Chapter 4: Motion Study and Manual Work Design

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Chapter 4 Motion Study and Manual Work Design

• Overview
• Principle of Motion Economy
• Motion Study
• Manual Work and Design Guidelines

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Principles of Motion Economy

• Achieve the maximum muscle strength at the of
  motion
• Achieve the maximum muscle strength with
  movements

Force-Velocity Relationship of Skeletal Muscle
Typical Relaxed Posture Assumed by People in
Weightless Conditions
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Principles of Motion Economy

• Use to assist workers
• Design tasks to optimize capability
• Use for tasks requiring strength

Static Strength Positions
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Principles of Motion Economy

• Stay below of maximum voluntary force

T1.2/(f-0.15)0.618-1.21 Where T enhance
time(min) f required force
Static muscle endurance-exertion level
relationship with 1SD ranges depicted
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Principles of Motion Economy

• Use short, frequent, and intermittent
• Design tasks so that most workers can do them

Percentage of maximum isometric strength that can
be maintained in a steady state during rhythmic
contraction
Changes in maximal isometric strength with age
in women and men
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Principles of Motion Economy

• Use low force for movements
• Do not attempt precise movements control
  immediately after heavy work
• Use ballistic movements for speed
• Begin and end motions with
  simultaneously
• Move hands symmetrically and simultaneously
• Use the natural of the body
• Use motions
• Work with simultaneously
• Minimize

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Classifications of Movements
Principles of Motion Economy

• Use the practical classification of
  movements

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Motion Study

• Definition analyze employed in doing a job
• Purpose
• Eliminate or reduce movements
• Facilitate and speed movements
• Basic Motions (therbligs)
• Effective Therbligs
• RE reach M move G grasp RL release
• PP preposition U use A assemble
• DA disassemble
• Ineffective Therbligs
• S search SE select P position I inspect
  H hold
• UD unavoidable delay AD avoidable delay
• R rest to overcome fatigue PL plan

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Two-hand Process Chart

• Shows all and made by left and right
  hands, and the between the basic divisions of
  accomplishment
• Purpose and the given operation
• Application highly manual operation
• How ?
• Establish the best
• Study , , of therbligs and

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Manual Work and Design Guidelines

• Energy expenditure and workload guidelines
• The energy expenditure on a task
• Ë 4.9V(0.21-EO2)
• Time required for rest to allow body to recover
• R (W 5.33)/(W 1.33)
• Where
• Ë energy expenditure (kcal/min)
• V volume of air inspired (liters/min)
• EO2 fraction of O2 in expired air (0.16-0.18)
• R time required for rest ( of total time)
• W average energy expenditure during work
  (kcal/min)
• W lt5.33 no need to rest

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Example of Energy Costs of Various Types of Human
Activity
Note energy costs are given in kilocalories per
minute
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Manual Work and Design Guidelines

• Heart rate guidelines
• An indirect measure of energy expenditure
• An acceptable increase 40 beats/min (compare
  between average working heart rate to resting
  heart rate)
• typical resting heart rate
• Heart rate creep an increase in heart rate
  during steady-state work

Linear increase in heart rate with physical
workload
Heart rate for two different workloads
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Manual Work and Design Guidelines

• Subjective ratings of perceived exertion Borgs
  Rating of Perceived Exertion (RPE) Scale
• Rating Verbal Anchor
• 6 No exertion at all
• 7 extremely light
• 8
• 9 very light
• 10
• 11 light
• 12
• 13 somewhat hard
• 14
• 15 hard
• 16
• 17 very hard
• 18
• 19 extremely hard
• 20 maximal exertion

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Manual Work and Design Guidelines

• Low Back Compressive Forces
• The internal force of the erector spinae muscle
• 2 FM 30 50
• FM muscle force
• The total compressive force exerted on the disk
• FCOMP FM 50 800
• A compressive force of 770lb (350kg) is
  considered the danger threshold

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NIOSH Lifting guidelines

• The Recommended Weight Limit (RWL)
• RWL(lbs) LC HMVMDMAMFMCM
• ? Load Index LI Load Weight/RWL ? prioritizing
  job ? redesign
• Where
• LC load constant 51 lbs
• HM horizontal multiplier 10/H
• VM vertical multiplier 1-0.0075V-30
• DM distance multiplier 0.82 1.8/D
• AM asymmetry multiplier 1-0.0032A
• FM frequency multiplier
• CM coupling multiplier
• H horizontal location of the load cg forward
  of the midpoint between the ankles, 10 ? H ? 25
  inches
• V vertical location of the load cg, 0 ? V ?
  70 inches
• D vertical travel distance between the origin
  and destination of the lift,
• 10 ? D ? 70 inches
• A angle of asymmetry between the hands and
  feet (degree) 0o ? A ? 135o

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Example 1

• Before recent automotive design changes, it was
  not unusual to have to lean forward and extend
  the arms while placing an object into the trunk
  of a car. Assume the occupant lifts a 30-lb box
  from the ground into a trunk. Being lazy, the
  occupant simply twists 90 degrees to pick up the
  box from the ground level (V0) at a short
  horizontal distance (10in). The vertical travel
  distance is the difference between the vertical
  location of the box at the destination (assume
  the bottom of the trunk is 25 inches from the
  ground) and the vertical location of the box at
  the origin (V 0), yielding D 25. Assume that
  this is a one-time lift therefore FM 1. also
  assume that the box is fairly small and compact,
  but has no handles (the coupling is fair).
  Assuming a larger reach (H25 inches) into the
  trunk because of the bumper and high trunk lip,
  no twisting, the distance traveled remaining the
  same and the coupling remaining fair. Determine
  RWL and Li for the origin and the destination

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Solution

• RWLORG 51(10/10)(1 0.00750-30)
  (0.821.8/25)(1-0.003290)(1)(0.95)23.8
• RWLDEST 51(10/25)(1-0.007525-30)
  (0.821.8/25)(1-0.00320)(1)(0.95)16.6
• LI 30/16.6 1.8 gt 1 (hazardous) ? improve ?

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Multitask Lifting Guidelines

• Compute a Single Task RWL (STRWL) for each task
• Compute a Frequency Independent RWL (FIRWL) for
  each task by setting FM 1
• Compute a Single Task LI (STLI)
• Compute a Frequency Independent LI (FILI)
• Compute the Composite Lifting Index CLI for the
  overall job by rank-ordering the tasks according
  to decreasing physical stress
• CLI STLI1 ??LI
• Where
• ??LI FILI2(1/FM1,21/FM1)FILI3(1/FM1,2,31/FM
  1,2) ..

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Example 2

• Consider the three-task lifting job as the
  following table
• Task number 1 2 3
• Load weight (L) 20lb 30lb 10lb
• Task frequency (F) 2 1 4
• FIRWL 20 20 15
• FM 0.91 0.94 0.84
• STRWL 18.2 18.8 12.6
• FILI 1.0 1.5 0.67
• STLI 1.1 1.6 0.8
• New task number 2 1 3

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Solution

• The task with the greatest LI is new Task 1 (old
  Task 2) with STLI 1.6
• The sum of the frequencies for new Tasks 1 and
  2 is 1 2 3
• The sum of the frequencies for new Tasks 1, 2
  and 3 is 1 2 4 7
• The new frequency multipliers are
• FM1 0.94, FM1,2 0.88, and FM1,2,3 0.70
• The combined lifting index is
• CLI 1.6 1 x (1/0.88 1/0.94) 0.67x(1/0.7
  1/0.88) 1.9

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Safe lifting procedure