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. 

 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.

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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.

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. 

 
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. 

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

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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.

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.

Demonstration

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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.

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.

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.

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.

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.

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

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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.

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

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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.

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 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.

Recommendations

(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

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.

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.