IE 486 Work Analysis

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IE 486 Work Analysis Design II

• Lecture 13 Intro to Biomechanics
• Dr. Vincent G. Duffy
• Thursday March 1, 2007

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IE486 Lecture 13 - QOTD

• Q.1. Is it reasonable to expect that the whole
  is equivalent to the sum of the parts? Yes/No?
• Q.2 Which is more costly, low back injury or
  CTD/upper limb?
• Q.3 What is the recommended weight limit for
  lifting in the given example?
• 5, 10, 50 lbs.?

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Recall from Ch. 10 Engineering Anthropometry
Human Variability

• Q.1. Is it reasonable to expect that the whole
  is equivalent to the sum of the parts?
• A tall person may have short arms. A person with
  long torso may have short legs.
• You can not measure one body part and extrapolate
  to know the remainder re fit.

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Administrative see updated schedule with minor
revisions highlighted in yellow

• Today and after Spring Break

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What is the estimated cost of ignoring issues
related to the biomechanics of work?

• As of 1991, 27-56B (low back alone) according to
  Pope, et al. 1991.
• Other recent and related statistics are presented
  in NORA (National Occupational Research Agenda)
  and other documents on the NIOSH webpage.
  http//www.cdc.gov/niosh/nora/
• Related statistics are presented in Wickens et
  al. 2004 Waters et al. 1993 1999

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Q.2 Which is more costly, low back injury or CTD?

• Re Upper-extremity cumulative trauma disorders
  (CTDs).
• Where repetitive hand and arm exertions are
  prevalent, CTDs of the upper extremities are
  common and can be even more costly than low-back
  problems.

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Briefly discuss the psychophysical method of
assessing static muscle strength.

• Subjects adjust load upward and downward after
  each trial in a simulated task situation until
  they believe the load has reached THEIR maximum
  capacity. It is be self-report (subjective
  rating).
• It is suggested (according to Chaffin and
  Andersson, 1991) that psychophysical methods,
  even considering trouble with the method based on
  motivation/cooperation, etc., may be the most
  accurate method of estimating a persons
  strength.
• And ECE 511 Prof. Hong Tan, Psychophysics.

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Consider Figure 11.1. Find the moment about the
elbow for a single segment biomechanical model of
the forearm in which the hand is holding a load
of 25kg (rather than 20kg).
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Fundamentals. 1. A mass in motion (or at rest)
remains in motion (or at rest) until acted upon
by an unbalanced external force. 2. Force is
proportional to the acceleration of a mass (eg.
At rest, use gravity). Any action is opposed by
a reaction of equal magnitude. That is why we
can assume that the sum of moments around the
elbow is zero.

• Since we are assuming the single-segment model,
  you can refer to the original figure 11.1 with
  modifications as follows.

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The moment about the elbow is 46.98 Nm.

• This comes from the sum of moments around elbow
  0
• sum M 16N0.18m(unchanged wt of forearm/hand)
  259.8/20.36
• which is the new load divided by weight of each
  hand (by 2) multiplied by gravity and multiplied
  by distance from hand to elbow (unchanged).

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Briefly discuss low back problems in relation to
seated work.
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Briefly discuss low back problems in relation to
seated work.

• Most people do not maintain an erect posture for
  long, but adopt a slumped posture.
• The slumped position produces wedging of disks in
  lower back and can pressurize soft tissues in the
  spine causing low-back MSDs.

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What is the purpose of the NIOSH lifting
equation? What is AL and MPL and what is the
difference between them?

• According to the National Institute for
  Occupational Safety and Health (NIOSH, 1981), the
  purpose is to analyze lifting demands on low
  back.
• It allows the user of the analysis tool to
  establish a Recommended weight limit (RWL) for a
  specific task that nearly all healthy workers
  could perform for a substantial period of time
  without increased risk of developing
  lifting-related low-back pain.
• The AL is the action limit a weight limit above
  which a small portion of the population may
  experience increased risk of injury whereas the
  Maximum permissible limit (MPL) is three times
  the action limit (AL).
• MPL is considered the weight limit at which most
  people would experience a high risk of back
  injury (for those lift conditions).

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What is the difference between the original NIOSH
lifting equation (1981) and the revised version
from 1991?

• Eg. 1981 equation did not consider asymmetric
  lift.
• In 1991 the Lift Index (LI) is also used to
  quantify the degree to which a lifting task
  approaches or exceeds the RWL.

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NIOSH Lifting Guide (Revised 1991)

• NIOSH lifting equation is the ratio of load
  lifted to RWL
• LI L / RWL
• (a ratio if LIgt1, adjust task task likely to
  pose increased risk for some workers if LIgt3,
  most workers at high risk for low back pain
  injury)
• For a given expected load to be lifted given
  task, compute the RWL
• RWL LC x HM x VM x DM x AM x FM x CM

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NIOSH Lifting Guide (Revised 1991)

• compute the RWL
• RWL LC x HM x VM x DM x AM x FM x CM
• LCload constant
• Max. recommended weight under optimal conditions
  eg. Symmetric lift, occasional lift, no torso
  twist, good coupling, lt25cm vertical distance of
  lift
• HMhorizontal multiplier
• (moment) disc compression force increases as
  horizontal distance between load spine
  increases.
• Therefore, max. acceptable weight limit should be
  decreased from LC as horizontal distance increases

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NIOSH Lifting Guide (Revised 1991)

• compute the RWL
• RWL LC x HM x VM x DM x AM x FM x CM
• VM vertical distance multiplier
• Lifting from the floor is more stressful than
  lifting from greater heights.
• Thus, allowable weight for lift is a function of
  the originating height of the load.
• DM distance multiplier
• Physical stress increases as vertical distance of
  lift increases.
• AM asymmetric multiplier
• Asymmetric lift involves torso twist and is more
  harmful to spine than symmetric lift. Therefore
  allowable load to be lifted should be reduced
  when lift includes asymmetric lifts.

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NIOSH Lifting Guide (Revised 1991)

• compute the RWL
• RWL LC x HM x VM x DM x AM x FM x CM
• FM frequency multiplier
• Reflects effects of lifting frequency on
  acceptable lift weights.
• CM coupling multiplier
• Difficulty of grab. Effected by whether load has
  handles.

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NIOSH Lifting Guide (Revised 1991)

• compute the RWL
• RWL LC x HM x VM x DM x AM x FM x CM
• Components Metric US
• LCload constant 23kg 51 lb.
• HMhorizontal multiplier 25/H 10/H
• VM vertical distance multiplier (1-.003(V-75)
  1-.0075(V-30)
• DM distance multiplier .824.5/D .821.8/D
• AM asymmetric multiplier 1-.0032A 1-.0032A
• FM frequency multiplier see table 11.2
• CM coupling multiplier see table 11.3

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Figure for NIOSH Lifting Analysis (consider QOTD
3. Compute the RWL)
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• H16
• V44
• D18
• A80degrees
• F3 lifts/minute
• CGood coupling
• Job duration 8 hrs/day
• Wt. Lifted 15 lbs.

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Six multipliers that can be calculated to get
Recommended Weight Limit (RWL)

• HM 10/H
• VM1-.0075x(V-30)
• DM.821.8/D.
• AM1-.0032xA
• FM (from table)
• CM( from table)
• RWL51xHMxVMxDMxAMxFMxCM

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Six multipliers that can be calculated

• HM 10/H 10/16.625
• VM1-.0075x(V-30)1-.0075x(44-30)
• .895
• DM.821.8/D.821.8/18.92
• AM1-.0032xA1-.0032x80.744
• FM.55 (from table at 3lifts per min. Vgt30)
• CM1.0 (good coupling from table)
•  
• RWL51xHMxVMxDMxAMxFMxCM

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Six multipliers that can be calculated

• HM 10/H 10/16.625
• VM1-.0075x(V-30)1-.0075x(44-30)
• .895
• DM.821.8/D.821.8/18.92
• AM1-.0032xA1-.0032x80.744
• FM.55(from table at 3lifts per min. Vgt30)
• CM1.0 (good coupling from table)
•  
• RWL51xHMxVMxDMxAMxFMxCM
• 51x.625x.895x.92x.744x.55x1.0
• 10.74 (lbs)

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Lift index

• LI L/RWL 15/10.741.4
• some workers would experience an increase in risk
  of back injury because the lift index is gt1.0.
• some precautions should be taken to minimize the
  risk of injury, and the job may need to be
  redesigned to lower the LI.