RI 9684

REPORT OF INVESTIGATIONS/2011

Practical Demonstrations of
 

Ergonomic Principles
 

Department of Health and Human Services 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 
Report of Investigations 9684 

Practical Demonstrations of Ergonomic Principles 

Susan M. Moore, Ph.D., Janet Torma-Krajewski, Ph.D., C.I.H., C.P.E., 

Lisa J. Steiner, M.S., C.P.E. 

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

National Institute for Occupational Safety and Health
 

Pittsburgh Research Laboratory
 

Pittsburgh, PA
 

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

copied or reprinted. 

Disclaimer 

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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 author(s) and do not necessarily 

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

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DHHS (NIOSH) Publication No. 2011-191 

July 2011 

SAFER • HEALTHIER • PEOPLETM 

ii 
CONTENTS
 

ACKNOWLEDGEMENTS ...........................................................................................................v
 

GLOSSARY ...............................................................................................................................vi
 

SECTION 1: Introduction ........................................................................................................... 1
 

Target Users and Audiences............................................................................................... 1
 

Format of Demonstration Descriptions................................................................................ 2
 

Suggested Supplies............................................................................................................ 2
 

Graphics ............................................................................................................................. 3
 

SECTION 2: Neutral Postures.................................................................................................... 4
 

Principles ..........................................................................................................................19
 

SECTION 4: Hand-Tool Selection and Use...............................................................................28
 

Principles ..........................................................................................................................28
 

SECTION 5: Fatigue Failure and Back Pain..............................................................................37
 

Principles ..........................................................................................................................37
 

SECTION 6: Moment Arms and Lifting......................................................................................41
 

Principles ..........................................................................................................................41
 

APPENDIX A: Suggested Supplies...........................................................................................47
 

APPENDIX B: Useful Images for Handouts...............................................................................52
 

REFERENCES .........................................................................................................................56
 

iii 
FIGURES
 

Figure 1. Neutral and awkward wrist postures............................................................................ 5
 

Figure 2. Neutral and awkward elbow postures. ......................................................................... 6
 

Figure 3. Neutral and awkward shoulder postures. .................................................................... 6
 

Figure 4. Neutral and awkward back postures............................................................................ 7
 

Figure 5. Example of a portable EMG device (showing electrodes on skin) that indicates 

muscle activity by emitting audible signals......................................................................... 8
 

Figure 6. Wrist postures and electrode placement for portable EMG device. ............................. 9
 

Figure 7. Negative, flat, and positive tilt positions for a keyboard. .............................................10
 

Figure 8. Electrode placement on the upper arm.......................................................................11
 

Figure 9. Electrode placement for the shoulder.........................................................................12
 

Figure 10. Neutral, abducted, and flexed (reaching) shoulder postures.....................................13
 

Figure 11. Electrode placement for the back (line indicates location of spine). It is important
 

that the electrodes are placed on the muscles as shown in the photograph. If the 

electrodes are placed too high on the back, the demonstration will not work properly. .....14
 

Figure 12. Neutral, moderately flexed, and highly flexed postures of the back. .........................15
 

Figure 13. Hand dynamometer showing how wrist angle affects force production for neutral,
 

ulnar deviation, and radial deviation wrist postures...........................................................18
 

Figure 14. Pinch (lateral) grip and power grip............................................................................20
 

Figure 15. Example of the maximum forces generated for a pinch grip (lateral) and a power
 

grip ..................................................................................................................................22
 

Figure 16. Example of a pinch grip (lateral) and the resulting maximum force...........................24
 

Figure 17. The power grip is shown for five different grip widths. The narrowest grip is 

Grip 1; the width increases for each subsequent grip, with Grip 5 being the widest grip. ..26
 

Figure 18. Maximum–force output for each grip width. Note that, for this participant, Grip 2
 

had the highest force production. .....................................................................................27
 

Figure 19. Evaluating the effect of tool-handle diameter............................................................30
 

Figure 20. Examples of situations in which a pistol grip and inline grip would be useful as a 

means for keeping the wrist in a neutral posture...............................................................32
 

Figure 21. Two types of pliers, one with a spring that reduces forceful exertion when opening
 

the jaw, and one without a spring. ....................................................................................34
 

Figure 22. Example of one-handed and two-handed drilling.....................................................36
 

Figure 23. Image of vertebrae, disc, and endplates. ................................................................37
 

Figure 24. A pen cap that is bent multiple times visually shows fatigue; a paper clip shows the
 

result of failure. The graph (generalized for bone) illustrates how the same load, lifted 

many times, may ultimately, over time, lead to failure.......................................................40
 

Figure 25. These schematics illustrate how increasing the distance between the worker and
 

the object being lifted increases the overall moment (i.e., torque) for which the back 

muscles must compensate by expending more force. ......................................................42
 

Figure 26. A moment-arm simulator showing that more force/weight (W; arrow indicates 

direction of force) is needed to balance the “see saw” if the moment arm (L) is shorter 

on one side of the fulcrum as compared to the other side.................................................44
 

Figure 27. Moment-arm simulator with dial scale showing that, as the moment arm is
 

increased, the resulting force acting on the scale increases. ............................................46
 

Figure A-1. Dimensions for the moment-arm simulator. ............................................................49
 

Figure A-2. Examples of the suggested supplies for the demonstrations...................................51
 

iv 
ACKNOWLEDGEMENTS
 

The technical contributions of Dr. Sean Gallagher pertaining to methods and materials used 

for the demonstrations and his assistance writing the background for the lower back 

demonstrations (Section 5) are acknowledged. Additionally, the assistance of Jonisha Pollard and 

Mary Ellen Nelson in obtaining photographs of the demonstrations and the artistic work of 

Alexis Wickwire are also acknowledged. The authors also appreciate Alan Mayton’s and Patrick 

McElhinney’s willingness to participate in the demonstrations video. Videotaping and editing of 

the videos were professionally completed by Charles Urban. 

v 
GLOSSARY
 

Awkward posture. Deviation from the natural or ―neutral‖ position of a body part. A neutral 

position places minimal stress on the body part. Awkward postures typically include reaching 

overhead or behind the head; twisting at the waist; bending the torso forward, backward, or to the 

side; squatting; kneeling; and bending the wrist. 

Cumulative injury (overuse injury). Cumulative injuries develop from repeated loading of 

body tissues over time. Such injuries include overuse sprains/strains, herniated discs, tendonitis, 

and carpal tunnel syndrome. 

Disorder. A medical condition that occurs when a body part fails to function properly. 

Ergonomics. The science of fitting workplace conditions and job demands to the capabilities of 

workers, and designing and arranging items in the workplace for efficiency and safety. 

Fatigue failure. The weakening or breakdown of material subjected to stress, especially a 

repeated series of stresses. 

Force. The amount of physical effort a person uses to perform a task. 

Inline grip. A hand tool with a straight handle that is parallel with the direction of the applied 

energy. 

Moment (torque). The tendency to produce motion about an axis. 

Moment arm. The perpendicular distance between an applied force and the axis of rotation. For 

muscles, this is the perpendicular distance between the line of action of the muscle and the center 

of rotation at the joint. 

Musculoskeletal disorders (MSDs). Illnesses and injuries that affect one or more parts of the 

soft tissue and bones in the body. The parts of the musculoskeletal system are bones, muscles, 

tendons, ligaments, cartilage, and their associated nerves and blood vessels. 

Neutral body posture. The resting position of body parts. 

Pinch grip. A grasp in which one presses the thumb against the fingers of the hand and does not 

involve the palm. 

Pistol grip. A tool handle that resembles the handle of a pistol and is typically used when the 

tool axis must be elevated and horizontal or below waist height and vertical. 

Power grip. A grasp where the hand wraps completely around a handle, with the handle running 

parallel to the knuckles and protruding on either side. 

vi 
Repetitive. Performing the same motions repeatedly over time. The severity of risk depends on 

the frequency of repetition, speed of the movement, number of muscle groups involved, and 

required force. 

Risk factor. An action and/or condition that may cause an injury or illness, or make an existing 

injury or illness worse. Examples related to ergonomics include forceful exertion, awkward 

posture, and repetitive motion. 

Stress. Demand (or ―burden‖) on the human body caused by something outside of the body, such 

as a work task, the physical environment, work-rest schedules, and social relationships. 

Traumatic injury. Injuries that are acute, that may result from instantaneous events such as 

being struck by objects and that often require immediate medical attention. These types of 

injuries are often sustained through accidents. 

vii 
SECTION 1: INTRODUCTION
 

Musculoskeletal disorders (MSDs) often involve the back, wrist, elbow, and/or shoulder, 

and occur when workers are exposed over time to MSD risk factors, such as awkward postures, 

forceful exertions, or repetitive motions. These exposures sometimes occur due to poorly 

designed workstations, tasks, and/or hand tools [Chaffin et al. 2006; Sanders and McCormick 

1993; Silverstein et al. 1996, 1997]. Workers must understand the nature of MSD risk factors 

and how to avoid exposure to them. In a classroom setting, trainers may discuss ergonomic 

principles and show examples of MSD risk factors with photographs or videos. However, 

supplementing training with practical, hands-on demonstrations may further reinforce these 

ergonomic principles and help workers understand the importance of avoiding exposure to MSD 

risk factors. Moreover, demonstrations that allow for worker participation result in a greater 

understanding of the impact exposures to particular MSD risk factors have on workers’ bodies. 

This document consists of a series of demonstrations designed to complement training on 

ergonomic principles. A description of the materials needed and step-by-step methodology are 

included in this document. Each demonstration highlights worker participation and uses 

relatively inexpensive materials. 

The demonstrations are organized by type of ergonomic principle. Five general topics are 

addressed: 

Neutral compared with non-neutral postures 

Grip types 

Hand-tool selection and use 

Fatigue failure and back pain 

Moment arms and lifting 

The demonstrations show the effects of posture, work methods, workstation design, tools, 

tasks, and location of materials on worker exposure to MSD risk factors. Many of the 

demonstrations are appropriate supplements to the NIOSH-developed training ―Ergonomics and 

Risk Factor Awareness Training for Miners,‖ which is provided to mining employees and 

downloadable from the NIOSH mining website: 

http://www.cdc.gov/niosh/mining/pubs/pubreference/outputid2748.htm [NIOSH 2008]. 

Target Users and Audiences 

This document was developed for individuals who intend to provide training on ergonomic 

principles that focus on MSD risk-factor exposures. It was designed for trainers of all 

experience levels including the beginning trainer. The demonstrations are designed to be 

performed by both the trainer and the worker. Each demonstration reinforces specific ergonomic 

principles and teaches the worker how and why to avoid MSD risk factors. Additionally, 

individuals involved in the purchase and selection of new and/or replacement tools may benefit 

from many of the demonstrations because they highlight the importance of considering 

ergonomic principles before purchasing tools. 
Format of Demonstration Descriptions 

Each section of this document begins with a discussion of an ergonomic principle and its 

role in avoiding MSD risk factors, followed by a series of demonstrations that may be used to 

show how the principle can be incorporated into the work environment. Each demonstration 

starts with clear objective statements and concludes with take-home messages that participants 

should incorporate into their everyday thinking. These demonstrations encourage audience 

participation because discussing how the principle plays a role in a worker’s specific workplace 

is important for promoting understanding. Each demonstration includes the following 

information: 

Objectives of the demonstration 

List of suggested supplies needed to conduct the demonstration 

Step-by-step demonstration methodology 

Take-home messages that should be emphasized during the demonstration 

To assist the trainer in knowing how to use some of the suggested supplies (e.g., portable 

EMG device, hand dynamometer), a series of brief video clips on DVD are included with this 

document. The DVD also contains video clips that show how to perform the demonstration and 

the results you should receive when using the portable electromyography (EMG) device. 

Suggested Supplies 

A complete list of required supplies for performing the demonstrations is provided in 

Appendix A. As previously mentioned, each demonstration description includes a list of 

supplies specific to that demonstration. Most of the supplies are available at hardware stores for 

a reasonable cost. 

A portable EMG device is recommended for use with several of the demonstrations (Please 

see Appendix A for more information regarding the purchase of this device including cost and 

potential manufacturers). An EMG device is a rudimentary instrument that can be used to make 

relative comparisons of muscle activity by measuring the electrical activity of a muscle. The 

muscle emits an electrical signal when it undergoes one of two types of contractions— 

concentric (i.e., when the muscle shortens as it contracts) or eccentric (i.e., when the muscle 

lengthens as it contracts). When a contraction of the muscle is detected, the device emits an 

audible signal of beeps; the frequency of these beeps increases as the measured activity of the 

muscles beneath the electrodes increases. For example, if you place the electrodes on the inside 

of the forearm and ask the participant to contract the forearm muscles to 50% of their maximal 

effort, you will hear the device beep at a specific frequency. If you then ask the participant to 

contract his or her forearms to a maximal level, the frequency of the beeps will increase. 

One problem with measuring muscle activity using electrodes placed on the skin is that the 

electrodes may measure what is referred to as ―crosstalk.‖ Crosstalk is produced when an 

electrode measures a signal over a nonactive or nearby muscle. For example, if you apply the 

electrodes to the inside of the forearm of the participant and have the participant flex his or her 

wrist (i.e., move the palm towards the inside of the forearm), the forearm muscles being 

measured are contracting and the EMG device will emit the audible signal. However, if you 

have the participant extend his or her wrist (i.e., move the palm as far away from the inside of 

the forearm as they can), the EMG device will still emit an audible signal even though the 

2
 
muscles the electrodes reside above are not contracting. This is an example of crosstalk where 

the electrodes are detecting activity from the muscles on the other side of the forearm. The 

electrodes could also be detecting a small eccentric contraction if the participant is using the 

inner forearm muscles to control the rate at which the wrist is extended. If the trainer is not 

aware of these issues, the trainer and the participants may be confused by the seemingly 

mistaken readings. The trainer should practice the demonstrations provided in this document 

prior to attempting them in front of an audience to minimize any occurrences where this 

―crosstalk‖ could cause confusion for the audience members. 

Another limitation in using electrodes is that the amount of electrical activity produced by 

the different muscles of the body varies. Therefore, the EMG device provides the user with the 

ability to select from several different scales. You may need to adjust the scale in order for the 

device to be sensitive enough to detect changes in muscle activity for your muscles of interest. 

You must use the same scale the entire time you are measuring the electrical activity of a 

specific muscle group; otherwise, you will not be able to make a direct comparison to the 

muscle activity before and after an event. 

Graphics 

Appendix B includes several images that may be useful to show in a PowerPoint 

presentation when conducting the demonstrations. Electronic files for these graphics are found 

on the DVD provided with this document. 

3
 
SECTION 2: NEUTRAL POSTURES 

Principles 

Use neutral postures: 

o	 Maximum muscle force producible in neutral postures is 

greater than maximum muscle force producible in 

awkward postures. 

o	 Fatigue occurs sooner when working in awkward 

postures. 

o	 Working in extreme awkward postures (near extreme 

ranges of motion) causes stress on muscles and joints. 

A neutral posture is achieved when the muscles are at their resting length and the joint is 

naturally aligned. For most joints, the neutral posture is associated with the midrange of motion 

for that joint. When a joint is not in its neutral posture, its muscles and tendons are either 

contracted or elongated. Joints in neutral postures have maximum control and force production 

[Basmajian and De Luca 1985; Chaffin et al. 2006]. Neutral postures also minimize the stress 

applied to muscles, tendons, nerves, and bones. A posture is considered ―awkward‖ when it 

moves away from the neutral posture toward the extremes in range of motion. 

For the most part, a worker is capable of producing his or her highest amount of force when 

a joint is in its neutral posture. As the joint moves away from the neutral posture, the amount of 

force the muscles can produce decreases because some of the muscle fibers are either contracted 

or elongated [Clarke 1966; Kumar 2004]. Also, when you bend your wrist, the tendons of the 

muscles partially wrap around the carpal bones in the wrist. Because the bones do not act as a 

perfect pulley, a loss in the force that can be produced will occur. Furthermore, losses in force 

are also experienced due to friction [Ozkaya and Nordin 1999]. Thus, in order for a worker to 

produce the same force in an awkward posture as they do in the neutral posture, the worker’s 

muscles must work harder and expend more energy. Working in an awkward posture, therefore, 

is a MSD risk factor that should be avoided. This is an extremely important principle because 

working closer to one’s maximum capability, especially without rest, may result in an earlier 

onset of fatigue and, over time, may also increase the risk of MSDs [Chaffin et al. 2006]. 

Ideally, tasks and workspaces should be designed so that work is conducted at approximately 

15% or less of maximum capacity [Chaffin et al. 2006]. 

Therefore, to minimize the level of effort as a percentage of the maximum capacity, you 

should help workers use the neutral posture of their joints. However, some joint motion must 

occur because remaining in a static posture for too long produces several negative consequences 

and should be avoided. When a worker remains in a static posture, the prolonged application of 

a load by the muscles can result in fatigue. Also, not moving muscles for a time impedes blood 

4
 
flow, which is needed to bring oxygen and crucial nutrients to the muscles and to remove 

metabolic waste products. Static postures are avoided when work is dynamic, with the muscles 

and joints periodically moving. With this in mind, workstations, tasks, and hand tools should be 

designed to enable workers to use primarily neutral postures and postures that are in relative 

proximity to the neutral posture. Care should be taken to ensure that awkward postures are not 

frequent and that high forces are not required while in awkward postures. Figures 1–4 show 

neutral and awkward postures for the joints (e.g., wrist, elbow, shoulder, and back). 

Figure 1. Neutral and awkward wrist postures. 

These 

topics will be discussed in more detail in this document. 

Special considerations are made for the back and hand. Even though the neutral posture of 

the back technically occurs with the back slightly forward flexed, lifting in a flexed posture can 

place unwanted forces on the spine itself. Lifting tasks should be performed while the back is 

not flexed, and the nonflexed posture is often called the neutral posture of the back. The neutral 

posture for the hand is achieved when the fingers are in a slightly flexed (relaxed) position 

[Bechtol 1954]. 

5
 
Figure 2. Neutral and awkward elbow postures. 

Figure 3. Neutral and awkward shoulder postures. 

6
 
Figure 4. Neutral and awkward back postures. 

The following demonstrations are designed to highlight the effect that awkward postures 

have on muscle activity for the wrist, elbow, shoulder, and lower back. 

7
 
Effects of Postures on Muscle Activity 

Objectives 

To understand the 

effect of neutral and 

awkward postures 

on muscle activity 

for the wrist, elbow, 

shoulder, and lower 

back 

Supplies 

Portable EMG device indicating muscle activity via audible 

sound (Figure 5) 

 

Figure 5. Example of a portable EMG device (showing 

electrodes on skin) that indicates muscle activity by emitting 

audible signals. 

Step-by-Step Demonstration Method (Wrist). See 

Introduction video for EMG placement; see 

WristFlexionExtension video. 

1. Place the electrodes on the forearm (see supplemental 

video clips for more information on electrode 

placement). 

2. Instruct participant to place his or her wrist in the 

neutral posture and then extend the wrist until it is fully 

extended and it is clear that the wrist is in an awkward 

posture (Figure 6). 

3. Note that the frequency and volume of the sounds 

produced by the portable EMG device increase as the 

joint moves away from the neutral posture and extends 

into an awkward posture (indicating more muscle 

activity). 

Neutral Compared with Awkward Postures

8
 
4.

5.

Effects of Postures on Muscle Activity 

Ask the audience to identify tasks at their worksite where they use extended wrist 

postures or other awkward postures of the wrist (excessive flexion, extension, and 

radial/ulnar deviations). 

Discuss with the audience whether it would be possible to use a neutral posture to 

perform these tasks. If not, ask the audience why they are limited to awkward postures. 

Identify changes to workstation design, tasks, tools, or location of materials that may 

allow neutral postures to be used. 

Figure 6. Wrist postures and electrode placement for portable EMG device. 

Neutral Compared with Awkward Postures 

9
 
Effects of Postures on Muscle Activity

NOTE: This demonstration can be used to train workers who use keyboards because it 

focuses on evaluating wrist posture with the keyboard placed at different positions, 

including flat, positive, and negative tilt (Figure 7). 

Figure 7. Negative, flat, and positive tilt positions for a keyboard. 

Neutral Compared with Awkward Postures

10
 
Effects of Postures on Muscle Activity

Step-by-Step Demonstration Method (Elbow). See BicepCurl video. 

1. Place the electrodes on the upper arm (Figure 8). 

2. Instruct the participant to assume an elbow posture with a 90° angle (neutral) 

(Figure 8). 

3. Note the intensity of the sounds from the portable EMG device. 

4. Instruct the participant to raise the forearm so that the elbow angle is less than 

90° (flexion). As the elbow angle is decreased, the intensity of the sounds from 

the portable EMG device will increase (indicating more muscle activity). 

5. Ask the audience to identify tasks that require them to use awkward postures for 

the elbow and how these postures might be avoided. 

Figure 8. Electrode placement on the upper arm. 

Neutral Compared with Awkward Postures 

11
 
Effects of Postures on Muscle Activity

Step-by-Step Demonstration Method (Shoulder). See ShoulderRaise and 

ShoulderReach videos. 

1. Place the electrodes on the shoulder (Figure 9). 

2. Instruct the participant to assume a neutral shoulder posture (Figure 10). 

3. Note the intensity of the sounds from the portable EMG device. 

4. Instruct the participant to raise his or her arm (abduction) so that it is parallel to 

the ground. As the shoulder angle is increased, the intensity of the sounds from 

the portable EMG device will increase (indicating more muscle activity). 

5. Instruct the participant to reach above their head as if to change a light bulb. As 

the shoulder angle is increased further, the intensity of the sounds from the 

portable EMG device will also increase (indicating more muscle activity). 

6. Ask the audience to identify tasks that require them to use awkward postures for 

the shoulder and how these postures might be avoided. 

Figure 9. Electrode placement for the shoulder. 

Neutral Compared with Awkward Postures 

12
 
Effects of Postures on Muscle Activity 

Figure 10. Neutral, abducted, and flexed (reaching) shoulder postures. 

Neutral Compared with Awkward Postures 

13
 
Effects of Postures on Muscle Activity 

Step-by-Step Demonstration Method (Low Back). See BackFlexionNoWeight 

video. 

1. Place the electrodes on the low back. (Figure 11). 

2. Instruct the participant to slowly lean forward with the back at about a 45°–60° 

angle, and note the increase in the intensity of the sounds from the portable EMG 

device (Figure 12). The muscle group being tested is referred to as the erector 

spinae, which undergoes an eccentric contraction as the trunk flexes forward. 

The erector spinae helps control the rate at which the torso is lowered by acting 

against the abdominal muscles that are performing a concentric contraction to 

flex the trunk. However, make sure the participant does not flex until their torso 

is fully horizontal since the erector spinae is not as active once the torso comes to 

rest. The decreased activity, as indicated by a decrease in audible EMG signals, 

may be confusing to the audience. Before performing this demonstration in front 

of a group, practice determining the position of the torso when the activity of the 

erector spinae diminishes. This will help you to advise the participant to avoid 

going beyond this position. Although muscle activity has decreased at postures 

near full flexion, the spine continues to be loaded in an undesirable manner. 

Figure 11. Electrode placement for the back (line indicates location of spine). It is 

important that the electrodes are placed on the muscles as shown in the photograph. If 

the electrodes are placed too high on the back, the demonstration will not work properly. 

Neutral Compared with Awkward Postures 

14
 
Effects of Postures on Muscle Activity 

Take -Home 

Messages 

Whenever possible, 

workers should 

work in a neutral 

posture to reduce 

muscle activity and 

reduce fatigue. 

3. Ask the participant to slowly return to a standing 

position. During this motion, the erector spinae are 

undergoing a concentric contraction in order to raise 

the weight of the torso upward against the force of 

gravity. However, when the participant is back in an 

upright posture, this activity will diminish as will the 

frequency of the audible signal from the EMG device. 

4. Ask the audience to identify tasks at their worksite that 

require them to work with their backs in flexed 

postures, and how these postures might be avoided. 

Figure 12. Neutral, moderately flexed, and highly flexed 

postures of the back. 

Neutral Compared with Awkward Postures 

15
 
Wrist Angle and Grip Strength 

Objectives 

To increase 

awareness of how 

posture affects force 

production, 

capabilities, and 

worker fatigue 

To discuss tasks that 

require workers to 

use awkward 

postures while 

exerting force 

Supplies 

Hand dynamometer (Figure 13) 

Stopwatch 

Step-by-Step Demonstration Method. See PowerGrip 

video. 

1. Place the hand dynamometer in the participant’s hand 

and make sure his/her hand is in the neutral posture as 

shown in Figure 13. 

2. Instruct the participant to squeeze with maximum 

force, and record the force he/she was able to produce. 

3. Instruct the participant to rotate his or her wrist into a 

position of radial deviation (awkward posture). 

4. Instruct the participant to squeeze with maximum 

force, and record the force he or she was able to 

produce. The force should be less than the force 

obtained with the wrist in the neutral posture. 

5. Instruct the participant to rotate his or her wrist into a 

position of ulnar deviation (awkward posture, see 

Figure 13). Some people may not have sufficient range 

of motion to move their wrist into this posture. Before 

performing this demonstration, determine if the 

participant selected for the demonstration can achieve 

this posture. 

6. Instruct the participant to squeeze with maximum 

force, and record the force he/she was able to produce. 

The force should be less than that produced with the 

wrist in the neutral posture. 

7. Ask a few more audience members to participate. The 

trend will be the same for all participants even though 

the maximum forces they can produce will vary. 

Neutral Compared with Awkward Postures 

16
 
Wrist Angle and Grip Strength 

8. Now ask each participant to exert a force of 20 lbs while in the neutral posture, 

and hold that force as long as possible. Record the length of time the participant 

maintains the posture. Then, ask the participants to do the same using wrist 

positions with radial and ulnar deviations; because fatigue occurs sooner in these 

postures, the length of time the force can be maintained for these postures should 

be less than the time the force can be maintained using a neutral posture. 

9. Ask the audience to identify tasks at their worksite where awkward postures of 

the wrist occur. 

NOTE: Dramatic results will also be seen if this demonstration is performed with the 

wrist in extension or flexion as shown in Figure 1. 

Neutral Compared with Awkward Postures 

17
 
Wrist Angle and Grip Strength 

Take -Home 

Messages 

When body joints 

are in awkward 

postures, maximum 

force produced 

decreases. 

Muscle fatigue will 

occur earlier when 

working in an 

awkward posture 

instead of a neutral 

posture. 

Figure 13. Hand dynamometer showing how wrist angle affects 

force production for neutral, ulnar deviation, and radial 

deviation wrist postures. 

Neutral Compared with Awkward Postures 

18
 
GSECTION 3: GRIP TYPES
 

Principles 

Force generated with a pinch grip is about 15%–25% of force 

generated with a power grip. 

Use a power grip when higher forces are required.
 

Use a pinch grip when precise movements are needed, and the
 

force required is low (< 2 lbs).
 

Research shows the design width of power grips should be
 

1.75 to 3.75 inches. 

In general, an object can be grasped using one of two methods: a pinch grip or a power grip 

(Figure 14). A power grip curls the fingers toward the palm; a pinch grip presses the thumb 

against the fingers of the hand or an object, and does not involve the palm. The amount of force 

that can be generated depends on the type of grip and the width of the grip. 

Three types of pinch grips can be used: 

Tip pinch—using only the tips of the fingers and thumb (holding a bead) 

Chuck pinch—using the thumb and first two fingers (holding a pencil) 

Lateral pinch—using the thumb and side of the first finger (holding a key) 

For a given force, using a pinch grip is biomechanically more stressful than using a power 

grip. The amount of force one is capable of exerting is greater for the power grip than for the 

pinch grip. A general rule of thumb is that the force generated with a pinch grip is about 15%– 

25% of the force generated with a power grip, depending on the type of pinch grip and the 

worker’s individual force capability. The amount of force exerted also varies among the three 

types of pinch grips. When using a tip pinch, the force exerted is 71%–72% of the lateral pinch 

force; when using a chuck pinch, the force exerted is 98% of the lateral pinch force. The amount 

of force generated by power grips and pinch grips also varies depending on the width of the 

grip. For a power grip, the maximum force is generated with a grip width of 1.75–3.75 inches. 

For a pinch grip (type not specified by citation), the maximum force is generated with a grip 

width of 1–3 inches [Chengalur et al. 2004]. 

19
 
A pinch grip provides more control because the thumb joint is highly movable and precise. 

In contrast, minimal control is associated with the power grip as the fingers move as one entity 

and only in one direction (flexion). For these reasons, pinch grips are typically used for shortduration, low-force, and precision tasks because they require minimal force exertion but high 

control (e.g., tightening or removing eyeglass screws). In general, tasks that are done repeatedly 

and require 2 lbs or more of force should not involve pinch grips. For example, tasks that 

require using a power drill are ideally suited to the use of a power grip because the neutral 

posture for the fingers is a slightly flexed position [NIOSH 2004]. 

Figure 14. Pinch (lateral) grip and power grip. 

NOTE: Grip type is greatly influenced by hand-tool design. Therefore, a separate section in 

this document (see Section 4) has been devoted to hand-tool selection and use, and follows this 

section on grip types. 

20
 
Power Grip Compared with Pinch Grip 

Objectives 

To increase 

awareness that: 

Maximum force 

generated using a 

power grip is 

greater than when 

using a pinch grip. 

Force production 

capabilities differ 

among individuals 

for both pinch and 

power grips. 

Pinch grips should 

be avoided when 

possible because it 

places high demands 

on the hand and 

produces less force 

than a power grip. 

Supplies 

Hand dynamometer that measures pinch-grip strength 

(Figure 15) 

Hand dynamometer that measures power-grip strength 

(Figure 15) 

Step-by-Step Demonstration Method. See 

LateralPinchGrip, TipPinchGrip, ChuckPinchGrip, and 

PowerGrip videos. 

1. Place dynamometer for pinch grip measurements 

between the participant’s thumb and forefinger as 

shown in Figure 15 (lateral). 

2. Instruct the participant to exert maximum force. 

3. Note the maximum force produced. 

4. Place the dynamometer for power-grip measurements 

in the palm of participant’s hand, as shown in Figure 

15. 

5. Instruct the participant to exert maximum force. 

6. Note the maximum force produced. 

7. Ask several participants to exert maximum forces using 

both pinch (lateral, tip, and chuck) and power grips. 

Record these results. 

8. Discuss the differences between the maximum forces 

generated for the different types of grips. 

9. Discuss the differences among individuals in 

generating forces using all types of grips. 

Grip Types 

21
 
Power Grip Compared with Pinch Grip 

Take -Home 

Messages 

A pinch grip should 

be used only for 

precision tasks that 

require minimal 

forces to be 

generated. 

In general, pinch 

grips should be 

avoided for any 

length of time, 

regardless of the 

force required. 

A power grip should 

be used for tasks 

that require larger 

forces, that do not 

require high degrees 

of precision and 

dexterity. 

10. Discuss specific tasks that would be appropriate for 

using either type of grip. 

11. Identify specific tasks the worker performs that uses 

the different types of grips, and determine if the grip is 

appropriate for the task requirements. 

 

Figure 15. Example of the maximum forces generated for a 

pinch grip (lateral) and a power grip. 

Grip Types 

22
 
Pinch Grip Strength and Applications 

Objectives 

To increase 

awareness of 

individual maximum 

capabilities for a 

pinch grip 

To discuss tasks that 

are performed with 

a pinch grip 

To understand force 

and repetition 

requirements of the 

task 

Supplies 

Hand dynamometers that measures pinch grip strength 

(Figure 16) 

Step-by-Step Demonstration Method 

1. Place hand dynamometer between participant’s thumb 

and forefinger (lateral pinch grip) as shown in 

Figure 16. 

2. Instruct the participant to exert maximum force. 

3. Note the maximum force produced. 

4. Ask several other audience members of different sizes 

(i.e., weight, height) and/or gender to perform steps 1– 

3 and compare their maximum forces; this 

demonstrates the effects of anthropometry (i.e., size 

variability among people). Encouraging audience 

members to compete for the largest force production 

often increases participation and friendly competition. 

Make sure to point out to the audience how much the 

forces produced varied across the group. 

5. Instruct the participants to exert 2 lbs of force to 

provide them with a general understanding of what it 

feels like to exert that level of force. 

6. Ask the audience to identify tasks at their worksite 

where they use a pinch grip and exert more than 2 lbs 

of force. Repetitive tasks for which a pinch grip is used 

should also be avoided. Ask the audience to identify 

repetitive tasks at their worksite for which they use a 

pinch grip. 

7. Discuss the possibility of using a better tool or 

workstation design to avoid using pinch grips. 

Grip Types 

23
 
Pinch Grip Strength and Applications 

Take -Home 

Messages 

Maximum forces 

exerted with a pinch 

grip vary among 

workers. 

A pinch grip should 

not be used when 

high forces or 

repetition are 

required. 

Pinch grips should 

be used only for 

tasks that require 

small forces (< 2 

lbs). 

Figure 16. Example of a pinch grip (lateral) and the resulting 

maximum force. 

Grip Types 

24
 
Power Grip: Effect of Grip Width 

Objectives 

To increase 

awareness of how 

grip width affects 

maximum strength 

for a power grip 

To increase 

awareness of 

individual maximum 

capabilities for a 

power grip 

To discuss whether 

or not the grip width 

necessary to 

perform tasks is 

appropriate 

Supplies 

Hand dynamometer that allows grip strength to be evaluated 

for multiple grip widths (Figure 17) 

Step-by-Step Demonstration Method. See PowerGrip, 

NarrowGrip, and WideGrip videos. 

1. Place the hand dynamometer in the participant’s hand, 

and instruct him or her to place the wrist in a neutral 

posture (Figure 17). 

2. Measure the maximum force the participant can produce 

using three to five different grip widths. If using only 

three different grip widths, use grips 1, 3, and 5 as shown 

in Figure 17. 

3. Record the force produced for each grip width and 

compare these values across the different grip widths. 

You should notice that for very wide grips and for very 

narrow grips, the participant will not be able to produce 

as much force as with the intermediate grips (Figure 18). 

4. Ask several other audience members of different sizes 

(i.e., height, weight) and/or gender to perform steps 1–3, 

and compare their maximum forces; the forces produced 

will vary, showing the effect of anthropometry). 

However, the maximum force for each participant should 

be produced for a grip width of 1.75 to 3.75 inches. 

5. Ask the audience to identify tasks at their worksite where 

a power grip is required at or near their minimum or 

maximum grip width capacity. Determine whether or not 

these tasks require workers to exert forces near their 

maximum capabilities. 

6. Discuss with the audience how they may use a better tool 

or workstation design to avoid using power grip widths 

that are too narrow or too wide. 

Grip Types 

25
 
Power Grip: Effect of Grip Width 

Figure 17. The power grip is shown for five different grip widths. The narrowest grip is 

Grip 1; the width increases for each subsequent grip, with Grip 5 being the widest grip. 

Grip Types 

26
 
Power Grip: Effect of Grip Width 

Take -Home 

Messages 

Maximum force 

produced with a 

power grip varies 

with grip width. 

Maximum forces 

exerted for a power 

grip vary among 

workers. 

Tools and 

workstations should 

be designed so that 

workers may use 

optimum power grip 

widths (1.75–3.75 

inches). 

Figure 18. Maximum–force output for each grip width. Note that, 

for this participant, Grip 2 had the highest force production. 

Grip Types 

27
 
SECTION 4: HAND-TOOL SELECTION AND USE
 

Principles 

Select tools that allow neutral postures to be used.
 

Use tools with handles designed for a power grip.
 

Use tools with handles that are appropriately sized and shaped
 

for the user’s hand.

Use tools with built-in features (e.g., springs that open tool 

handles) that minimize forceful exertions required to use the 

tool. 

When operating heavy tools, ensure they accommodate using 

both hands to support the tool's weight. 

Hand-tool design can play an important role in the reduction of MSDs. A tool that is 

designed with consideration for the worker’s tasks can greatly reduce the worker’s exposure to 

risk factors for MSD. However, using a poorly designed tool or an inappropriate tool negatively 

impacts the entire body by dictating the postures assumed by the worker to complete the task, 

and increasing the resulting forces exerted by the worker. Such tools can also directly apply 

unwanted forces or vibrations to other body parts. 

Several factors should be considered when purchasing or selecting a hand tool. The topics 

discussed in the above sections all play a role in whether or not a hand tool is designed with the 

worker or task in mind. Some of these points will become clearer after performing the 

demonstrations in this section. Before performing these demonstrations, consider the following 

questions related to the safety of the tools you use: 

Does the orientation of the handle allow the worker to use neutral joint postures?
 

Does the size of the handle allow for the midrange of grip (1.75–3.75 inches; hand in the
 

shape of a ―C‖) width when using a power grip? 

Does the handle extend past the palm?
 

Is the handle shape contoured to fit the palm?
 

If a pinch grip is required, is the force the worker must exert < 2 lbs.?
 

For heavier tools, such as power tools, do the features of the tool allow the worker to 

support the tool’s weight with both hands? 

To obtain a detailed checklist for hand-tool selection, refer to the NIOSH publication ―Easy 

Ergonomics: A Guide to Selecting Non-Powered Hand Tools‖ [NIOSH 2004]. 

28
 
Tool-Handle Size and Shape 

Objectives 

To understand why 

handle size is 

important when 

selecting tools 

Supplies 

One screwdriver, whose handle has a diameter size that 

complements the hand and a comfortable, appropriate shape 

One screwdriver, whose handle has a smaller diameter size that 

does not complement the hand and does not have a 

comfortable, appropriate shape 

Wood block with screw 

Clamp to affix wood block to tabletop 

Step-by-Step Demonstration Method 

1. Clamp wood block and screw to tabletop. 

2. Instruct the participant to grasp the appropriately sized 

screwdriver that has the larger diameter handle. 

Instruct the participant to show the audience how 

he/she is gripping the screwdriver (Figure 19). 

3. Repeat the previous step using the other screwdriver, 

with a smaller diameter handle that does not 

complement the hand. Discuss observed differences in 

gripping the two screwdrivers. 

4. Instruct the participant to drive a screw with both 

screwdrivers. Ask them which one feels more 

comfortable in his or her hand and is easier to grasp 

when the screw starts to provide resistance. The handle 

with the larger diameter that complements the hand 

should make it easier for the participant to apply torque 

to drive the screw. 

5. Discuss with the audience the fact that selecting a hand 

tool with a handle that complements the hand reduces 

the effort needed to accomplish the task, thus reducing 

fatigue and the required muscle activity; this, in turn, 

reduces discomfort while using the tool. 

Hand-Tool Selection and Use 

29
 
Tool-Handle Size and Shape 

Take -Home 

Messages 

Tools whose handles 

are sized and shaped 

to complement the 

hand, require less 

effort to use, thereby 

reducing the muscle 

fatigue that leads to 

discomfort. 

Figure 19. Evaluating the effect of tool-handle diameter. 

Hand-Tool Selection and Use 

30
 
Tool-Handle Orientation 

Objectives 

To demonstrate how 

tool-handle 

adjustability or 

orientation may 

allow for neutral 

postures to be 

adopted 

To encourage 

workers using and 

purchasing tools to 

consider how the 

tool will be applied 

and whether or not 

a different tool, or 

tool configuration, 

would be more 

appropriate 

Supplies 

Screwdriver (battery-powered) with pistol-grip and inline-grip 

capabilities (Figure 20) 

Wood block with screw in block 

Clamp to hold wood block in place while screw is being driven 

with screwdriver 

Step-by-Step Demonstration Method 

1. Clamp the wood block to a tabletop so that the block is 

perpendicular to the table and the screw is driven parallel 

to the tabletop. 

2. Place the screwdriver in the inline position, and instruct 

the participant to begin driving the screw. 

3. Place the screwdriver in the pistol-grip position, and 

instruct the participant to begin driving the screw. 

4. Ask the participant if he or she can feel a difference 

between the two techniques. Ask the audience which grip 

would be best to use for this task (answer: pistol grip). 

5. Clamp the wood block to a tabletop so that the block is 

parallel to the table and the screw is driven perpendicular 

to the tabletop. 

6. Place the screwdriver in the inline position, and instruct 

the participant to begin driving the screw. 

7. Place the screwdriver in a pistol-grip position, and 

instruct the participant to begin driving the screw. 

8. Ask the participant if he or she feels a difference between 

the two techniques. Ask the audience which grip would 

be best to use for this task (answer could vary depending 

on height of participant relative to the tabletop which 

affects his/her wrist and shoulder angle). 

Hand-Tool Selection and Use 

31
 
Tool-Handle Size and Shape 

Take -Home 

Messages 

Adjustability in 

tools, or multiple 

tool designs, is 

important because it 

allows for neutral 

postures to be 

adopted 

When selecting or 

purchasing a tool, 

consider the ability 

of the tool’s handle 

to be adjusted in 

multiple positions to 

keep the wrist in a 

neutral posture. 

Figure 20. Examples of situations in which a pistol grip and 

inline grip would be useful as a means for keeping the wrist in a 

neutral posture. 

Note: This demonstration can be done without actually driving 

a screw into a wood block. You can ask the participant to 

simulate driving a screw into a tabletop and into the wall. The 

differences in postures can be observed and discussed by the 

audience. 

Hand-Tool Selection and Use 

32
 

 

Features to Reduce Forceful Exertions 

Objectives 

­

To increase 

awareness that some 

tools have design 

features that reduce 

forceful exertions 

when the tool is used 

to perform a task 

Supplies 

Spring-loaded, needle-nose pliers (Figure 21) 

Needle-nose pliers that are not spring loaded (Figure 21) 

Portable EMG device with audible sounds to indicate muscle 

activity (optional) (Figure 5) 

Step-by-Step Demonstration Method 

1. If using a portable EMG device, place the electrodes on 

the forearm as shown in Figure 5. 

2. Adjust the output sounds to a range where minimal 

sounds are heard when the participant wiggles his or 

her fingers. 

3. Instruct the participant to close and then open the 

spring-loaded, needle-nose pliers. 

4. Note the intensity of the sounds from the portable EMG 

device. 

5. Instruct the participant to close and then open the 

needle-nose pliers that are not spring loaded. 

6. The intensity of the sounds from the portable EMG 

device will be increased when using the nonspring 

loaded pliers compared to the spring-loaded pliers. 

When using pliers for a work activity, individuals often 

use their dominant hand to both open and close the 

pliers. Spring-loaded pliers remove the need to open 

the pliers and reduces the force requirements on the 

hand. 

7. Ask the participant if he or she can feel a difference 

between the two tools. 

Hand Tool Selection and Use 

33
 
Features to Reduce Forceful Exertions 

Take -Home 

Messages 

Select tools with 

features that reduce 

forceful exertions 

when preparing the 

tool for use and 

operating the tool 

during the task. 

8. Discuss with the audience that, even though a tool may 

be spring loaded, it may be still difficult to use if the 

resting-grip width is large (see Section 3 concerning 

grip widths). 

9. Discuss with the audience other design features that 

reduce forceful exertions, such as counterbalances, 

ratcheting tools, keyless drill chucks. 

Figure 21. Two types of pliers, one with a spring that reduces 

forceful exertion when opening the jaw, and one without a 

spring. 

NOTE: Another example of reduced forceful exertions as a 

result of design features is the insertion of a bit into a 

screwdriver that requires manual tightening of the chuck with a 

key, as compared to a screwdriver with a chuck that is simply 

pushed down and then released. 

Hand-Tool Selection and Use 

34
 
One- and Two-Handed Tools 

Objectives 

To demonstrate that 

muscle activity 

decreases when 

forces are 

distributed across 

both arms instead of 

just one arm 

To inform workers 

that they should 

purchase and use 

tools with 

appropriate power 

because too much 

power for the job 

may result in 

difficulty controlling 

the tool, increased 

fatigue, and poorquality 

workmanship 

Supplies 

Electric drill with capability to hold drill with one hand or with 

two hands using an additional handle (Figure 22) 

Wood block with a screw 

Clamp to affix wood block to a table 

Step-by-Step Demonstration Method 

1. Instruct the participant to hold the drill at waist height 

and then at shoulder height with one hand, and note the 

degree of effort required to hold the drill for both. 

2. Instruct the participant to grab the additional handle 

with the second hand, and again hold the drill at waist 

and then shoulder height. It should be easier with both 

hands on the tool because the muscles from both arms 

are now being used to hold the drill; this reduces the 

force produced by each individual muscle. 

3. Explain to the audience that, because force in each 

muscle has decreased, fatigue will set in later than if 

the task was performed with only one hand. 

4. Clamp down the wood block with the screw. 

5. Instruct the participant to drive the screw while holding 

the drill with only one hand. 

6. Instruct the participant to drive the screw while holding 

the drill with both hands. 

7. Ask the participant if he or she feels more control when 

using two hands. 

Hand-Tool Selection and Use 

35
 
Features to Reduce Forceful Exertions

Take -Home 

Messages 

Select tools that are 

properly sized in 

overall dimensions, 

weight, and power 

for the specific task. 

Too much weight 

and power can 

increase fatigue in 

the worker, and 

result in poorquality 

workmanship. 

When purchasing 

heavy power tools, 

consider features 

that allow the tool to 

be held with both 

hands. 

When operating 

heavy tools, take 

advantage of 

features that allow 

for greater control 

of the tool and less 

fatigue. 

8. Ask the audience if they perform tasks that should be 

performed with a tool that has the one- and two-handed 

design feature. 

8. Discuss the concept relating to the tool’s power and the 

worker’s ability to control the tool—as the power of the 

tool increases, a worker’s ability to control the tool 

decreases; this may result in poor-quality workmanship. 

Also, buying tools with excessive power may have 

negative consequences for the worker by requiring greater 

muscle exertions and causing an earlier onset of fatigue. 

Figure 22. Example of one-handed and two-handed drilling. 

Hand-Tool Selection and Use 

36
 
SECTION 5: FATIGUE FAILURE AND BACK PAIN
 

Principles 

Repeated lifting, even at submaximal levels, may eventually 

lead to damage of the spine (fatigue failure). 

Substantially reducing loads placed on the spine can greatly 

minimize the risk of fatigue failure. 

The spine consists of a column of bones called vertebrae that are separated by flexible discs 

(Figure 23). 

Figure 23. Image of vertebrae, disc, and endplates. 

The discs serve as cushions and allow the spine to assume many postures. 

Degeneration of these discs is a common source of back pain, which is thought to result from a 

loss of disc nutrition [Adams et al. 2006]. Because discs do not have a blood supply, they rely 

on obtaining their nutrition from the adjacent bones (or vertebrae). Normally, nutrients flow 

from the vertebrae to the disc through structures called vertebral endplates. The endplates are on 

the top and bottom of each vertebrate. Unfortunately, these endplates may fracture if an 

excessive force or repeated loads are placed on them by the contracting back muscles, as 

occurs when lifting [Brinckmann et al. 1989]. 

Researchers believe that endplate fractures usually occur through repeated loading, by a 

process known as fatigue failure [Bogduk 1997; Brinckmann et al. 1988; Adams et al. 1995, 

2006; Gallagher et al. 2005; Marras 2008]. Fatigue failure begins when a load causes a small 

crack in a vertebral endplate. Subsequent loads (e.g., repeated lifting) will cause this crack to 

expand, leading to a large fracture [Brinckmann et al. 1988]. The body heals this fracture with 

scar tissue, but the scar tissue does not allow nutrients to get to the disc, causing it to degenerate 

[Bogduk 1997]. As the disc degenerates, fissures and tears in the disc will begin to appear. 

When these fissures or tears extend to (or occur in) the outer portions of the disc, a painful 

inflammation may occur. Unfortunately, because the disc has a decreased blood supply, repair 

of the tissues is a slow process [Bogduk 1997]. At the same time, the disc often continues to 

become loaded during activities of daily living, which may result in additional damage to the 

disc, even while repairs are ongoing. The slow healing, combined with continuous loading and 

trauma to the tissues, is thought to lead to a vicious cycle of chronic pain and inflammation 

[Barr and Barbe 2004]. 

37
 
Fatigue Failure 

Objectives 

To introduce 

workers to the 

concept of fatigue 

failure 

To reinforce the 

importance of 

minimizing object 

weight, lever 

arm, barriers, and 

repetition of manual 

lifting tasks 

Supplies 

One pen cap 

One paper clip for each audience member 

Step-by-Step Demonstration Method 

1. Take intact pen cap and bend ―tail‖ once. 

2. Show audience members that there is a discoloration at 

the spot where the bending occurred, which is a visual 

example of subfailures occurring. 

3. Continue to bend the pen cap about five times, and then 

show the audience that the discoloration has expanded. 

4. Explain to the audience that, if you continue to bend 

the pen cap, it would eventually fail. 

5. Explain to the audience that, for some materials (e.g. 

paper clip, vertebrae), fatigue failure is not visible. 

6. Distribute one paper clip to each audience member. 

7. Ask the audience to bend the paper clip back and forth, 

and count the number of cycles it can withstand before 

breaking. 

8. Ask various audience members how many cycles it 

took before the paper clip failed; emphasize that the 

number of cycles varies for the paper clips as no one 

paper clip is exactly the same as another. This is also 

true for people and their vertebrae. Just like with the 

paper clips, some worker’s will experience fractures in 

their vertebrae very quickly as others require many 

cycles despite undergoing the same loading conditions. 

9. Show the graph in Figure 24 to the audience. 

Fatigue Failure and Back Pain 

38
 
Fatigue Failure 

10. Explain that every type of material has an ultimate load (i.e., the load at which it 

fails when that load is applied only once). The graph in Figure 24 illustrates the 

amount of loads the spine can handle without breaking. Because every spine is 

unique, the ultimate load varies somewhat for each spine. Thus, the y-axis of this 

graph represents the percentage of ultimate load. The x-axis represents the 

number of cycles a load was applied. For example, if you applied a load to a 

spine that was 80% of its ultimate load, you would be able to apply that load 

100 times before the spine would fail. Likewise, if you applied a load that was 

only 50% of its ultimate load, you could apply that load 1,000 times before 

failure. If you applied a load that was only 30% of its ultimate load, you could 

complete an infinite number of loading cycles without the spine ever failing. 

11. Explain to the audience that this means they are not ―doomed to having a back 

injury.‖ Rather, if the load applied to the spine is decreased substantially, they 

could perform their job an infinite number of times and never injure their spine. 

You may also increase your core strength to better handle loads—a balanced 

body in terms of abdominal and back strength makes a more stable core when 

trained together. 

12. Discuss ways to reduce the load applied to the spine, such as decreasing the 

weight of the object, reducing the moment arm (see Section 6), removing 

barriers, and eliminating twisting and back flexion. 

Fatigue Failure and Back Pain 

39
 
Fatigue Failure 

Take -Home 

Messages 

Often, the vertebrae 

of the back can have 

multiple subfailures 

that are not visible 

but can result in 

complete failure 

over time. 

The number of 

cycles that lead to 

failure of the 

vertebrae varies 

across the 

population. 

Efforts should be 

made to 

substantially 

decrease loading of 

the spine. 

Figure 24. A pen cap that is bent multiple times visually shows 

fatigue; a paper clip shows the result of failure. The graph 

(generalized for bone) illustrates how the same load, lifted 

many times, may ultimately, over time, lead to failure. 

Fatigue Failure and Back Pain 

40
 
SECTION 6: MOMENT ARMS AND LIFTING
 

Principles
 

Reduce the weight of the object being lifted.
 

Keep loads close to the body when lifting.
 

The best way to prevent low-back pain is to prevent the initial fatigue failure of the vertebral 

endplates. In general, for a given task, if the forces exerted by back muscles are high (e.g., in 

heavy lifting), fatigue failure will occur more quickly. However, if forces produced by the lowback muscles are decreased, the risk of injury also decreases [Brinckmann et al. 1988]. 

Forces produced by the lower back muscles can be reduced by minimizing the weight being 

lifted or carried. However, those forces can also be reduced by minimizing the moment (see 

Glossary) or by minimizing the moment arm (i.e., lever arm). When lifting an object, as shown 

in Figure 25, the moment arm is the horizontal distance between the object and the person. As 

this distance increases, the moment (i.e., torque), involving the worker’s back also increases. 

The muscles of the lower back must produce more force to counteract this moment so that the 

person does not fall forward. Even light objects can cause large forces in the lower back if those 

objects are lifted or carried farther away from the body. 

Weight and moment arm are not the only considerations in determining forces produced by 

the lower back muscles. Other factors, mostly related to the object being lifted, should also be 

briefly mentioned. The size and shape of the object and the handholds on the object affect the 

worker’s lifting style. Also, the existence of physical barriers that separate the worker from the 

object to be lifted plays a role in the forces exerted in lifting the object because barriers force a 

worker to hold an object farther away from his or her body while the worker moves the object 

over the barrier. A barrier often requires the worker to lift or hold an object incorrectly. The 

distribution of the weight across the object itself is also a consideration because an awkward 

weight distribution can also cause the worker to lift and carry the object incorrectly. Detailed 

information about these factors can be found in Waters et al. [1993] and [NIOSH 1994]. 

41
 
Figure 25. These schematics illustrate how increasing the distance between the worker and the 

object being lifted increases the overall moment (i.e., torque) for which the back muscles must 

compensate by expending more force. 

42
 
Moment Arms 

Objectives 

To introduce 

workers to the 

concept of moments 

and moment arms 

Supplies 

A moment-arm simulator (Figure 26) 

Three rectangular blocks of equal weight 

Step-by-Step Demonstration Method. See Introduction 

video. 

1. Place two of the three blocks on opposite sides of the 

fulcrum at equal distance from the fulcrum of the 

moment-arm simulator. 

2. The moment-arm simulator should be perfectly 

balanced. 

3. Move one of the blocks to twice the distance from the 

fulcrum. 

4. Note that the ―see-saw‖ will tip towards the block that 

is furthest from the fulcrum. This occurs because the 

moment arm (i.e., the distance from fulcrum) is larger 

for this block. Thus, the moment, or torque, produced 

by this block is greater than that of the second block. 

5. Add a second block to the side of the moment-arm 

simulator with the shortest distance from the fulcrum. 

6. Note that the moment-arm simulator will now balance 

again, indicating that it is capable of withstanding 

twice as much force because the moment arm is half 

as long. 

7. Discuss with the audience the point that the weight of 

the object is not the only consideration in producing 

forces on the body—as the horizontal distance 

increases, the resulting moment also increases. 

Moment Arms and Lifting 

43
 
Moment Arms 

Take -Home 

Messages 

During manual 

material handling, 

reduce the moment 

arm as much as 

possible by reducing 

the load on the 

lower back and 

keeping the load 

close to the body. 

Design workstations 

and storage facilities 

that allow the 

worker to keep 

objects close to his 

or her body when 

lifting them. 

Figure 26. A moment-arm simulator showing that more 

force/weight (W; arrow indicates direction of force) is needed to 

balance the “see saw” if the moment arm (L) is shorter on one 

side of the fulcrum as compared to the other side. 

Moment Arms and Lifting 

44
 
Moment Arms and the Low Back 

Objectives 

To introduce the 

effect of moment 

arm on forces 

exerted by lower 

back muscles when 

lifting or carrying 

an object 

To emphasize that 

the weight of an 

object is not the only 

consideration in 

determining forces 

produced by the 

lower back; the 

position of the 

weight of the load in 

relation to the body 

also affects the 

forces and stresses 

in the low back 

To discuss factors 

that may increase 

the moment arm 

and the resulting 

forces exerted by the 

low-back muscles 

Supplies 

A moment-arm simulator made from aluminum (Figure 27) 

Spring scale 

Metal weights 

Step-by-Step Demonstration Method. See Introduction 

video. 

1. Place known weights midway between the fulcrum and 

the end of the horizontal bench. 

2. The moment-arm simulator should be perfectly 

balanced as the spring scale undergoes loading. 

3. Note the force in the spring scale. 

4. Move the weights farther away from the fulcrum. 

5. Again, the moment-arm simulator should be perfectly 

balanced; however, the force in the spring scale should 

increase. 

6. Discuss with the audience that the only change made 

was the moment arm, indicating that weight is not the 

only factor affecting how much force must be exerted. 

7. Relate the spring scale to the low-back muscles, 

fulcrum to the vertebrae, and weight to an object being 

carried. 

8. Discuss with the audience how the forces exerted by 

the low-back muscles must increase as an object is 

moved farther away from the pelvis. 

Moment Arms and Lifting 

45
 
Moment Arms and the Low Back 

Take -Home 

Messages 

The length of the 

moment arm and 

weight of the object 

both affect the 

forces exerted by the 

lower-back muscles. 

The size and shape 

of the object lifted 

or carried, existence 

of barriers, and 

design of 

workstations are all 

factors that affect 

the moment arm of 

an object being 

lifted or carried. 

9. Discuss with the audience the factors that may 

increase the moment arm when attempting to 

lift/carry objects—size and shape of the object, 

existence of a barrier, methods used to complete 

tasks, or design of workstations. 

Figure 27. Moment-arm simulator with dial scale showing that, 

as the moment arm is increased, the resulting force acting on 

the scale increases. 

Moment Arms and Lifting 

46
 
 

APPENDIX A: SUGGESTED SUPPLIES
 

All supplies needed for the demonstrations are included in the following list. Purchasing 

information is also provided, although most of the supplies can be purchased at hardware 

1 stores . 

1 The National Institute for Occupational Safety and Health does not endorse any manufacturer or supplier of these products. When using the 

general guidelines provided above, it is the responsibility of the end user to assess the products they intend to purchase and use. 

47
 

Hand dynamometer (grip type)—evaluates grip strength for multiple grip widths (Figure 

A-2 A and B). 

o	 This device measures the hand force generated by a power grip (i.e., where the 

user curls the fingers towards the palm). The force generated is displayed on a 

dial or on a digital output. If attempting to locate this device using an Internet 

search engine (e.g., www.google.com, www.yahoo.com, www.ask.com), the 

following phrase may be helpful when searching for a vendor—―hand 

dynamometer grip width.‖ You may also consider adding the keywords 

―adjustable‖ or ―multiple‖. This item costs from $225 to $375, depending on the 

manufacturer and the number of grip widths the dynamometer can evaluate. 

Among common suppliers of dynamometers, identified from an internet search, 

are Baseline Tool Company, Medline Industries, and Sammons Preston Rolyan. 

o	 This device may also be used to evaluate pinch grip, by adjusting the width of 

the grip to its minimum. 

Hand dynamometer (pinch type)—evaluates pinch grip strength (Figure A-2C). 

o	 This device measures the force generated by a pinch grip (i.e., where the user 

presses the thumb against the index finger). The force generated is displayed on a 

dial or on a digital output. If attempting to find this device using an Internet 

search engine (e.g., www.google.com, www.yahoo.com, www.ask.com), the 

following phrase may be helpful when searching for a vendor—―dynamometer 

pinch grip.‖ You may also consider adding the keywords ―strength‖ or ―price.‖ 

This item costs from $250 to $350 depending on the manufacturer and the 

maximum force measured by the device. Among common supplier of pinch type 

dynamometers, identified from an internet search, are Dynatronics, Baseline 

Tool Company, and Jamar. 

Traditional screwdrivers—varying handle diameters (Figure A-2D). 

o	 Traditional screwdrivers are designed with a hard, plastic handle. Many 

manufacturers offer screwdrivers with various handle diameters and working-end 

size (i.e., a larger handle diameter corresponds to a larger working-end size). 

However, other manufacturers attempt to keep the handle diameter as large as 
possible. Screwdrivers with different handle diameters, but similar-sized working 

ends, should be obtained for the demonstrations. These items may be purchased 

at any standard hardware store for about $20 each. 

Screwdriver—can convert to either an inline or pistol grip (Figure A-2 E and F). 

o	 Screwdrivers that can be adjusted from an inline to a pistol grip are available at 

most standard hardware stores. This item costs approximately $50. 

Electric drill—can be held with both hands when additional support is needed (Figure 

A-2 G and H). 

o	 Electric drills that have a second handle to allow support from both hands may 

be purchased at any hardware store. This item costs approximately $100 to $200. 

Needle-nose pliers—with and without a spring-loaded handle (Figure A-2 I and J). 

Needle-nose pliers may be purchased at a hardware store. This item may be purchased 

for approximately $20. 

Portable electromyography (EMG) device—battery-operated device with audible 

feedback where the intensity of the sounds produced by the device increases with 

increased muscle activity (Figure A-2K). 

o	 When using electrodes placed on the surface of the skin (with tape similar to a 

Band-Aid), the amount of muscle activity may be measured when a worker is at 

rest and while performing a task. The amount of muscle activity experienced by 

the worker is conveyed with an audible sound. As muscle activity increases, so 

does the intensity of the sound produced by the device. If attempting to locate 

this device using an Internet search engine (e.g., www.google.com, 

www.yahoo.com, www.ask.com), the following phrase may be helpful when 

finding a vendor—―portable EMG audible‖. This item costs from $300 to $450 

depending on the manufacturer. For the purpose of this document, the ―pocket 

ergometer‖ from AliMed was used; however, NIOSH does not endorse any 

1 specific manufacturer . However, a supply of disposable surface electrodes must 

also be purchased. These typically come in packs of 50, 100, or 1,000. A pack of 

100 costs about $10. If using an Internet search engine, the following phrase may 

be helpful—―EMG disposable surface electrode.‖ Among the common suppliers 

of electrodes, identified from an internet search, are Nikomed USA, Inc. and 

Biopac Systems, Inc. 

o	 The product directions should be followed for the specific device purchased. In 

general, the device will likely consist of a small box, a cable with three leads at 

the end, and a package of electrodes. The small box houses the signal processing 

and output capabilities of the unit. Three electrodes should be placed on the skin 

above the same muscle or muscle group. Once affixed to the skin, the leads of 

the cable should be connected to the electrodes. The cable will likely consist of 

two red leads and one black lead. The black lead should be connected to the 

48
 
electrode that is midway between the other two (Figure A-2K). The red leads 

should be connected to the electrodes on either side. Once the device is turned 

on, the audible output may need to be adjusted based on the amount of activity 

associated with the specific muscle or muscle group being evaluated. 

Standard weights or custom-made blocks (3.5 in x 5 in x 1 in) – four steel blocks 

weighing approximately 5 lbs each. 

o	 Standard weights may be purchased at any store that sells supplies for weight 

training. 

o	 Custom steel blocks can be constructed from steel stock purchased at most larger 

hardware stores. Some also offer the service of cutting these items to size for 

their customers. Other materials may also be used for this task as long as the 

weight and size of all blocks are uniform. 

Dial scale—must have the ability to attach to objects at either end of the scale (Figure A

2L). 

o	 This item is best purchased using an online source. Use an Internet search engine 

(e.g., www.google.com, www.yahoo.com, www.ask.com). The following search 

phrase may be helpful when finding a vendor—―hanging spring dial scale.‖ This 

item may cost from $10 to $50, depending on the manufacturer and the 

maximum force that the device measures. For these demonstrations, a 10-lb 

capacity or greater is recommended. Among the common suppliers of this 

device, identified from an internet search, are Detecto, Global Industrial, 

Calibex, and Salter Brecknell Mechanical Scales. 

Moment-arm simulator that is 40 in long (20 in from fulcrum to end x 5 in width)—must be 

able to support the weight of the four steel blocks (Figures A-1 and A-2L). 

­

Figure A-1. Dimensions for the moment-arm simulator. 

49
 
o	 This item may be custom-made. Suggested dimensions complement the 

dimensions provided above for the steel blocks. Additionally, this size allows the 

device to be viewed by all audience members when training is given in a room 

that is the size of a standard classroom. The ―see-saw‖ should be able to move 

freely about its fulcrum. Aluminum stock may be purchased at most larger 

hardware stores; some stores also offer the service of cutting these items to size 

for their customers. Alternatively, this device may be made from wood at a less 

expensive cost. Materials other than aluminum may be used. Aluminum was 

suggested due to its relatively light weight and low cost. You should mark the 

locations along the see-saw that are 10 inches and 15 inches from the fulcrum as 

a visual aid for placing the steel blocks during the demonstrations. 

o	 Other similar devices may be purchased for less than $100. An internet search 

found that common suppliers included Fisher Scientific. 

Pen cap—must be plastic and easy to bend (Figure A-2M) 

o	 Remove from the top of a pen 

Paper clip—standard size (Figure A-2N) 

o	 Metal, uncoated paper clips 

50
 
Figure A-2. Examples of the suggested supplies for the demonstrations. 

51
 
APPENDIX B: USEFUL IMAGES FOR HANDOUTS
 

52
 
53
 
54
 
55
 
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57
 
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