IE 486 Work Analysis & Design IIIE 486 Work Analysis & Design II

Lecture 13 – Intro to BiomechanicsLecture 13 – Intro to BiomechanicsDr. Vincent G. DuffyDr. Vincent G. Duffy

Thursday March 1, 2007Thursday March 1, 2007

IE486 – Lecture 13 - QOTDIE486 – Lecture 13 - QOTD

Q.1. Is it reasonable to expect that the Q.1. Is it reasonable to expect that the ‘whole’ is equivalent to the ‘sum of the ‘whole’ is equivalent to the ‘sum of the parts’? Yes/No?parts’? Yes/No?

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

Q.3 What is the recommended weight limit Q.3 What is the recommended weight limit for lifting in the given example?for lifting in the given example?– 5, 10, 50 lbs.?5, 10, 50 lbs.?

Recall from Ch. 10: Engineering Recall from Ch. 10: Engineering

Anthropometry & Human VariabilityAnthropometry & Human Variability Q.1. Is it reasonable to expect that the Q.1. Is it reasonable to expect that the ‘whole’ is equivalent to the ‘sum of the ‘whole’ is equivalent to the ‘sum of the parts’?parts’?– A tall person may have short arms. A person A tall person may have short arms. A person

with long torso may have short legs. with long torso may have short legs. – You can not measure one body part and You can not measure one body part and

extrapolate to know the remainder re: fit.extrapolate to know the remainder re: fit.

Administrative: see updated schedule with Administrative: see updated schedule with minor revisions highlighted in yellowminor revisions highlighted in yellow

Today and after Spring BreakToday and after Spring Break

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

As of 1991, $27-56B (low back alone) As of 1991, $27-56B (low back alone) according to Pope, et al. 1991. according to Pope, et al. 1991.

Other recent and related staOther recent and related statisticstistics are presentedare presented in NORA (National Occupatiin NORA (National Occupationalonal Research Research Agenda) and other documenAgenda) and other documents on the NIOSHts on the NIOSH webpage. webpage. http://www.cdc.gov/http://www.cdc.gov/niosh/nora/niosh/nora/

Related statistics are presented in Wickens et al. Related statistics are presented in Wickens et al. 2004; Waters et al. 1993; 1999 2004; Waters et al. 1993; 1999

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

Re: Upper-extremity cumulative trauma Re: Upper-extremity cumulative trauma disorders (CTDs). disorders (CTDs).

Where repetitive hand and arm exertions Where repetitive hand and arm exertions are prevalent, CTDs of the upper are prevalent, CTDs of the upper extremities are common and can be even extremities are common and can be even more costly than low-back problems.more costly than low-back problems.

Briefly discuss the ‘psychophysical’ Briefly discuss the ‘psychophysical’ method of assessing static muscle method of assessing static muscle

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

Consider Figure 11.1. Find the moment Consider Figure 11.1. Find the moment about the elbow for a single segment about the elbow for a single segment biomechanical model of the forearm in which biomechanical model of the forearm in which the hand is holding a load of 25kg (rather the hand is holding a load of 25kg (rather than 20kg). than 20kg).

Fundamentals. 1. A mass in motion (or at rest) remains in Fundamentals. 1. A mass in motion (or at rest) remains in motion (or at rest) until acted upon by an ‘unbalanced’ motion (or at rest) until acted upon by an ‘unbalanced’ external force. 2. Force is proportional to the acceleration of external force. 2. Force is proportional to the acceleration of a mass (eg. At rest, use gravity). Any action is opposed by a mass (eg. At rest, use gravity). Any action is opposed by a reaction of equal magnitude. That is why we can assume a reaction of equal magnitude. That is why we can assume that the sum of moments around the elbow is zero. that the sum of moments around the elbow is zero.

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

The moment about the elbow is 46.98 Nm.The moment about the elbow is 46.98 Nm.

This comes from the sum of moments around elbow =0 This comes from the sum of moments around elbow =0

sum M =16N*0.18m(unchanged – wt of forearm/hand) +25*9.8/2*0.36 sum M =16N*0.18m(unchanged – wt of forearm/hand) +25*9.8/2*0.36

which is the new load divided by weight of each hand (by 2) multiplied by 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).gravity and multiplied by distance from hand to elbow (unchanged).

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

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

Most people do not maintain an erect posture for long, Most people do not maintain an erect posture for long, but adopt a slumped posture. but adopt a slumped posture.

The slumped position produces wedging of disks in lower The slumped position produces wedging of disks in lower back and can pressurize soft tissues in the spine causing back and can pressurize soft tissues in the spine causing low-back MSDs.low-back MSDs.

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

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

What is the difference between the What is the difference between the original NIOSH lifting equation (1981) and original NIOSH lifting equation (1981) and

the revised version from 1991?the revised version from 1991?

Eg. 1981 equation did not consider Eg. 1981 equation did not consider asymmetric lift. asymmetric lift.

In 1991 the Lift Index (LI) is also used to In 1991 the Lift Index (LI) is also used to quantify the degree to which a lifting task quantify the degree to which a lifting task approaches or exceeds the RWL.approaches or exceeds the RWL.

NIOSH Lifting Guide (Revised 1991)NIOSH Lifting Guide (Revised 1991)

NIOSH lifting equation is the ratio of load lifted to RWLNIOSH lifting equation is the ratio of load lifted to RWL LI = L / RWLLI = L / RWL

(a ratio; if LI>1, adjust task; task likely to pose increased risk (a ratio; if LI>1, adjust task; task likely to pose increased risk for some workers; if LI>3, most workers at high risk for low for some workers; if LI>3, most workers at high risk for low back pain & injury)back pain & injury)

For a given expected load to be lifted & given task, compute For a given expected load to be lifted & given task, compute the RWLthe RWL

RWL= LC x HM x VM x DM x AM x FM x CMRWL= LC x HM x VM x DM x AM x FM x CM

NIOSH Lifting Guide (Revised 1991)NIOSH Lifting Guide (Revised 1991)

compute the RWLcompute the RWL– RWL= LC x HM x VM x DM x AM x FM x CMRWL= LC x HM x VM x DM x AM x FM x CM

LC=load constant LC=load constant – Max. recommended weight under optimal conditions eg. Max. recommended weight under optimal conditions eg.

Symmetric lift, occasional lift, no torso twist, good coupling, Symmetric lift, occasional lift, no torso twist, good coupling, <25cm vertical distance of lift<25cm vertical distance of lift

HM=horizontal multiplierHM=horizontal multiplier– (moment) disc compression force increases as horizontal (moment) disc compression force increases as horizontal

distance between load & spine increases. distance between load & spine increases. – Therefore, max. acceptable weight limit should be decreased Therefore, max. acceptable weight limit should be decreased

from LC as horizontal distance increasesfrom LC as horizontal distance increases

NIOSH Lifting Guide (Revised 1991)NIOSH Lifting Guide (Revised 1991)compute the RWLcompute the RWL

– RWL= LC x HM x VM x DM x AM x FM x CMRWL= LC x HM x VM x DM x AM x FM x CM

VM = vertical distance multiplierVM = vertical distance multiplier– Lifting from the floor is more stressful than lifting from greater heights. Lifting from the floor is more stressful than lifting from greater heights. – Thus, allowable weight for lift is a function of the Thus, allowable weight for lift is a function of the originatingoriginating heightheight of of

the load. the load.

DM = distance multiplierDM = distance multiplier– Physical stress increases as Physical stress increases as verticalvertical distance of liftdistance of lift increases. increases.

AM = asymmetric multiplierAM = asymmetric multiplier– Asymmetric lift involves torso twist and is more harmful to spine than Asymmetric lift involves torso twist and is more harmful to spine than

symmetric lift. Therefore allowable load to be lifted should be reduced symmetric lift. Therefore allowable load to be lifted should be reduced when lift includes asymmetric lifts.when lift includes asymmetric lifts.

NIOSH Lifting Guide (Revised 1991)NIOSH Lifting Guide (Revised 1991)

compute the RWLcompute the RWL– RWL= LC x HM x VM x DM x AM x FM x CM RWL= LC x HM x VM x DM x AM x FM x CM

FM = frequency multiplierFM = frequency multiplier– Reflects effects of lifting frequency on acceptable lift Reflects effects of lifting frequency on acceptable lift

weights. weights.

CM= coupling multiplierCM= coupling multiplier– Difficulty of grab. Effected by whether load has Difficulty of grab. Effected by whether load has

handles.handles.

NIOSH Lifting Guide (Revised 1991)NIOSH Lifting Guide (Revised 1991)

compute the RWLcompute the RWL RWL= LC x HM x VM x DM x AM x FM x CMRWL= LC x HM x VM x DM x AM x FM x CM

ComponentsComponents MetricMetric US USLC=load constant LC=load constant 23kg23kg 51 lb. 51 lb.

HM=horizontal multiplierHM=horizontal multiplier 25/H25/H 10/H 10/H

VM= vertical distance multiplier VM= vertical distance multiplier (1-.003(V-75) 1-.0075(V-30)(1-.003(V-75) 1-.0075(V-30)

DM= distance multiplierDM= distance multiplier .82+4.5/D.82+4.5/D .82+1.8/D.82+1.8/D

AM= asymmetric multiplierAM= asymmetric multiplier 1-.0032A1-.0032A 1-.0032A1-.0032A

FM= frequency multiplier FM= frequency multiplier see table 11.2 see table 11.2CM= coupling multiplierCM= coupling multiplier see table 11.3 see table 11.3

Figure for NIOSH Lifting AnalysisFigure for NIOSH Lifting Analysis (consider QOTD 3. Compute the RWL)(consider QOTD 3. Compute the RWL)

Outgoing J-conveyor

Incoming conveyor 16” 62”

8”

36”

H=16”H=16”

V=44”V=44”

D=18”D=18”

A=80degreesA=80degrees

F=3 lifts/minuteF=3 lifts/minute

C=Good couplingC=Good coupling

Job duration: 8 hrs/dayJob duration: 8 hrs/day

Wt. Lifted: 15 lbs.Wt. Lifted: 15 lbs.

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

HM= 10/H HM= 10/H

VM=1-.0075x(V-30)=VM=1-.0075x(V-30)=

DM=.82+1.8/D=.DM=.82+1.8/D=.

AM=1-.0032xA=AM=1-.0032xA=

FM= (from table)FM= (from table)

CM=( from table)CM=( from table)

RWL=51xHMxVMxDMxAMxFMxCMRWL=51xHMxVMxDMxAMxFMxCM

Six multipliers that can be calculated:Six multipliers that can be calculated:

HM= 10/H = 10/16=.625HM= 10/H = 10/16=.625VM=1-.0075x(V-30)=1-.0075x(44-30)VM=1-.0075x(V-30)=1-.0075x(44-30) =.895=.895DM=.82+1.8/D=.82+1.8/18=.92DM=.82+1.8/D=.82+1.8/18=.92AM=1-.0032xA=1-.0032x80=.744AM=1-.0032xA=1-.0032x80=.744FM=.55 (from table at 3lifts per min. V>30”)FM=.55 (from table at 3lifts per min. V>30”)CM=1.0 (good coupling; from table)CM=1.0 (good coupling; from table)  RWL=51xHMxVMxDMxAMxFMxCMRWL=51xHMxVMxDMxAMxFMxCM ==

Six multipliers that can be calculated:Six multipliers that can be calculated:

HM= 10/H = 10/16=.625HM= 10/H = 10/16=.625VM=1-.0075x(V-30)=1-.0075x(44-30)VM=1-.0075x(V-30)=1-.0075x(44-30) =.895=.895DM=.82+1.8/D=.82+1.8/18=.92DM=.82+1.8/D=.82+1.8/18=.92AM=1-.0032xA=1-.0032x80=.744AM=1-.0032xA=1-.0032x80=.744FM=.55(from table at 3lifts per min. V>30”)FM=.55(from table at 3lifts per min. V>30”)CM=1.0 (good coupling; from table)CM=1.0 (good coupling; from table)  RWL=51xHMxVMxDMxAMxFMxCMRWL=51xHMxVMxDMxAMxFMxCM =51x.625x.895x.92x.744x.55x1.0=51x.625x.895x.92x.744x.55x1.0

=10.74 (lbs)=10.74 (lbs)

Lift indexLift index

LI = L/RWL = 15/10.74=1.4 LI = L/RWL = 15/10.74=1.4 some workers would experience an increase in some workers would experience an increase in risk of back injury because the lift index is >1.0. risk of back injury because the lift index is >1.0.

some precautions should be taken to minimize some precautions should be taken to minimize the risk of injury, and the job may need to be the risk of injury, and the job may need to be redesigned to lower the LI.redesigned to lower the LI.