Contents

The Basics

Classes of Levers

Muscles

Risk Factors

The Fingers and Hand

The Wrist

The Elbow

The Shoulder

The Neck

The Back

Conclusion

References

Acknowledgment

 

 

© Copyright Washington Ergonomics 1994. All rights reserved. Rev 1 1998


Copyright Waiver of First Sale Doctrine
Washington Ergonomics materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser. Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.


Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations. Other or additional safety measures may be required under particular circumstances.

 


Table of Contents

Introduction

The intent of this book is to help those involved in the design, construction or operation of a workplace to better understand how the human machine functions. Knowing the strengths and weaknesses of the human body will help readers design and maintain safer and more productive workplaces.  Also, utilizing these concepts will be a useful foundation for any workplace ergonomics improvement effort.

 I have found over years of ergonomics training that most people understand the design of mechanical systems better than the design of the human body.  For example, almost everyone understands how a hinge works, but many find an anatomical view of the human elbow confusing.  Therefore, this book compares joints, tendons, ligaments, and muscles of the body to common machine designs.  The simplified drawings of anatomy will provide the reader with a basic understanding of the capabilities and the limitations of the human body.

 The reader must also be aware that the ideal workplace design will not guarantee prevention of all injuries.  The individual doing the task also represents a risk factor.  Variables such as individual height, weight, strength, and physical fitness can influence the chance of injury.  Additional programs such as safety and health training, early symptom recognition and reporting, proper medical management and other programs aimed at injury reduction must also be implemented. 

The Basics

 Being asked to think of the human body as if it were a machine sounds rather callous.  However, most people understand the strengths and limitations of machines, but they do not have the same understanding of the human body.  This lack of understanding of human body mechanics often results in less consideration being given to people than to machines.  Therefore this book will demonstrate the similarities between machines and the human body with the hope that increased understanding will lead to increased respect for the body�s strengths and limitations in the workplace. 

Both a machine and the body use joints to enable movement.  In the human machine, a joint is a junction between two or more bony surfaces.  Ligaments are the supports that hold the bones of a joint together.  Muscles provide the power that moves the joint, and tendons are the cables that attach the muscles to the bones. The signal to move is carried to the muscle by nerves (See Figure 1).

Figure 1  Mechanical interpretation of a joint.

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

Table of Contents

Joints Work Like Levers

Engineering has defined three types of levers to describe the mechanical movement between two pieces of structure.  These three types of levers are referred to as class one, class two, and class three lever systems.  Figure 2 illustrates each of the three classes of levers, the conditions that typically use these levers, and a human joint that has a similar design. 

CLASS OF LEVER

COMPARISON

HUMAN JOINT

 
Figure 2  Three classes of levers.

The following table compares the advantages and disadvantages of each class of mechanical lever.  Similar advantages and disadvantages apply to human joints. 

CLASS OF LEVER

ADVANTAGE

DISADVANTAGE

Class One

Stable and strong.  Increasing the length of the lever arm will increase the mechanical advantage.

Slow acting and limited flexibility.

Class Two

Stable and strong.  Increasing the length of the lever arm will increase the mechanical advantage.

Slow acting and limited flexibility.

Class Three

Fast movement and large range of motion.  Increasing the length of the lever arm will decrease the mechanical advantage.

The amount of force applied must always exceed the weight of the load.

The body contains each of the three lever systems.  However, there are very few class one or two lever systems.  The majority of joints are class three levers. This is the class of lever that gives us the flexibility and speed of movement in the elbow and shoulder joint to throw a ball.  Unfortunately, the price of this flexibility is weakness, instability and the potential for an injury such as a dislocated shoulder joint.

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

Table of Contents

Muscles Pull on Joints Like Single Action Solenoids

Muscles can only pull, they cannot push.  They are similar to a single action solenoid (See Figure 3).  For example, when electrical current is applied to the solenoid, the operating arm will pull with force.  Since this is a single action cylinder, returning the arm to its original position requires that the electrical current be removed and an external force be applied that will pull the arm back out.  Muscles work the same way.  The muscle can only apply force to a bone, such as the forearm, while that muscle is contracting. 

 
 

Figure 3  Muscles can only pull, they cannot push.

 

If muscles can only pull and they cannot push, try to determine how you are able to stick out your tongue.  There are muscles in the neck that thrust the tongue out of the mouth as they contract.

Since muscles can only provide power while contracting, they must work in pairs.  One set of muscles, called the agonist, must contract for the initial movement.  Then the agonist must relax while another set of muscles, called the antagonist, contracts to return the bones to their original position (See Figure 4).  The flexor muscles pull to move the hand downward from the neutral wrist position.  To return the hand to the neutral wrist position, the extensor muscles must pull.  Seldom are the two muscle groups (agonist and antagonist) of equal size or strength.  Notice that the flexor muscles on the underside of your forearm arm are much larger than the extensor muscles on the top of your forearm arm.  Are you stronger moving your hand downward or moving it upward?  The majority of people are stronger moving their hand downward because their flexor muscles are larger and more developed than their extensor muscles.


Recommendation

Always consider the direction of movement when designing for the application of force.  This is necessary because it is normally more efficient to move a joint in one direction than it is in the other.  Therefore, design the task so that the majority of force is applied only in the stronger of the two directions.  For example, it is easier to close the fingers than to open the fingers.  That may be part of the reason why opening scissors can be more stressful to the fingers and hand than closing the scissors.

Range of Motion

While muscles and solenoids are similar in action, there are some important differences.  Normally, muscles are strongest when in the middle of their range of motion (See Figure 5).  As the muscle moves (extended or contracted) from the neutral position it loses strength.  This is why it is important to consider where force must be applied within a muscle�s range of motion.

Also, the length of time and/or the frequency that a muscle must provide force will affect its ability to work.  If the muscle is moved too frequently, the result is dynamic fatigue.  Digging a ditch leads to dynamic fatigue.  If the muscle must maintain a constant force, the result is static fatigue.  Holding the handle of a heavy suitcase for a long period of time leads to static fatigue.

There is also a second type of static fatigue.  This is the static fatigue caused by not moving a muscle.  Muscles must move to maintain the proper blood circulation within that muscle and to other parts of the body.  Without a normal amount of movement, the muscle will not receive the proper blood supply and eventually will atrophy and lose strength.  Have you noticed the loss of strength and reduced muscle size after removing a cast from a broken arm or leg?

To keep muscles healthy and reduce fatigue, work should be designed to allow the muscles occasionally to move easily throughout their full range of motion.  This will reduce static fatigue.  If a task requires force, the task should be designed so the worker can apply the force in the power range of motion and in the stronger direction.  Look at Figure 5 and notice the power range, then try to determine which is the stronger direction of movement within that range.  Are you stronger moving your hand toward the shoulder or away from the shoulder?  Toward the shoulder is the stronger direction for most people.

Figure 5  Muscles are strongest in the middle of their range of motion.

Demonstration


Use your left hand to hold the first two fingers of your right hand (see Figure 6).  Squeeze your hand tightly enough so that you have difficulty pulling your fingers free.  Then, bend your left wrist to a 900 angle and squeeze the two fingers of your right hand again (see Figure 7). With your wrist bent, you will notice a considerable loss of grip force, and you will be able to pull your fingers free easily.  You may even notice a slight pain in your left forearm as the muscles strain to maintain your grip. 


Figure 6  Left wrist in
neutral position.

When your wrist is straight, you are able to apply maximum gripping force with your fingers.  This is because the tendons that connect the muscles of the forearm to the fingers are in a straight line, and the muscles of the forearm are neither contracted nor extended.  When your wrist is bent, the muscles of the arm must work harder because the force these muscles can apply is reduced.  Notice that when you bend your wrist downward, the muscles of your lower forearm are contracted and the muscles of the upper forearm are stretched (Figure 8).  Working with a bent wrist reduces efficiency and increases the chance of injury to the tendons, nerves and/or muscles of the forearm.

Figure 7  Left wrist out of neutral position.


Figure 8  Working with the hand flexed reduces strength.

Recommendation

Consider the advantages that are gained when you modify the workplace to allow the operator to use her or his muscles in the power range and optimum direction of movement.  Not only will the operator be able to apply more force, but there will also be less chance of discomfort or injury.

 


 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

Table of Contents

Overview of Basic Ergonomic Risk Factors

There are four basic, interrelated risk factors involved in the chance of injury to the tendons, muscles, or ligaments of a joint.  These are:

1.      Position:  Every joint has an ideal position.  This ideal position avoids applying stress (stretching and/or bending) to the associated tendons, nerves and blood vessels.  Also the muscles used to maintain that position are neither extended or contracted.  For example, your wrist is in the ideal position when it forms a straight line with your forearm.  The more the wrist is bent (deviated) from this straight line position the poorer that position becomes.  Poor position causes a loss of strength and efficiency and increases the chance of injury.  If this poor position is coupled with high repetition, high force or long duration, the chance of injury is greatly increased.  However, if this position is used with low repetition, low force, and for only a short period of time, the chance of injury is low.

2.      Force:  Muscles must contract to apply a certain amount of force.  The definition of low, medium and high forces is difficult to define accurately because these levels often vary with the individual doing the task.         

Working muscles at a low force is good for the muscles and generally safest.  The application of increased force tends to increase risk of injury.  However, even low or moderate force can lead to injury if it is coupled with poor position, high repetition or a long duration.  

External force can also be a risk factor, such as a contact force caused by sharp edges on handles, tables or other objects.  These external forces can increase the chance of injury by pressing on tendons, nerves, blood vessels or even the skin and muscles.

3.      Repetition:  Injury can result from either too much or too little repetition of movement.  Not moving muscles leads to �static� fatigue, poor circulation, and eventually muscle atrophy.  Constant movement of muscles leads to �dynamic� fatigue.  Both static and dynamic loading of muscles, coupled with poor position, unacceptable force, or long duration, can lead to an increased chance of injury.

4.      Duration:  The chance of injury is increased if the task is performed long term.  For example, raising your arms over your head is normally only a problem if your arms remain overhead for a long period of time or if there is high force or high frequency involved.

Caution: Any new task usually requires a period of time for the muscles and tendons to adjust to the new demands.  The first time a person performs a new task there is likely to be mild fatigue to those muscles not normally used.  For example, the first softball game of the season will generally cause sore, tired muscles and tendons.

There are four additional risk factors to consider.  These are:

 1.      Individual:  The individual doing the task also represents a risk factor.  Each person has her or his own unique characteristics such as height, reach, strength and physical fitness.  Therefore the chance of injury varies, even for the same task, when performed by different individuals.  However, modifying the individual is usually not considered an acceptable intervention.  For example, using backbelts or wrist braces is not considered ergonomic intervention and should be left to the discretion of the individual and her or his health care provider.

2.      Environment:  The environment in which the work is performed also represents a risk factor.  For example, cold and damp environments may contribute to the chance of injury.

3.      Vibration:  There are two types of vibration.  Segmental vibration is a vibration that affects a small portion of the body.  Certain powered hand tools cause segmental vibration to the hand or arm.  Whole body vibration affects the entire body.  Driving a truck, forklift, or flying a helicopter are good examples of whole body vibration. 

4.      Work Organization:  Some less obvious risk factors may also contribute to the chance of injury.  Shift schedules, overtime, machine pacing, performance tracking, personal problems, time pressures and other factors can all affect physical and mental stress and lead to the chance of injury.

One equation for determining the chance of injury may be expressed as follows: 

CI = (PFRD) (IEVW)

CI =

Chance of Injury

P =

Position
For this text the word �position� refers to the angle of a single joint.  For example, if you bend your wrist, you will move it to a different position.

F =

Force

R =

Repetition

D =

Duration

I =

Individual

E =

Environment

V =

Vibration

W =

Work Organization

Several workplace analysis systems have attempted to assign values and weighting factors to this type of equation.  For example, the severity of each of the risk factors �position� through �work organization� could be rated on a scale of 1 to 10 and the number inserted into the equation.  Once the math is completed, it could be assumed that a higher number represents an increased chance of injury.  However, because the relationship between each of these factors is so complex, there is no reliable way of using the formula to predict accurately the chance of injury.  However, with increased understanding of each of these risk factors, the reader can identify those areas that need improving and attempt to reduce any or all of the risk factors.  Reducing these risk factors is far more important than rating the risk factors, especially if your goal is a safer and more productive workplace.

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

 

 

Table of Contents

The Fingers and Hand

 

The large muscles that provide the power to curl the fingers and thumb for gripping are located in the forearm.  These muscles are connected to the fingers with long tendons (See Figure 9). The small muscles located in the hand spread and close the fingers and help with certain complex movements of the thumb and little finger (See Figure 9).

Figure 9  Muscles pull tendons to open and close the fingers.

 

The muscles and tendons shown in Figure 9 are on the palm side of the hand and are used to close the fingers.  To open the fingers, there is a similar design of muscles and tendons on the back side of the hand.  The tendons that connect the muscles of the forearm to the fingers on the back side of the hand are interconnected (See Figure 10).  This interconnection improves the ease of making certain movements, but makes others difficult.  For example, grip the first two fingers of your right hand as you did in the demonstration shown in Figure 6 and squeeze.  Then, lift only the little finger of your gripping hand, as if you were
using it to point, and try to grip again.  You should notice a significant reduction of grip strength.  Imagine the extra effort required if a tool handle did not allow the little finger to close with the rest of the fingers.

The fingers are used for gripping objects with power or for precision movements.  There are three main categories of grips: pinch, power and open.


Figure 10  Some tendons are interconnected.

 

1.      Pinch:  Fingertips are  close to or touching the thumb, useful for accurate or fine adjustments.  This is a very precise, but weak position (See Figure 11).  Pinch grip tasks, such as a dental hygienist cleaning your teeth, create conflicting demands of precision and force.  The dental hygienist must use a small diameter tool for the required precision, yet often must apply a high force.

2.      Power:  Hand forms a fist, creating the strongest grip, useful for carrying a suitcase.  As the name implies this is the most powerful position of the hand.  Precision movements are difficult when using this grip (See Figure 12).

Open:  Fingers and thumb are extended and not opposed.  This is the grip you would use to carry a large box that does not have handles.  The hand is fairly strong in this position, but because the thumb and fingers are not opposed there is no clamping action, and the object could easily slip from the hand (See Figure 13). 

 


Figure 11
Pinch grip


Figure 12
Power grip


Figure 13
Open grip

The large muscles that provide the power to move the hand up, down, and side to side are located in the forearm.  These muscles are connected to the hand with a series of tendons (See Figure 14).

Figure 14  Muscles in the forearm are connected to the fingers and hand.

 

On the palm side of the hand there are two tendons to each finger and one tendon to the thumb.  The tendon that extends to the end of the finger helps you curl each joint of your finger to make a fist.  The tendon to the middle joint helps you flex the entire finger, as you would to play a piano or tap your fingers on the table. 

The one tendon that runs from the muscles in the forearm to the thumb helps to flex the thumb.  The reason there is only one tendon coming from the muscles of the forearm is the need for flexibility.  The other muscles that help move the thumb are located in the base of the thumb.  This close proximity of muscles to the thumb provides the needed flexibility.

The joints of the fingers and hand are either a class one, two or three lever system, depending on the position of the fingers and the position of the load within the hand.  For example, if the load is concentrated toward the wrist, it could be a class one lever system (See Figure 15).  If the load is concentrated toward the center of the hand, it could be a class two lever system.  And if the load is at the end of the fingers, it could be a class three lever system.

 

Figure 15  The fingers and hand are a class one, two or three lever system depending on the position of the load in the hand.

When the load is concentrated toward the end of the fingers, it is a class three lever system, flexible but weak.  This would be similar to a �pinch� grip, such as holding a pencil.  If the load is toward the wrist, and the fingers can curl around it, a �power� grip can be used.  This is a class one or two lever system, stronger but less flexible. 

 

Recommendation

The pinch grip is useful for picking up a small object or manipulating small precise tools, but can be very stressful to the muscles and joints when lifting or manipulating heavy objects.  Think of carrying a bowling ball with your fingers in the finger holes for any length of time.

Using the power grip as opposed to the other grips makes a significant difference in a person�s ability to lift or carry an object.  For example, carrying a 35 pound suitcase with a good handle is much different than carrying a 35 pound sack of sand without a handle. 

Figure 16 illustrates examples of the finger�s range of motion.

Figure 16  Range of motion for the fingers.

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

 

 

Table of Contents

The Wrist

The wrist joint is formed by the bones of the forearm (radius and ulna bones) and the eight bones of the wrist.  This joint allows the hand to move up and down, and from side to side.  However, it does not allow the hand to twist (See Figure 17).  The common movement of turning a screwdriver is often thought of as twisting the wrist.  However, this twisting actually takes place at the elbow joint.  Try turning a screwdriver while holding your forearm still.

 

Figure 17  Anatomical view of wrist joint.

 

The tendons that close the thumb and fingers (flexion) pass through an area in the base of the wrist called the carpal tunnel.  The word carpal simply means �wrist.�  The carpal bones form a small tunnel through which nine tendons and the median nerve pass (See Figure 18).  Continuous and/or stressful movements of the wrist or fingers can cause these tendons to swell (inflame) and place pressure on the median nerve.  If this pressure is severe or remains for a long period of time, there is a potential for damage to the nerve.  This pressure on the nerve causes numbness or tingling in the fingers similar to the feeling you have when you wake up after having slept on your hand.  The difference is this numbness does not go away by simply shaking your hand.

 

 

Figure 18  Nine tendons and the median nerve pass through the carpal tunnel.

While the wrist is capable of a variety of movements, there is a price to pay for this flexibility.  As the wrist moves from its neutral (straight) position to a bent position, the following occurs:

1.      Grip strength is reduced because the muscles must contract to bend the wrist, moving these muscles out of their ideal range of motion (See page 7, Range of Motion). 

2.      Muscle fatigue is increased because muscles that are contracted are less efficient and therefore must work harder to provide the same force.

3.      The tendons that pass through the carpal tunnel are more likely to inflame due to the additional friction caused by the bending of the tendon.  This inflammation may place pressure on the median nerve causing numbness and tingling.


Recommendation

Tasks that require bending the wrist and/or frequent movement of the fingers can lead to swelling of the tendons that pass through the carpal tunnel.  The result can be pressure on the median nerve leading to numbness and tingling.  Activities that result in numbness or tingling in the hand should be modified or eliminated.  For instance, consider adjusting furniture or equipment to allow the task to be performed while the wrist is straight. 

 

WARNING:  Numbness and tingling are symptoms of a potentially dangerous injury.  Medical attention is recommended if the symptoms are severe or remain for several days.

See Figure 19 for examples of the wrist�s range of motion.

 

Figure 19  Range of motion for the wrist.

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

 

 

Table of Contents

The Elbow

 

The elbow is actually three joints.  The first joint is similar to a door hinge.  It allows the arm to bend and straighten.  The other two joints are between the upper arm bone (humerus) and the two bones of the forearm (ulna and radius).  These two joints allow the forearm to rotate.  The combination of the three joints allows the forearm to move up and down and to rotate, all at the same time (See Fig 20).

 

Figure 20  Anatomical view of elbow joint.

 

Muscles seldom act alone when moving a joint.  In the case of elbow flexion (forearm moved upward toward the biceps), the two prime movers are the biceps brachii and the brachialis muscles (See Figure 20).  Because of this interrelationship between muscles and the way these muscles are attached to the bones of the forearm, there are certain positions of the elbow joint that are stronger than others.  For example, with the palm of the hand facing up (supination), the hand and forearm can support a heavier load than when the hand is facing down (pronation).  This strength difference is due to the way the tendons of the biceps brachii and brachialis muscles are attached to the bones of the forearm.  When the hand is up, the connection is ideal.  Then, as the hand rotates down, the radius bone of the forearm must rotate (Figure 21), and the tendon attachment point is pulled off to the side and becomes less efficient.  You can feel this by holding your right forearm at 900 with your palm facing upward. 


Figure 21  Radius bone must rotate.

 

Now, place the fingers of your left hand on the tendon that connects the biceps muscle to the bones of your forearm (Figure 22).  Next, rotate your hand downward and feel the tendon of the biceps move as the bones rotate.

The result of this movement is a loss of strength. As a test, hold your forearm at 90 degrees with your hand facing upward (supination) and apply a downward pressure to that hand.  Next, rotate your hand so that it is facing downward (pronation) and apply the same downward pressure to the back of your hand.  You will notice a significant loss of strength when the hand is pronated.

 

 

Figure 22  Feel the tendons rotate as your hand rotates.

The elbow is a third class lever.  Because the biceps muscle is attached near the elbow joint, the stability and strength of the arm is greatly reduced (See Figure 23).  However, this design has the important advantage of flexibility and speed.  Without this flexibility, fly fishing would be difficult and the results disappointing.

 

Figure 23  The elbow is a class three lever.

 

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

 

Table of Contents

The Shoulder

 

The shoulder is a very flexible and complex joint.  It allows the arm to move in a elaborate combination of directions.  For example, think of all the motions required of the shoulder to throw a baseball.  Also, the entire shoulder joint can be moved, such as when shrugging your shoulders.  Lifting a container from the floor to a high shelf is an example of a complex shoulder movement.

The three bones that make up the shoulder joint are the humerus, clavicle, and scapula (Figure 25).  The intersection between the humerus and scapula bones forms a shallow ball and socket joint that is held together by four muscles called the rotator cuff (Figure 26). 


Figure 25 
The bones of the shoulder.

 

Figure 26  The four SITS muscles of the rotator cuff.

 

The four muscles of the rotator cuff are often referred to as the �SITS� muscles.  Pronounce the names of the muscles listed in Figure 26 to see why this acronym is used.

In most other joints, ligaments provide the strength to hold bones together.  However, in the case of the shoulder, muscles and tendons hold the bones of the shoulder together (humerus to the scapula).  This provides exceptional flexibility, but at the expense of stability.  The shoulder is easily dislocated for two reasons:

1.      Muscles and tendons stabilize the joint instead of stronger, but less flexible, ligaments.

2.      The ball and socket connection between the humerus and scapula is very shallow.


Shoulder movement is more complex than most people realize.  Moving the entire shoulder is a complicated action, required for such activities as throwing a ball.  Try to throw a ball without moving the shoulder itself forward.  These shoulder motions are the result of the six muscles that stabilize the scapula (Figure 27).

 

 

Figure 27  Six muscles stabilize and move the scapula bone.

 

Sometimes the entire shoulder joint will rise even when no motion is required. Often people find that as they work and concentrate their shoulders will rise as muscles contract due to tension.  This is especially true if they sit for extended periods while working, for example, someone writing a book on anatomy!  To observe this tendency, ask someone working at a computer terminal if you can place your hands on their shoulders.  Then ask them to relax.  Often, as they relax, their shoulders will drop down several inches.

As a result of this shoulder muscle tension, many people complain of sore and stiff upper backs and necks.  The cause of the pain is not heavy lifting, but static tension in the shoulder muscles.  Simple stretching exercises could reduce many of these problems.  Remember, it is important to stretch a muscle before it becomes fatigued.  Once the muscle is fatigued, the recovery time is much greater.  That is why a little exercise done often is usually more effective than a lot of exercise done less frequently.


The scapula and humerus form a class three lever system (See Figure 28).  Notice that the SITS muscles attach near the top of the humerus bone.  Think of the amount of force that these SITS muscles must apply to hold the arm out from the body.

 

 

Figure 28  The shoulder joint is a class three lever system.

 

The connection between the clavicle and the sternum (See Figure 29) is the only place that the bones of the shoulder attach to the rest of the body structure using ligaments.  The other bones of the shoulder use muscles that act as the suspension system and don�t rely on ligaments for attachment.  Figure 27 shows the muscles that tie the scapula to the body structure.  This design helps to provide the extreme flexibility needed in the shoulder.

Place your fingers at the point where the clavicle attaches to the sternum and move your arm in a throwing motion.  Feel the motion as the entire shoulder moves about this pivot point.  Try to throw a ball without pivoting the shoulder about the sternum.


Figure 29 
Connection of shoulder to body structure.

 

Recommendations

Many tasks require the dynamic use of the shoulder muscles such as lifting, throwing or carrying.  However, there are other tasks, such as typing, soldering, small parts assembly, or using a microscope, that cause static fatigue to these same muscles.  In either case, the person often goes home with stiff and sore muscles.  To reduce static fatigue, do simple frequent stretching to increase blood circulation and reduce muscle fatigue.  In the case of dynamic shoulder movement such as continuous lifting, keep the elbows close to the sides of the body to minimize stress on the shoulder muscles and reduce the risk of injury.

 See Figure 30 for examples of the shoulder�s range of motion.

* Safest Range:  Numbers for the �safest range� have been intentionally left out.  These numbers are difficult to quantify and differ between individuals.  The green bar only indicates the general size and direction of a safe range.

 (Adapted from American Academy of Orthopedic Surgeons, 1965)

Figure 30  Range of motion for the shoulder.

 

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

 

Table of Contents

The Neck

The neck is a combination of many joints.  It includes a pivot joint between the head and spine and one joint between each vertebrae of the spine.  The pivot joint allows the head to rotate, as when you shake your head �no.�  This pivot joint is a cylinder shaped bone that moves within a ring-shaped bone (See Figure 31).  The joints between the individual vertebrae of the neck allow the head to move forward, backward, and side to side (See Figure 31).  By themselves, each of the disks between the vertebrae provides only a small amount of movement, but as a group the amount of movement potential is significant.

Figure  31  The neck is a combination of many joints. 


The head comprises about 5 to 7% of the total body weight.  Therefore, a 200 pound person�s head could weigh as much as 14 pounds.  That�s the weight of a bowling ball.  Numerous muscles in the neck and back provide the necessary flexibility and stability required to support the head.  When the head is directly over the spine, the spine has a normal curve and the supporting muscles are at rest.  When the head is not directly over the spine, the muscles must work hard to support the weight of that bowling ball.

 The connection between the head and the cervical spine is a class one lever system (See Figure 32).  The class one lever is stable, but not very flexible.  The additional flexibility needed for the head is provided by the cervical spine.

Figure 32  Head to neck is a class one lever system.

 

Recommendations

Avoid spending long periods of time with the head held at an angle.  This can cause static muscle fatigue which leads to muscle pain.  Also, all the nerves that run down the arms exit the cervical spine (neck) between the vertebrae in this area.  Therefore, the potential for pinching these nerves is increased as the head is tilted. 

Tilting the head backward is generally the worst of the head postures.  The muscles that support the head in this position are smaller and fatigue quickly.  Also, because tilting the head backward increases the curvature of the cervical spine, there is an increased chance of pinching the nerves of the spinal column.  Think of the stiffness and soreness in your neck and shoulders after painting a ceiling.

 See Figure 33 for examples of the neck�s range of motion.

 

 (Adapted from American Academy of Orthopedic Surgeons, 1965)

*Safest Range:  Numbers for the �safest range� have been intentionally left out.  These numbers are difficult to quantify and differ between individuals.  The green bar only indicates the general size and direction of a safe range.

Figure 33  Range of motion for the neck.

 

� Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

Copyright Waiver of First Sale Doctrine
Washington Ergonomics� materials are fully protected by United States copyright laws and are solely for the non-commercial use of the purchaser.  Purchaser agrees that such materials shall not be rented, leased, loaned, sold, transferred, assigned, broadcast in any media form, publicly exhibited or used outside the organization of the purchaser or reproduced, stored in a retrieval system or transmitted in any form by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Washington Ergonomics.

Disclaimer
Although the information and recommendations contained in this publication have been compiled from sources believed to be reliable, Washington Ergonomics and the author make no guarantee and assume no responsibility for the correctness, sufficiency or completeness of such information or recommendations.  Other or additional safety measures may be required under particular circumstances.

 

 

Table of Contents

The Back

The spine is a group of 24 building blocks called vertebrae.  The vertebrae are separated by disks that are similar to jelly doughnuts, a soft center surrounded by an outer shell.  Movement is made possible by compressing each of these jelly doughnuts (disks) a small amount.  The compression of the disks enables the back to bend forward, backward, side to side, and to twist.

When the spine is in its normal position, there are three basic curves, the cervical, the thoracic and the lumbar (See Figure 34).  If these normal curves are maintained, the upper body weight is spread evenly over the surface of each disk and the supporting muscles are relaxed.  However, when the back bends, the normal curves are lost and the weight will be unevenly concentrated on a smaller portion of the disks.

  

 

Figure 34  The spine consists of three curves.

 

 

The muscles that support the back are a complex structure, but can be separated into two basic functions.  The first function is to allow the upper body to bend at the hips, while maintaining the normal curves of the spine (See Figure 35).  The second function allows the spine to curve at each individual vertebra (See Fig 36).


Figure 35  Bending at the hips.


Figure 36  Curling the back.

 

The muscle group primarily responsible for pulling the upper body to a vertical position is called the erector spinae muscles.  These muscles run from the hips to the ribs and even to the neck.  These muscles are relatively powerful (See Figure 37).  The power and size of these muscles is the reason why many programs that address proper lifting technique recommend that you keep the back in the normal curves while lifting. 

Another series of muscles must apply significant force when the spine is curved, as in Figure 36, or bends from side to side, or is twisted.  These are the muscles that connect one vertebra to another (See Figure 38).  Each of the vertebra has three projections, small lever arms, to which the muscles/tendons and ligaments attach.  The projections on each side of the vertebra are called the transverse process, and the single rearward projection is the spinous process.  If you run your finger up and down your back you will feel the spinous processes.

 


Figure 37  The erector spinae muscle group.

 

Figure 38  Small muscles between vertebrae.

 

No single joint could provide the amazing flexibility and maintain the stability that is required of the back.  Therefore, each disk between the vertebrae provides a small amount of motion.  The sum of this motion gives the back the needed flexibility.  Once again, if this flexibility is abused, through high force and/or repetition, the chance of injury is increased.


The muscles and ligaments that support the spine are complex, but the structure can be thought of as a class one lever system (See Figure 39). 

 

 

Figure 39  Back is a class one lever.


Recommendations

When a person lifts an object, the back functions much like the boom on a crane (See Figure 40).  The same type of questions that would be asked to protect the crane from damage should be asked to protect the human�s back.  


Figure 40  The human back is similar to a crane

Questions For Crane

Questions For Human

What is the capacity of the crane? 

Manufacturers rate the lifting capacity of each model crane.

What is the capacity of the human? 

Each person has a different lifting capacity.  This capacity could be based on factors such as:

1.      Physical strength.

2.      Physical conditions such as prior injury.

3.      Other factors unique to that individual.

What is the angle and length of the boom? 

The capacity of the crane changes as the boom is moved from straight up.  There are some cranes that will tip over under their own weight if the boom is extended straight out.

What is the angle of the back during the lift? 

Just like the crane, as the person bends the amount that can be lifted safely is reduced.

From what point will the load be lifted and where will it be placed? 

Some loads are difficult to reach because of obstructions between the crane and the position of the load.

Will the person be required to twist or twist and bend? 

Twisting places high forces on the small muscles between the vertebrae.

How stable is the connection between the wire rope and the load, and the crane and the ground? 

Not all loads have good connection points and whether the crane is on concrete or mud makes a difference.

How easy is it to grip the object and how stable is the footing? 

Both the grip and the stability of the footing influence the ease of lifting.

 

See Figure 41 for examples of the back�s range of motion.

 

(Adapted from American Academy of Orthopedic Surgeons, 1965)

* Safest Range:  Numbers for the �safest range� have been intentionally left out.  These numbers are difficult to quantify and differ between individuals.  The green bar only indicates the general size and direction of a safe range.

Figure 41 Range of motion for the back.

 

© Copyright Washington Ergonomics 1994.  All rights reserved. Rev 1 1998

 

 

Table of Contents

Conclusion

The human body is designed to be a very flexible machine.  Each joint provides an amazing combination of flexibility, strength and range of motion.  Because the body can do so much, it is often abused by being asked to do too much.  Actions that violate the �risk factors� decrease efficiency, increase the chance of fatigue and create greater potential for injury.  Understanding the basic design of the body and its capabilities and limitations will help us meet the goal of a safer and more productive working environment.

 

References

 

American Academy of Orthopedic Surgeons. 1965.  Joint Method of Measurement and Recording. American Academy of Orthopedic Surgeons, Chicago.

ANSI Z-365. April 17, 1995.  Control of Work-Related Cumulative Trauma Disorders - Part 1: Upper Extremities.  (DRAFT).

Brough, William R.  1991.  Evaluating Your Workplace - Lifting.  Brough Plus Associates, Seattle.

Chaffin, Don B. and Andersson, Gunnar B.J.  1991.  Occupational Biomechanics.  John Wiley and Sons, Inc., New York.

Eastman Kodak Company. 1986.  Ergonomic Design for People at Work. 
Volume 2 Van Nostrand Reinhold Company, New York.

Kapit, Wynn and Elson, Lawrence M.  1993.  The Anatomy Coloring Book.  Second Edition.  Harper Collins College Publisher, New York.

Loeper, Jennifer.  1985.  Range of Motion Exercise.  Sister Kenny Institute, Minneapolis.

Pratt, Neal E.  1991.  Clinical Musculoskeletal Anatomy.  J.B. Lippincott Company, Philadelphia.

Wilson, D.B. and Wilson, W.J.  1983.  Human Anatomy.  Oxford University Press, New York.

 

 

 

Table of Contents

Acknowledgement

 

The author would like to thank both Ms. Stephanie Brough and Ms. Pia Aronsson for their research and initial work on this book.  Without their contributions this book would never have been completed.  I would like to thank Jean Marie Brough, Ms. Regina Barker and Mr. Phil Sloane for their dedicated editing work and extend my appreciation to my other friends and colleagues who were willing to help edit this material. 

 

I would also like to thank the wonderful artists at TechPool Studios for the use of their illustrations throughout this book. 

 

Illustrations based on LifeART Super Anatomy 1 & 3 Images

Copyright © 1991, 1994 by TechPool Corp. USA

 

The stylized human figure appearing on the cover of this publication is the trademark of Ergonomics, Inc. and is used with the permission of my friend and colleague, Mr. Ian Chong.

 

Contact

William R. Brough, CPE

Washington Ergonomics, Inc.

1118 South 287th Place, Suite 101

Federal Way, WA 98003

Phone or FAX: (253) 839-4624

E-mail: bill@waergo.com

Web site: www.waergo.com
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