Manual Material Handling

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Manual Material Handling
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Introduction

• Research has been conducted in the area of MMH
  for about 40 years continued research in this
  area today
• Research has entailed establishment of acceptable
  handling limits using different approaches, the
  application of ergonomic principles to job
  design, employee placement and employee training.
• Manual lifting represents a major cause of injury
  to industrial workers and a major cost to
  industry.
• Age and gender variables do not affect back
  injury rates significantly (Laughery and Schmidt,
  1984)
• Possible job assignment based on age and gender
• Older workers generally more experienced with the
  job
• Contradictory evidence from the Bureau of Labor
  Statistics Supplementary Data System (SDS)
  indicates a significant decrease in injury claims
  with age (possible self selection)

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MMH Injury Frequency and Cost

• Back (lower back) injuries occur with alarming
  frequency (Caillet, 1981, estimates 70M Americans
  suffered back injuries with increasing incidence
  of 7M annually).
• 5M people partially disabled because of back
  injury, 2M not able to work at all. Estimates
  that 6.5M Americans will lie in bed on any given
  day because of back pain with increases of 1.5M
  annually (Keim, 1981,1984)
• Cost is estimated at 19-25.5 of all WCC due to
  back pain
• See Figure 1 for age and gender specific data
• See Table 1 for WCC based on type of exposure

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• Nearly 50 of all back strain/sprains
  precipitated by manual lifting of objects
• Low back pain (Khalil et al., 1984) is the second
  largest pain problem behind headaches. White
  (1983) reported that more than 70M people in USA
  see physician annually complaining of lower back
  pain. Loesser (1979) estimated that 170M working
  days lost each year.
• For work injury and cost statistics see figure 2
  (National Safety Council, 1972-1984)
• Figure 2 shows the work injury and cost
  statistics associated with injury.
• This figure shows two intriguing facts regarding
  MMH-related injuries.
• First, despite improved medical care, increased
  automation in industry, and more extensive use of
  pre-employment exams, only a marginal decline in
  worker injuries is observed.
• Second, the cost of injuries has increased at an
  alarming rate. More recent data shows that the
  increased health care costs have actually
  increased this number significantly in the last
  decade

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Affected Industrial Populations

• Back pain categorized into occupational and
  non-occupational origins. Occupational
  definition if the origin of back pain and work
  tasks can be established
• Nurses have more back injuries than most
  occupational groups (Jensen, 1985, 1986 Klein et
  al., 1984) and are particularly vulnerable to
  low-back pain
• The prevalence, incidence, and lost work time
  were high, and interference with effective work
  was great.
• Construction and Mine workers are also have
  considerable back injury (See Table 3)

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Variables in MMH

• Worker Task Environment System Concept
• The Worker Component
• The Task Component
• The Environment Component
• Interactive Effects of System Components
• System Response Measures

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Worker Task Environment System Concept

• MMH system consists of three components
• Worker
• Task
• Environment
• System may be closed or feedback where system
  has a closed loop structure and its outputs
  (responses) influence inputs in such a way that
  the goal sought is achieved (negative feedback).
• In the context of MMH, goal may be to select a
  workload that does not lead to excessive fatigue
  or injury. If workload (input) exceeds a certain
  level, it will trigger responses (output) that
  will be unsafe and will lead to excessive fatigue
  or injury.

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Lifting Limits in MMH

• First publicized set of weight-lifting limits was
  produced by the International Labor Organization
  (ILO) in 1962
• Specific safe weight limits were specified for
  men and women of different ages
• Frequency of lifting, size of the object, etc.,
  were not considered in setting these limits.
• The ILO limits (due to their lack of
  comprehensiveness or poorly stated guidelines)
  appeared to have little effect on reducing the
  incidence of musculoskeletal injury and illness
  in industry (NIOSH, 1981).
• Review of ILO load lifting limit (1988) revealed
  a large variation in interpretation and use of
  the ILO recommendations across countries
• Led to the development of the NIOSH Work
  Practices Guide to Manual Lifting and the
  following factors
• Epidemiology of musculoskeletal injury
• Biomechanics concepts
• Physiological principles
• Psychophysical (including muscular strength)
  population lifting limits

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Scope of NIOSH Work Practices Guide for Manual
Lifting

• Adequate, consistent research findings have been
  available since 1981 to support a recommendation
  only on symmetric (two-handed) lifting of loads
  in the sagittal plane
• Resulting recommendations were limited to the
  following
• Lifts should be smooth, with no sudden
  acceleration effects
• Objects to be lifted should be of moderate width,
  with a hand separation of less than 75 cm.
• Lifting postures should be unrestricted, with no
  bracing of the torso
• Couplings should be good (Handholds should be
  secure and the shoe-floor slippage potential
  low).
• Temperatures should be favorable to lifting

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NIOSH Job Attributes

• The 1981 NIOSH guide required the following four
  job attributes to be defined as the basis for
  recommending a weight-lifting limit
• Location of the center of mass of the object (or
  the geometrical center of the hand grip),
  measured horizontally from a point on the floor
  midway between the ankles (H).
• Location of the center of mass of the object (or
  the geometrical center of the hand grip),
  measured at the beginning (origin) of the lift
  (V) from the floor.
• Vertical travel distance of the hands from the
  origin oto the destination (i.e., release) of the
  object (D).
• Frequency of lifting (in lifts per minute),
  averaged over a period of lifting either of less
  than one hour or on an eight-hour basis (F).

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Additional NIOSH Job Attributes

• The angle of asymmetry, measured from the center
  of mass of the load to the bodys midsagittal
  plane (A)
• The coupling of the load, measured in three
  classes, depending on the difficulty of grasping
  the object (C).

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Basis for 1994 NIOSH-Recommended Weight-Lifting
Limit

• Intent of both 1981 and 1994 NIOSH lifting guide
  was to provide a quantitative method of
  determining the amount of weight that can be
  lifted for specific conditions defined by the H,
  V, D, F, A and C values determined by a job
  evaluation or simulation
• Expert panels assembled by NIOSH derived an
  analytical model that predicts, for a given H,
  V,D, F, A and C values, when either a
  biomechanical, psychophysical, or physiological
  population norm would be exceeded.

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• The population norms chosen by the 1991 NIOSH
  Expert Committee as the basis for the resulting
  recommended weight limit (RWL formerly the
  Action Limit in the 1981 guide) were meant to
  protect about 90 of workers.
• Biomechanically, the predicted maximum
  compressive forces on the L5/S1 disc would not
  exceed 3,400 N
• Physiologically, the metabolic energy expenditure
  rates (Kcal/min) would not exceed the following
  population limits developed by Rodgers et al.,
  (1991)

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Lift Location (V) cm Duration of Lifting Duration of Lifting Duration of Lifting
Lift Location (V) cm lt 1 Hour 1-2 Hours 2-8 Hours
V lt 75 4.7 Kcal/Min 3.7 Kcal/Min 3.1 Kcal/Min
Vgt75 3.3 Kcal/Min 2.7 Kcal/Min 2.2 Kcal/Min

• Psychophysically determined maximum acceptable
  weight-lifting limits would accommodate 75 of
  women and 99 of men (or 90 of the mix of men
  and women performing MMH jobs)
• RWL LC x HM x VM x DM x AM x FM x CM
• Lifting Index Weight of Object Lifted L
• Recommended Weight Limit
  RWL

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Table 8.2. Conversion of Lifting Task Descriptor
Variables H, V, D, A, F, and C to RWL Equation
Multiplier Variables. Frequency and Coupling
Conversions from Waters et al., 1994)
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Figure 8.3. Graphs of 1994 NIOSH multiplier
factors used in predicting RWL
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Example of NIOSH RWL Procedure

• Consult NIOSH publication before performing a job
  evaluation as this example does not include all
  of the specific rules for applying NIOSH
  procedures (e.g., lifting objects of different
  weight during the day, lifting to shelves of
  varying height, etc.).
• Example is of a worker unloading trays of dishes
  weighing 20 pounds (90 N) to a cart.
• Physical layout depicted in Figure 8.4 with
  pertinent data for the most extreme lift (e.g.,
  lowering the tray carefully to the lowest level
  on the cart
• Job analysis worksheet depicts the descriptive
  variables of the lifting task (see figure 8.4)
  which are converted to multiplier values by
  referring to the graphs in figure 8.3 with the
  resulting values shown
• The lifting index for both the origin (conveyor)
  and destination (cart) are derived

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Figure 8.4 Sample NIOSH Lifting Evaluation
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Status of the NIOSH Lifting Guide

• Keep in mind that NIOSH 1994 is an effort to
  control one aspect of MMH problems those
  associated with the simple act of two-handed
  lifting (and lowering) a load.
• Guide attempts to be more comprehensive than
  previous efforts in terms of job evaluation
  methods, criteria used for the derivation of
  limits, and control strategies

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• Too short a time period to discern effect on
  controlling musculoskeletal disorders in industry
  although anecdotal evidence supports the general
  approach set forth (Liles and Mahajan, 1985).
• Epidemiological study of 6,912 workers by Herin
  et al., (1986) found that the equation does a
  reasonably good job of predicting the incidence
  and severity of musculoskeletal injuries by
  evaluating the extremely stressful tasks that may
  exist in a job.
• Some statistics suggest a 33 reduction in
  long-term sick leave (Westgaard and Aaras, 1984
  Snook, 1988a). Mahone, 1994 believes that even
  better results could be obtained from a
  comprehensive program of job analysis and
  redesign.

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Alternative Recommendations for Evaluating Manual
Lifting Tasks

• Use of torso electorgoniometric system to
  continuously monitor the bending and twisting
  kinematics of the torso relative to the pelvis
• McGill et al., (1996) equation relating tolerance
  to compression forces on disks
• C 1067.6 1.219F 0.083F2 0.0001F3 3.229B
  0.119B2 -0.0001B3 0.862T 0.393T2 0.0001T3

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Design Criteria

• Strain/Stress Concept
• MMH Stresses
• Biomechanical Approach
• Biomechanical Analysis for MMH
• Physiological Approach
• Psychophysical Approach