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Manual Material Handling. Introduction. Research has been conducted in the area of MMH for about 40 years – continued research in this area today

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Manual Material Handling

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Manual material handling

Manual Material Handling



  • 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)

Mmh injury frequency and cost

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

Manual material handling

  • 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

Affected industrial populations

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)

Variables in mmh

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

Worker task environment system concept

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.

Lifting limits in mmh

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

Scope of niosh work practices guide for manual lifting

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

Niosh job attributes

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).

Additional niosh job attributes

Additional NIOSH Job Attributes

  • The angle of asymmetry, measured from the center of mass of the load to the body’s midsagittal plane (A)

  • The coupling of the load, measured in three classes, depending on the difficulty of grasping the object (C).

Basis for 1994 niosh recommended weight lifting limit

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.

Manual material handling

  • 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)

Manual material handling

  • 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

Manual material handling

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)

Figure 8 3 graphs of 1994 niosh multiplier factors used in predicting rwl

Figure 8.3. Graphs of 1994 NIOSH multiplier factors used in predicting RWL

Example of niosh rwl procedure

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

Figure 8 4 sample niosh lifting evaluation

Figure 8.4 –Sample NIOSH Lifting Evaluation

Status of the niosh lifting guide

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

Manual material handling

  • 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.

Alternative recommendations for evaluating manual lifting tasks

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

Design criteria

Design Criteria

  • Strain/Stress Concept

  • MMH Stresses

  • Biomechanical Approach

    • Biomechanical Analysis for MMH

    • Physiological Approach

    • Psychophysical Approach

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