There are several approaches to performing an “ergonomics evaluation”.   Some are superficial exercises at filling out forms with number scores that tell you only if the job is a high-medium-low risk for MSD, without specifically identifying and detailing the specific reasons for that risk (examples REBA and RULA).  Others are very complex math formulas that, again, tell you only if the job is at risk (example NIOSH lifting formula).   Another problem with these is they look only at the requirements of the job, the ergonomic design of the job.  We go beyond ergonomics to also examine worker behaviors, posture habits, body mechanics because worker behaviors are at least as important as job design.

We try to take a much more specific approach of seeking out specific MSD risks in job design and worker behaviors, which allows us to better identify and correct exactly what is wrong.   See our format for MSD risk analysis below...

------------------------------------------------------------------------------------------------------------------------

analyzing your jobs for MSD risks... ERGONOMIC MSD RISKS CHECKLIST: (OK to copy and use)

MSD RISKS ANALYSIS CHECKLIST...

Job _________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________

code: R = repetitive S = sustained F = forceful A = awkward X = present
____ Standing
____ Standing on ____ cement, ____ metal, ____vibrating surface
____ Foot switch operation
____ Holds _____ neck or ____ back in a ____ bent or ____ twisted posture
____ Kneeling or ____ squatting
____ Sitting
____ Poor chair design and/or: ____ Improper chair adjustment
____ Minimal variety of work postures; movement patterns; tasks

____ Forward head, rounded-shoulders, slouching posture
____ Lack of upper extremity weight bearing support
____ Keyboard, mouse or other data input
____ Shoulder elevation repeated or sustained
____ Elbows bent more than 90 degrees
____ Forearm supination
____ Loading across wrist (tennis elbow loading)
____ Wrist flexion or ____ wrist deviation
____ Grip ___________________________________________________________________________
____ Pinch __________________________________________________________________________
____ Combined thumb pinch with wrist deviation (deQuervain's risk)
____ Repeating same motion or cycle of motions every few seconds
Repetitions described__________________________________________________________________

____ Contact stress (edge or pressure or impact with hand or knee) ___________________________
____ Vibration ____during sitting _____during standing _____ in hand during grip
____ Improper tool selection and-or use __________________________________________________

Materials handling ____ lifting ____ carrying ____ pushing ____ pulling
Handling what _________________________________________________________________
____ Heavy.. wt = ______ average and ______ peak maximum
____ Frequent. frequency of lifts = ______ per hr; or ______ per day
____ Height lifted lowest ht = ______ highest ht = ______
____ Reach with load at _____ from body
____ Twisting with load ____ degrees twisted
____ Lifting/carrying across obstructions _________________________________________________
____ With difficult grip or _____ cumbersome shape
____ Workers using unsafe lifting techniques
____ Pushing or pulling _______________________________________________________________
Worker demographics ________________________________________________________________
Production demands _________________________________________________________________
Politics ____________________________________________________________________________
Claims Hx _________________________________________________________________________
Notes:

This analysis leads to suggesting corrective actions; selected from the Five "E's" of injury Elimination.

Ergonomics-Engineering ..Re-design work to reduce the required stresses of work. This works if design is faulty and if alternative design is available and it is affordable. Seeks to dilute stresses to a less toxic level. (Lifting example: reduce weight or frequency or lowest height or highest height or horizontal distance or arc twisted or cumbersome load, all to dilute severity of lifting requirements).  Here is an enlightenment:  Many, if not most, of your corrective recommendations for ergonomics improvements will actually come from the workers doing the jobs.  They are the experts.   We seek to facilitate this feedback from the workers and validate or modify their suggestions based on our expertise.  And give them credit for their suggestions.

Exposure Reduction... Reduce time exposure to work stress that cannot otherwise be re-designed: job rotation, job enlargement to increase task variety, sit-stand option to switch between standing and sitting, switching between alternative methods or movement patterns doing the job. (Lifting example: lifting work limited to 1-2 hours, then switch to non-lifting tasks for 1-2 hrs)

Exercise.. Specific stretches that restore perfusion to overworked tissues; politically difficult but very effective if done right and enforced by mgt. Great protection on jobs that are stressful but no redesign available. See our essay on this. (Lifting example: Hourly standing back-bends and hams stretches)

Enforcement.. Specific policies define exactly how work will be carried out. Effective if mgt holds supervisors accountable for enforcing policies to employees. Enforced proper lifting, use of list devices, movement patterns, how to work will be done for least stress, plus policies that reduce the cost and lost days from injuries that do occur. (Lifting example: supervisors enforce limits on weights lifted, use of devices, proper lifting technique, early pain reporting & restricted duty policies)

Education.. None of these work unless all parties are educated, not on what to do, but on why to do it. That is our Back School and Neck-Arm MSD School. (Lifting example: teach workers proper lifting techniques to correct work behaviors).

-------------------------------------------------------------------------------------------------------------------------------------

SPECIFIC M.S.D. DISEASES-INJURIES defined...

CARPAL TUNNEL SYNDROME (CTS): Pinched median nerve at wrist, affecting digits 1-2-3. Numbness, pain, loss of coordination, weakness in thumb side of hand. Extreme cases may need surgery, if EMG tests says damage is severe. CTS is often caused by or made worse by neck posture and stiffness problems, rather than by repetitive motion at the wrist. Median nerve passes through lateral neck muscles on its way to the wrist-hand. Tightness in these squeezes median nerve, causing it to swell all the way to the wrist where it gets pinched at carpal tunnel (double crush mechanism).

ROTATOR CUFF: Tendinitis of tendons that move and hold shoulder. Tendons run under end of collar bone and outer corner of shoulder blade to get to shoulder. This is narrow passage. Holding or moving shoulder elevated (reach) stresses tendons in this narrow track causing pressure, swelling, friction damage.

TENNIS ELBOW; GOLFERS ELBOW; EPICONDYLITIS: Muscle that work the wrist originate near elbow. Loads worked across wrist (grip, pull, lift at wrist-hand) will pull on these tendons where they originate near elbow, causing them to break down here.

DEQUERVAIN'S TENDINITIS: tendons that move thumb pass across wrist at base of thumb, passing through pulleys of ligament. They get worn here, causing tendinitis.

BACK STRAIN: Spine's bones connect at FACET JOINTS... are spaced by DISCS... all held together by LIGAMENTS, and are moved or held upright by MUSCLES. Many sensitive NERVES run through tight spaces between bones and discs, where they are vulnerable to pinching.
BENDING is leading cause of problems: it pulls on ligaments; it pulls on edges of discs; and weakens muscles that protect these structures.
TWISTING pinches edges of joints together and rips at edges of discs.
SITTING is major cause, as it is identical to bending, usually for hours.
STANDING leads to fatigue which allows spine to sag on its ligaments.
LIFTING just magnifies the stresses of bending, since most lifting is done with bending.
TIGHT HAMSTRINGS upsets mechanics between hips and low back during bending, stressing low back.

-----------------------------------------------------------------------------------------------------------------------------------

WHAT WE LOOK FOR AS RISK FACTORS (and why);

FORWARD HEAD POSTURE

Many workers demonstrate a forward head posture habit.  This is a very common work posture habit that significantly risks pain, fatigue, and MSD.  Forward head posture is a round shoulders posture.  This places the ligaments of the upper back on a sustained stretch, creating a mechanical and a nutritional stress leading to pain and swelling. 

The lateral neck muscles face excessive work demand and loss of flexibility.  This creates muscle strain and can lead to compression of nerves and blood vessels passing through these muscles on their way to the upper extremity.  This compression of nerves and blood vessels in the neck can increase the risk of MSD problems further down the upper extremity.  This is called thoracic outlet compression.  Reduced circulation to the upper extremities due to this posture reduces work tolerance and increases vulnerability to various MSD diseases. Compression of nerves also passing through the lateral neck muscles may risk carpal tunnel syndrome and other problems.

The posture muscles of the upper back and shoulder blades are also placed in an overworked but weakened position, risking muscle strain and reducing posture endurance.  Because the upper back is bent forward, the upper neck must bend backward to restore level vision.  This can compress the joints and soft tissues of the upper neck, risking headache and neck pain.  This posture protracts the shoulder girdle which increases pressure and tension on various tendons of the shoulder, risking shoulder tendon and bursa problems. Forward head posture can also change the posture of the jaw, leading to TMJ problems.  This can greatly stress MSD problems.

SUSTAINED SITTING

Sustained sitting can significantly add to risk of lower back, neck, and upper extremity MSD problems.  Sitting places the lower back in sustained forward bending, risking stretch weakness to lower back muscles, instability of ligaments and joints, and disc degeneration. Sustained forward bending at the lower back also tends to increase forward head posture, as already described.  The sedentary nature of sustained sitting jobs can deteriorate general musculo-skeletal condition, contributing to various health risks and reduced work tolerance.  Poor sitting habits, poor chair design, and poor chair adjustment add to these risks.

POOR CHAIR DESIGN

When the worker  is sitting on a chair that is of minimal ergonomic design, the described risks of sustained sitting and forward head posture will be increased.  The lack of adjustability and/or improper fit, and/or improper sitting habits may increase lumbar flexion and forward head posture effects of sustained sitting.

LACK OF UPPER EXTREMITY WEIGHT BEARING

This worker who cannot rest the arms on the work surface increases fatigue and MSD risks.  That is, the muscles of the neck and shoulders are sustaining a contraction to hold the upper extremities in the work position.  This may compress the nerves and blood vessels feeding the working upper extremity.  It also adds work load to the posture muscles of the upper body, contributing to fatigue.  Lack of upper extremity weight bearing support adds to the risk of MSD in the upper extremity, the risks described with forward head posture, excessive fatigue and its related reductions in productivity, quality and employee relations.

SUSTAINED STANDING

Sustained standing with a minimum amount of walking around is a risk for posture fatigue at the lower extremities, lower back, neck and shoulders.  The sustained muscle contractions required of sustained standing can impair stamina, work comfort, and posture.  Musculo-skeletal pain and other conditions can be aggravated by sustained posture, even if that posture is properly aligned.

CEMENT FLOORS

Standing on a cement or steel floor may magnify the posture fatigue risks of standing.  The density of these work surfaces greatly increases joint strain and muscle work to maintain posture functions.   Sustained standing on a cement floor may add to posture fatigue in the spinal muscles, resulting in excessive lumbar swayback or forward head posture.  These may contribute to lower back problems, leg pain and neck-arm MSD.

FORWARD BENDING

Sustained or repeated forward bending at the low back is one primary risk for lower back degeneration and strain.  Forward bending may place protective low back muscles in a position of stretch weakness, unable to provide load protection to discs, ligaments and to the muscles themselves.  There may be instability of joints, stretch weakness and tension to back ligaments, and damaging strain to lumbar discs.  Time spent in forward bending and ranges of forward bending define the degree of stress to low back structures.

TWISTING

Twisting of the lower back may compress the facet joint on one side and separate the facet joint on the other side.  This risks facet joint injury.  There may also be excessive pulling of the ligaments, risking ligament strain, along with excessive pinching into the outer layers of the lumbar discs, contributing to the risk of ruptured discs.

SHOULDER REACHING RISKS

Repetitive or sustained shoulder reaching creates possible mechanical strain to various tendons of the shoulder.  Reaching up in front of the body stresses the long head of the biceps tendon, risking biceps tendonitis.  Reaching out to the side may stress the rotator cuff, particularly the supraspinatus tendon.  Upper extremity reaching, even without a load, represents a mechanical stress to these small confined tendons. This stress is increased by time spent reaching and  length and height of the reach.

TENNIS ELBOW RISK

Pulling or lifting across the wrist in a manner that pulls or holds the hand back represents a mechanical strain to the ECR muscle.  This muscle is responsible for tennis elbow.  Forceful, repetitive or sustained contraction of this muscle may irritate where this muscle originates at the elbow.  This is tennis elbow or lateral epicondylitis. 

ELBOW FLEXION

Sustained or repetitive bending of the elbow to angles greater than ninety degrees can compress nerves and blood vessels passing by the front of the elbow, while producing tension on nerve structures passing behind the elbow.  This risks nerve entrapments or irritations and may aggravate MSD conditions further down the upper extremity.

SUPINATION

Turning or holding the forearm or hand in a palm up position (supination) creates tension in the flexor tendons originating at the medial epicondyle, running down the front of the forearm, through the carpal tunnel and into the hand.  Repetitive or sustained tension to this muscle and tendon group caused by supination may risk medial epicondylitis, flexor tendonitis and carpal tunnel syndrome.

DEQUERVAIN'S DISEASE

Sustained pinch with the thumb while the wrist is moving may cause tension and friction along the tendons of the long thumb extensors.  This may press or rub these tendons against bony edges at the wrist, causing DeQuervain's tendinitis. 

WRIST FLEXION

Tipping the hand down toward the palm may compress the wrist tendons passing through the carpal tunnel.  This risks carpal tunnel syndrome and tendonitis at the wrist.  The range of bending, frequency of movement, forces required and time spent in this position determine the extent of risks.  Superimposing grip or pinch during this movement or position increases the risk.

ULNAR DEVIATION OF THE WRIST

Tipping the wrist in the direction of the little finger may compress the carpal tunnel, risking carpal tunnel syndrome and tendinitis at the wrist.  The range of movement in this direction, frequency of this movement, and time spent in this position determine the extent of the risk.  Adding grip or pinch to this position greatly magnifies that risk.

GRIP

Hand grip creates tension of the flexor tendons in the forearm passing through the carpal tunnel.  Excessive demand on these muscles and tendons may risk tendinitis and carpal tunnel syndrome.  The factors affecting this risk include: force of grip; large grip diameter; small grip diameter; (grip diameter greater than two inches or smaller than two inches) and excessive holding time.  Excessive time spent gripped, however light the grip may be, is a significant risk factor.  Slipperiness, cold or vibration during grip further magnifies grip risks.

PINCH

The CTD risks of pinch are similar to that of grip.  Pinch, in fact, is an even greater MSD risk than grip.  Five pounds of pinch creates as much pressure in the carpal tunnel as twenty-five pounds of grip.  The factors that dictate the risk of pinch include: force of pinch, frequency of pinch, and the time spent pinched (holding time).  Excessive time spent pinched with an inadequate amount of time relaxed from pinch represents a significant risk factor for carpal tunnel syndrome and tendonitis in the wrist.

VIBRATION

Vibration during grip may increase reflex muscle tension in the wrist, increasing the risk of tendinitis and carpal tunnel syndrome.  There are also blood vessel and nerve responses to vibration, risking vibration white finger syndrome.

PRESSURE

Direct external mechanical compression of the carpal tunnel may irritate carpal tunnel tissues through mechanical trauma and obstruction of blood flow.  The force of this pressure and the time spent in contact with this force are determining factors.

TELEPHONE POSTURE RISKS

Holding the telephone handset braced between the neck and shoulder can produce a sustained contraction of the lateral neck muscles. This may compress the thoracic outlet and risk neck strain to the muscles, joints and ligaments. This can produce severe neck fatigue and pain problems. Even the use of a pad or bracket to help hold the handset to the shoulder is not adequate in reducing these stresses.

------------------------------------------------------------------------------------------------------------------------------------

Wrist-rests for keyboards...Yes or No?...

Some ergonomists worry about wrist positions with wrist rests. But they miss the point. Wrist position may not be as important as NECK posture. Neck muscles work to posture the head, neck, and working arms to control fingers on the keyboard. This posture work causes neck muscles to compress nerves and blood vessels passing through the neck on their way to the arm and wrist, risking CTS and tendinitis. The purpose of wrist rests is to ease neck-shoulder posture holding stress. This is far more important than wrist position. No matter what wrist position is used, some wrist structures will be stressed. Finding and staying with an ideal wrist position will not prevent CTS or tendinitis. Wrist position variety is far more important than wrist position perfection. Worrying about wrist position is not as important as is neck posture stress relief... the purpose of wrist rests. Arms rest lightly and intermittently on the padded wrist rest, not sustained there requiring awkward finger reaching. The object is to interrupt sustained neck-arm posture muscle holding with brief pauses for upper extremity weight-bearing.

Use of wrist rests greatly reduces neck muscle holding tension that typically squeezes blood vessels that feed the working wrist.  Neck muscles relax with wrist rests and allows better blood supply to working tendons at wrist

 

PROPER COMPUTER SETUP:

*Chair height-adjustable, forward tilt seat pan, lumbar support; change adjustments frequently for posture variety. *Monitor squarely in front, not to side; monitor height places vision angled down 5 to 15 degrees below horizontal. *Desk height places elbows at 90 degrees with fingers on keys. *Push equipment back to give space in front of keyboard for padded arm rest. *Paperwork on document holder near midline to avoid turning head. Vary this position for posture variety. *Mouse pushed in to allow arm to rest on surface. Perhaps switch often between mouse and trackball or touchpad for variety. *Shade to cut reflected glare. *Hourly chime from clock to remind to stop to do stretches at neck-arm-standing backbends at low back. *Telephone headset replaces handset if much phone work. *Pen-pencil padded to reduce pinch stress. *Stand to stretch often!!

See our publication PERSONAL ERGO GUIDE FOR COMPUTER WORK

-----------------------------------------------------------------------------------------------------------------------

HOW TO FIX THE IDENTIFIED RISKS...

The objective is not to eliminate identified risk factors.  That usually impractical and unattainable. The reality of our objective is to dilute the severity of one or more of the risks per job to increase the margin of work tolerance for the worker.  Symptoms arise out of an accumulation of risks over time (thus the term cumulative trauma disorder).  The idea is to reduce the intensity of some of the risks, as well as the time exposure to risks.   That reduces the toxicity of the total set of risks, thus reducing the risk of symptoms and disease.  We seek to dilute the severity of risks to make work less toxic and more tolerable.

As we examine work and develop preventive interventions, we build a list of risks, then consider how we may reduce the intensity or severity or time exposure to these risks.   We cannot expect to eliminate all risks.  We do not really need to eliminate all the risks, even if that were attainable, for success

Our ideal is to build a wish list of corrective recommendations for the worker and the employer to consider for selection and trial.  They will select interventions based upon cost, practicality, production, and (especially!!) their knowledge base about prevention that you have hopefully provided to them via Back School and Neck-Arm CTD School training programs and the education provided by the Work Risk Analysis Report you have provided to them.   We offer a list of choices, which expands success

I.Proper computer workstation set-up

Seating…The chair should be height-adjustable so that the worker can keep the feet placed flat on the floor while sitting fully back on the seat.  The objective is to minimize leg weight pressure under the distal thighs on the edge of the seat, in favor of the weight of the legs being carried primarily by the feet on the floor. 

But if this calls for lowering the chair to the point where the work surface is now too high, then bring the chair back up to where the hands are working at the correct height of the work surface.  Now place a foot rest to correctly carry the weight of the legs on the feet. 

The seat pan should not be too deep for the length of the worker's femurs.  The worker should be able to slide the buttocks fully back and still not touch the front of the seat pan with the back of the calf. 

The seat pan should be able to tilt slightly forward so that the front of the seat pan is slightly lower than the back of the seat pan.  This encourages the lower back to be held in its proper lordosis curve, minimizing lumbar flexion, and for the neck to be held upright, minimizing forward head posture.  The lumbar pad should fit the desired lumbar lordosis.   A posture wedge placed on the seat pan for chairs that do not adjust to tilt forward cost very little, but can improve neck and low back sitting posture.

But perfect seating posture is not good if it is sustained!  Posture variety and interruption is best. The worker must frequently be reminded to stand and do a back-bend for 5-10 seconds at least hourly, plus vary seat height and tilt for posture variety.

Surface… The table upon which the computer is placed should be at a height where the elbows work at 90 degrees of flexion.   There should be adequate space for the computer as well as the paperwork and associated materials to be kept within easy vision and reach.  Paperwork should be placed upright on a document holder to minimize neck flexion or rotation postures.

The table should be deep enough for the computer, monitor and keyboard to be placed to allow space in front of the keyboard for the operator to rest the forearms on a wrist rest .  This advice has been controversial, with some people preaching that wrist rests are bad.  This comes from incorrect assumptions.  The wrists are not confined firmly in place on the wrist rest.  Neither are they compressing themselves on an edge to the point of tissue stress. The forearms, instead, lightly rest intermittently on a well-padded surface, just enough to provide some intermittent relief of posture loading at the neck, shoulder girdle and thoracic outlet.  The traditional teaching that states that the upper extremities should never rest on the table is incorrect. The objective is reduced proximal posture loading.

The video monitor should be at a height that allows for the vision to be angled down approximately 5-15 degrees.  This would typically require the top surface of the monitor to be level with the operator's eyes.  This may be accomplished by placing books or a small box under the monitor.  The document holder should be used to place the paperwork as close to the monitor as possible.  Symmetry of head-neck posture is the objective.  There should not be unshaded windows or artificial lights directly behind the operator to cause reflection and glare off the screen.

Some may ask how the keyboard should be adjusted for tilt and wrist position.  Should the legs at the back of the keyboard be flipped down to tilt the keyboard forward to place a slight extension at the wrists? or not?  Should the wrists be slightly flexed, extended or neutral?   The are lots of "rules" that turn out to be purely assumptions and unsupported opinions about this and other posture issues. 

Our response is that it does not really matter all that much what position is chosen as "perfect" because perfect posture is bad for you, if you sustain that posture.  Variety of posture is far more critical than finding and maintaining "perfect" posture!  Now we should, of course, avoid extremes of position.  That is a rather safe assumption.  But we cannot seriously defend holding to a rule of 10-15 degrees of wrist flexion or extension or neutral position as "safe" protection from CTD problems at the wrist. 

Holding any position for prolonged periods will likely subject some tissues to a sustained load, leading to reduced nutrient pathway to some tissues, risking CTD.  Too often workers are subjected to recommendations to find a perfect position and then sustain that position because the rules say it is ergonomically "correct."   Time in a position (any position) is far more critical than the position itself. 

Our "rule" is: variety of posture is far more important than perfection of posture.

Frequently change the height of the seat, tilt of the seat pan, position of documents, tilt of the keyboard.  And the operator should get up from the work station every hour to perform standing back bends, scaleni stretching, relaxation response, ECR stretch and wrist flexor stretches.  The operator should also, during this stretch break, focus his or her eyes on a distant object so as to interrupt the sustained "posture" of visual focusing distance for eye relief.

Posture variety and stretch breaks are by far the most critical "ergonomic" interventions for the computer work station.

II.Repetitive task manufacturing jobs design strategies:

As stated earlier in this text, repetitive motion, per se, is not the truly critical issue.  The issue to consider first is the sustained posture demands required of repetitive motion job tasks.  One must usually maintain a sustained posture, such as sitting or standing at the work station, to carry out repetitive motion work.  It is this sustained posture component that is usually the most critical issue.  And sustained posture stresses are often easier to manage than are attempts to reduce repetitive motion demands.

Sustained posture requires sustained muscle contraction, sustained tendon tension, and sustained joint compression.  This creates mechanical tissue pressures that can readily exceed circulatory perfusion pressures, thus depriving these working tissues of their nutrient pathway.  This forces anaerobic metabolism, which rapidly burns up oxygen and glucose stores while creating large amounts of metabolic wastes.  These wastes cause irritation to the tissues, resulting eventually in inflammation.  This is, thus, the mechanical and chemical model of cumulative trauma disorder (CTD) as a nutrient pathway disorder.

Repeated movements are not innocent in this process.  But we have assumed the it to be a friction wear damage model.  This is likely not the true mechanism of damage of repetitive motion.  What is more likely is yet another process of interruption of nutrient pathway.  This time it is a process of repeated muscle contraction, tendon tension, and joint compression.  But what is significant is that when this is a repeated contraction, there is diminished relaxation of the contractile tissues between each cycle of contraction; the greater the frequency of contraction, the less the level of relaxation between each contraction.  The result is a sustained and often growing level of background tension during the repetitive motion process.  This, again, produces an interruption of nutrient pathway.

If we assume this to be the mechanism of MSD pathogenesis, we can see several possible avenues for prevention intervention.  It is very difficult to reduce repetitive motion in today's workplace.  Production needs simply will not allow this in most cases.  But we can find several acceptable methods of boosting tissue perfusion without reducing production, perhaps even increasing production due to enhanced performance and reduced fatigue of the working body.  This is where we begin to consider frequent but brief stretching of key tissues affected by posture and motion demands, job task rotation or sit-stand options to reduce time exposure to otherwise relentless stresses to working tissues as adjuncts or alternatives to often expensive ergonomic design changes.

We always start our intervention problem-solving with trying to correct fixable ergonomic design stresses.  These can alleviate critical stresses.  But ergonomic alternatives are often elusive and sometimes expensive.  So we always keep in mind stretches, rotation, sit-stand options, and self-care of the working body as alternative prevention strategies.   Stretching briefly and often can result in a relaxation of working tissues, allowing restoration of perfusion to replenish nutrients, flush wastes, and deliver materials needed for tissue repair. Task rotation and frequently switching between sitting and standing can shift workloads among a larger population of working tissues to disperse workloads and provide periods of reduced tissue loads for restoration of perfusion.

But let us consider the ergonomic issues. 

Seating is as described above in the discussion about computer workstations.  Chair should be height-adjustable to bring the weight of the legs primarily to the feet on the floor, rather than at the distal thigh on the front edge of the seat.  If this lowers the worker too low for the height of the work surface, then bring the chair up to place the elbows at 90 degrees for hand work and place feet on a footrest. The seat pan should tilt forward, improving lumbar lordosis and upright head posture.  Chair back should support lumbar lordosis.

The work surface should be at a height that allows the hands to work with the elbows at 90 degrees of flexion.  Adjustability would be nice, but is usually not available.  A forward tilt to work surfaces, where appropriate, may encourage a more upright head posture.  Seek to provide some intermittent upper extremity weight-bearing support (padded). Tools and materials should be arranged at the work surface to place the most commonly used items closest to the operator and closest to midline. 

Tasks that may require shoulder reach in flexion or abduction or lateral rotation may risk tendon stresses about the shoulder, particularly at the rotator cuff or biceps long head or bursae or associated soft tissues.  The degree of stress and MSD risk is determined by range, repetition, duration, or load of this reaching demand.  The prevention strategy is to reduce range or repetition or duration or load by at least a small amount of one or more of these factors, thus reducing the "toxicity" of the reach demand.

Tools or procedures should minimize grip, pinch, wrist flexion, ulnar deviation, forearm supination.   Repetitive, sustained, or forceful loading across the wrist in ways that may recruit the extensor carpi ulnaris (ECR) would risk lateral epicondylitis (tennis elbow).  Sustained, repeated, or forceful grip and pinch are to be minimized.  Thumb extension or lateral thumb pinch during wrist deviation may risk DeQuervain's Tenosynovitis.  Gripped surfaces that are slippery rather than tacky, larger or smaller than fingertip-on-thumbtip diameter, or vibrating may increase grip-pinch and wrist position stresses.

Work station or procedures that create flexion, extension, sidebending, or rotation at the cervical or upper thoracic spine may risk proximal stress responses at the spine as well as distal consequences further down the upper extremity.

Sustained sitting presents sustained flexion and loading risks at neck and low back, while sustained standing presents its own risks, consistent with fatigue effects on posture and flexibility and tissue perfusion.  The less available is posture variety, the greater these risks.

Vibration through the floor will exaggerate the posture loading stresses of standing and sitting due to fatigue of muscle and hydration changes in intervertebral discs (disc creep).  Vibration transmitted through tools or products in contact with the hands may risk vasospasm or excessive musculo-tendinous tension responses.  Vibration tends to inhibit the sensation of grip, thus the worker tends to grip harder, thus increasing musculotendinous tensions.

Wide task variety versus stereotyped repeated tasks will affect the severity of the described stresses.   The more repeated and stereotyped the tasks, the greater the MSD risks.  Jobs with a wide variety of movements and (especially!) work postures with have far less risk, unless heavy physical exertion is required.

Ergonomic interventions:

There are several no-cost options.  Perhaps the best is task rotation, whereby workers frequently switch between job tasks.  The greater the ergonomic risks, the more frequently tasks should be rotated (ranging from every four hours to every half-hour).   This suggestion is often met with immediate denial by employers and employees.  But many, many companies have said why not and , thus, succeeded in dramatically reducing their MSD claims simply by changing their workplace culture to require frequent job task rotation.  We often ask a company that resists this idea to simply ask themselves, "how much money do you want to save?  And how much is it worth to you to make these no-cost changes?" 

And productivity does not suffer with task rotation!   There may be a brief period of social disruption until people accept the need for change.  But productivity has been seen to increase in many companies who make this change, due to reduced worker fatigue.

Sustained sitting is a critical risk factor for neck-arm and low back MSD problems.  Simply purchasing new chairs is rarely effective by itself.  Adjustable seating is very valuable, but worker training and coaxing to adjust chairs properly, plus coaxing to frequently modify their adjustments for posture variety will radically reduce posture stresses of sustained sitting. 

The best advice costs nothing:  simply stand frequently as a break from sitting. 

The chair should be height-adjustable so that the worker can keep the feet placed flat on the floor while sitting fully back on the seat.  The objective is to minimize leg weight pressure under the distal thighs on the edge of the seat,  in favor of the weight of the legs being carried primarily by the feet on the floor. 

But if this calls for lowering the chair to the point where the work surface is now too high, then bring the chair back up to where the hands are working at the correct height of the work surface.  Now place a foot rest to correctly carry the weight of the legs on the feet. 

The seat pan should not be too deep for the length of the worker's femurs.  The worker should be able to slide the buttocks fully back and still not touch the front of the seat pan with the back of the calf. 

The seat pan should be able to tilt slightly forward so that the front of the seat pan is slightly lower than the back of the seat pan.  This encourages the lower back to be held in its proper lordosis curve, minimizing lumbar flexion, and for the neck to be held upright, minimizing forward head posture.  The lumbar pad should fit the desired lumbar lordosis.   A posture wedge placed on the seat pan for chairs that do not adjust to tilt forward cost very little, but can improve neck and low back sitting posture.

Standing jobs should provide footrest for standing posture variety.  Anti-fatigue floor mat or insoles can reduce standing fatigue that can lead to posture and flexibility problems.  A tall stool placed at the work station would provide posture variety by allowing the worker to switch between standing versus tall sitting or leaning on the stool.

The work surface should be at proper height and tilt, preferably with adjustable in height and tilt.  We seek to bring the work to a height whereby the worker's elbows are at 90 degrees during tasks. Available wood block may fit under legs to provide increased height or tilt.

Tools should be at the correct diameter (approximately the diameter formed by opposing index tip to thumb tip), have minimal vibration, and minimal weight.  Some tools may be connected to overhead counterbalancing devices that minimize tool weight.    Tools may be covered with antivibration material and-or material that improves tackiness of grip.  Tools should be selected to allow hands to work in neutral positions, minimizing flexion or deviation.  

Tasks should minimize prolonged grip, forceful grip, repeated grip.  Clamps or jigs may be used to hold objects being worked on, to replace the need for grip or pinch to objects otherwise held in hand.

III.Materials handling jobs:

Lifting, pushing, pulling, carrying all fall into the category of "materials handling" work.  Client companies and their workers often ask us to define a safe lifting limit or standard, e.g., a maximum permitted weight lifting limit.  But we cannot give such a defined lifting limit (30-50-70 pounds) with much validity.  It is not how much the load weighs; rather, it is how much it weighs taken in consideration with how frequently it is lifted, from how low to how high, at what distance from the body, over how much arc twisted, with how cumbersome the shape and handle of the weight.

The ergonomics (external demands of the job) are made up of each of these variables interacting.  One variable cannot be adequately considered without the others.  And this does not take into account the body mechanics selected by the worker regarding proper versus improper lifting behaviors. When seeking out and reducing lifting stresses, we need to consider the weight of the object lifted (average weight and peak weight), frequency it is lifted, how low it is lifted, how high it is lifted, how far horizontally it is handled from the body, amount of twisting with the load, and the coupling  grip efficiency between the worker and the load.

We first identify the level of demands for each of these parameters.  We then seek to reduce at least one or more of these, if only to a small degree.  The objective is not to eliminate the risks.  That is not practical.  Rather, we seek to dilute the level of demands on the working spine.   We may or may not be able to reduce the weight of the load.  We may or may not be able to reduce the frequency lifted.  We often can raise the lowest level lifted or lower the highest level lifted.  We may be able to get the load closer to the worker.  We may be able to arrange the lifting task to reduce the arc of twisting with the load.   Examples include placing a hydraulic or spring-operated lift table beneath a pallet that is being loaded or unloaded, so as to keep the loading surface at ideal waist level See illustrate examples.

Selected job design tactics:

1.Super-sack system: 50-100 pound bags of materials repeatedly by hand may be replaced with a super-sack , a one-ton bag that must be handled mechanically, with the materials mechanically dumped as needed.  The idea is to eliminate manual lifting of the material.

2.Lift table, with turntable:  product that is moved from conveyor line to be stacked on a palette, also known as a skid,  requires repeated bending-twisting-lifting output.  Placing the skid on a hydraulic lift table can keep lifting heights correct as the skid is piled higher.  The addition of a turntable under the skid allows the skid to turned as it is loaded so as to keep the side being loaded closest to the worker, avoiding reaching across the skid as it is stacked.

3.In the absence of a lift table, many workers simply place a few extra skids on one another, to raise the skid to be loaded to a better height, avoiding the lowest lifting heights.

4.Various items stored on shelves should be arranged such that the heaviest and most commonly handled items are stored at waist level for ideal lifting height.  Lighter and less frequently lifted items are arranged on progressively higher or lower shelves.