resulted 

in pressures that were six times greater than statically kneeling near full flexion. Furthermore, the 

estimated pressure on the knee was greater for kneeling near full flexion than for kneeling near 

90° of knee flexion and kneeling on one knee. Therefore, for lateral lifting tasks, kneeling near 

full flexion is not advised. Rather, workers should attempt to adopt a kneeling-near-90°-offlexion posture when physically possible. Alternatively, kneeling on one knee may be adopted; 

however, this will likely result in an earlier onset of fatigue. Furthermore, statically kneeling on 

one knee resulted in high shear forces which are likely to still exist while performing a lateral 

lift. 

34
 
Kneepads were not found to significantly reduce mean pressures applied to the knee for any 

posture where the knee was in contact with the ground. Although squatting does remove pressure 

from being applied to the knee, it is recommended that this posture should generally be avoided. 

Statically squatting resulted in high moments at the knee which, similar to kneeling near full 

flexion, could result in osteoarthritis or meniscal tears. Futhermore, squatting while performing a 

lateral lifting task resulted in large amounts of muscle activity which will likely lead to an earlier 

onset of fatigue. While statically kneeling on one knee, high shear loading was observed, making 

it likely that these shear forces are present when performing a lateral lift as well. Therefore, 

kneeling near 90° of knee flexion should be the primary choice for performing lateral lifting 

tasks, with kneeling on one knee as the secondary choice. When kneeling on one knee is 

necessary, mine workers should alternate the knee in contact with the ground. This posture, 

however, is not recommended for static tasks. 

Based on the data from the current study, several recommendations are made: 

1.	 Kneeling near full flexion is likely to be frequently adopted in environments where 

the vertical height is restricted; however, workers should avoid using this posture for 

prolonged periods of time. Rather, they should attempt to alternate between kneeling 

near full flexion and a posture that does not place the knee at a high risk for 

osteaoarthritis and meniscal tears such as kneeling near 90° of flexion. 

2.	 A kneel-assist device should be developed, by manufacturers, to reduce the possible 

negative consequences of kneeling near full flexion, which includes high flexion 

moments and increased levels of internal tibial rotation. Additionally, workers should 

focus on keeping their ankles straight when kneeling near full flexion, to reduce tibial 

rotations and thereby reduce loading to the meniscus. 

3.	 Squatting should be used sparingly and is not recommended. 

4.	 When performing static tasks, avoid kneeling on one knee. Instead, kneeling near full 

flexion or kneeling near 90° of knee flexion should be adopted. 

5.	 Kneepads must be redesigned such that they signifiantly reduce the pressure at the 

patella and, especially, at the PTT region. Manufacturers should obtain feedback from 

mine workers regarding commonly reported problems such as strap discomfort and 

coal particles trapped in the kneepad. Furthermore, manufacturers may want to 

consider a comprehensive approach whereby kneepads and other kneel-assist devices 

(e.g., foam wedge behind worn between the calf and thigh) are designed as one 

system. 

Several limitations to this study should be acknowledged. One limitation is the subjects were 

not experienced mine workers. Also, only two seam heights and three kneepad conditions were 

tested in a limited number of postures for this study. Furthermore, pressure data could not be 

determined for the dynamic postures (i.e., twisting at the waist) due to data collection limitations. 

Despite these limitations, information received through in-mine observations and feedback from 

mine workers indicated that the conditions tested would represent the majority of tasks 

performed by low-seam mine workers. A further limitation was the size of the gaps between 

sensing units of the sensor resulting from the need for the sensor to be curved in 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. Because the inferior border of the patella was the only 

35
 
portion experiencing loading, the impact of these larger gaps is likely minimal. However, hot 

spots (i.e. areas of high pressure) may have been missed in the PTT region. Therefore, the results 

of this study likely underestimated the peak and mean pressures in this region. Knee kinematics 

were determined in 3-D; however, due to the limitations of the pressure sensor, thigh-calf and 

heel-gluteus contact forces were obtained in 

2-D. These contact forces were measured normal to the tibia and thus the medial-lateral and 

superior-inferior shear forces caused by tissue deformation were neglected, thereby affecting the 

accuracy of the estimated shear forces, internal rotation, and varus moments. Due to the 

application of kneepads and the custom knee-pressure sensor, it was not possible to track the 

positions of the knee joint center during testing. Instead, an anatomical coordinate system was 

constructed to estimate the positions of these markers from the testing marker set worn by 

subjects during testing. This introduced mean errors of less than 4% for the kneeling postures 

and less than 9% for the squatting postures. However, due to the complexity of high-flexion 

kneeling, using markers to approximate the knee joint center for kneeling near full flexion and 

squatting may not have been feasible even without kneepads as femoral rollback causes an 

internal rotation of the tibia. When this occurs, the knee joint center may only be approximated 

by imaging techniques which were not employed in this study. In effect, some inaccuracies may 

be present with the tibial moments, as femoral rollback and knee joint center errors affect the 

moment arm calculations. However, the 3-D model used in this study may minimize these errors 

by accounting for tibial rotations. Finally, back muscles were not investigated in this study; 

however, the authors do not consider this a strong limitation as previous research has 

demonstrated that the knee is injured 1.7 times more often than the back in low-seam coal mines. 

In addition, extensive literature already exists regarding the lifting capacity of workers when in 

kneeling and squatting postures in regard to back stressors [Ayoub, et al. 1985a, Ayoub, et al. 

1985b, Gallagher and Hamrick 1992, Gallagher, et al. 2009, Gibbons 1989]. 

36
 
Conclusions 

Researchers should continue to investigate the tasks performed by low-seam mine workers 

and the postures associated with these tasks. The data obtained in future studies, coupled with the 

data reported in the current study, may then be used to develop an appropriate and practical 

postural rotation strategy for the low-seam mining occupation. Squatting and kneeling on one 

knee may be included in a postural rotation strategy upon occasion, but long bouts of exposure to 

these postures should be avoided. Some mining tasks may dictate the posture a mine worker is 

capable of adopting, making it difficult, or impossible, to use a postural rotation strategy. In 

these cases, mine workers should take occasional breaks during a shift to move their knees 

through the entire range of motion (i.e., flush the knee) [Moore, et al. 2008]. 

Manufacturers should consider a novel approach to the design of personal protective 

equipment for individuals who kneel for extended periods of time, including low-seam mine 

workers. A kneepad design that transfers pressure at the knee to other parts of the body that have 

larger surface areas (e.g., the shin) should be considered. Furthermore, manufacturers should 

consider the benefits of providing kneel-assist devices in addition to kneepads (e.g., foam wedge 

worn between calf and thigh). Kneeling near full flexion resulted in the least amount of muscle 

activity. Interviews with mine workers indicated that this posture is likely the most predominant 

posture used which may be a direct result of lower energy expenditures in this posture due to 

reduced muscular demands. Therefore, manufacturers should consider the development of 

devices that address knee loading during this specific posture. Such devices may include a 

cushion in the shape of a wedge worn at the ankle such that the applied flexion moment at the 

knee is reduced when kneeling near full flexion. Additionally, these devices may include a 

bracing feature to reduce the internal rotation of the tibia during kneeling, which may alleviate 

tension in the ligaments and reduce the loading on the medial compartment of the tibia. 

37
 
Acknowledgments 

The authors would like to acknowledge the contributions of Mary Ellen Nelson and Albert 

Cook for their assistance in data collection. 

38
 
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41
 
Appendix A 

For each segment, an anatomical and measured coordinate system was created from the 

motion capture data. The anatomical system was created from the anatomical standing T-pose 

and allowed the location of anatomical landmarks to be linked to the global reference frame. It 

was also used to determine the location of the ankle joint center (AJC), knee joint center (KJC), 

and hip joint center (HJC) as well as the location of the lower leg center of mass. A measured 

coordinate system was created from the anatomical standing T-pose as well as from each static 

trial, and was then used to link the testing markers to the locations of the markers that were 

removed. 

The anatomical coordinate system of the thigh (ATCS) was created using the left and right 

anterior superior iliac spine (L.ASIS & R.ASIS), knee, and thigh markers. 

knee lateral − knee medial 

r = x − axis 1 kneelateral − knee medial 

HJC − KJC 

r 2 = HJC − KJC 

r = r ×r y − axis 3 2 1 

r = r ×r z − axis 4 1 3 

The KJC was assumed to be midway between the medial and lateral epicondyles of the 

femur, measured by the medial and lateral knee markers. The location of the HJC was 

approximated using regression equations proposed by Bell et al. [1990] and adapted to fit the 

global reference frame of the laboratory. [Bell 1990] (Figure 22) 

42
 
Figure 22: Pelvis coordinate system highlighting the location of the right hip joint center 


 









+ = LASIS RASIS Origin 2 

x 

Origin + .36* PW

Origin − .19* PW

Origin − .3* PW

−
 LASIS
 RASISx =
 
 


 









=
 

PW
 

HJC
 

43 

The transformation matrix from the global reference frame to the ATCS (TTGA) was created 

from the unit direction vectors of the ATCS. 


 1 0 0 0 











       

T TGA = 







4 r

















r3 

















r

1 

















HJC
 

















The anatomical coordinate system of the shank (ASCS) was determined using the knee, 

ankle, and shank markers. The AJC was assumed to be midway between the medial and lateral 
malleoli, measured by the medial and lateral ankle markers. The transformation matrix from the 

GCS to the ASCS (TSGA) was created from the unit direction vectors of the ASCS. 

2 

1 

knee lateral − knee medial 

r =
 knee lateral − knee medial 

KJC - AJC
 r = z − axis 

KJC
 − AJC
 

r × − axis 3 = r1 r y 2 

r × − axis 4 = r3 r x 1 

 1 0 0 0 
 



















        







KJC
 















r4 

















r3 

















r1 









T SGA =
 

The ATCS and ASCS were oriented such that when standing the systems aligned with the 

GCS and the positive x-axis was in the lateral direction of the right leg, the positive z-axis was in 

the proximal direction, and the positive y-axis was in the anterior direction. (Figure 23) 

44
 

Figure 23: Orientation of the ATCS and ASCS 
A measured coordinate system (MCS) was created for the thigh and the shank using the 

marker clusters on the segments. The measured coordinate system of the thigh (MTCS) was 

created from the thigh, thigh front, and thigh rear markers. The transformation matrix from the 

GCS to the MTCS (TTGM) was also created with the right thigh front marker as its origin. 

1 

thigh thigh front − r = z − axis 

thigh thigh front −

thigh front thigh −
 rear 

r2 =
 

thigh front thigh − rear 

r × − axis 3 = r2 r x 1 

r × − axis 4 = r1 r y 3 

 1 0 0 0 
 



















       

thigh front 























r3 

















r4 

















r

1 









T TGM =
 

The measured coordinate system of the shank (MSCS) was created from the shank, shank 

front, and shank rear markers. The transformation matrix from the MSCS to the GCS (TSGM) was 

created with the right shank front marker as its origin. 

45
 

shank shank front − r − axis 1 = z

shank shank front −

shank front shank −
 rear 

r =
 

shank front shank − rear 

r − axis r = 2 ×r x 1 

r = r1 ×r y 3 − axis 4 

3 

2 

 1 0 0 0 

       

r1 r4 r3 shank front T SGM =





















































































To determine measured marker locations in the ATCS, TTMA was created; for the ASCS, TSMA 

was created. 

46
 

TTMA 

TSMA 

] 1 ] [TTGA 

− TTGM =
 [

1 ] [TSGA ] − TSGM = [
Appendix B
 

Aij : Matrix of the cells that make up the patella region (see Figure 4) 

Bij : Matrix of the cells that make up the PTT region (see Figure 4) 

i : sample number 

j : cell number 

n : total number of samples 

m : total number of sensing units in B (PTT) 

q : total number of sensing units in A (patella) 

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

both regions. 

m 

∑Bij n
 j =1

∑ q m 

i=1 ∑ Aij +∑Bij 

n 

j =1 j=1

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

 

m
 

∑Bij
 

∑

n 

j=1 

n 

i=1 m

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

 

n 

∑max( Bi ) 

n 

i=1 

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

 

n 

∑(stdev (Bi ))2 

n 

i =1 

NOTE: Equations for the patellar tendon and tibial tubercle region are shown. Patella equations 

are similar. 

47
 
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DHHS (NIOSH) Publication No. 2011-176 

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