IE 486 Work Analysis & Design II Lecture 13 – Intro to Biomechanics Dr. Vincent G. Duffy Thursday March 1, 2007

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.?

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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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 Other recent and related statistics are presented in NORA (National Occupational Research Agenda) and other documents on the NIOSH webpage. Related statistics are presented in Wickens et al. 2004; Waters et al. 1993; 1999

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.

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 person’s strength. And ECE 511 Prof. Hong Tan, Psychophysics.

Consider Figure 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).

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.

The moment about the elbow is Nm. 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 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).

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.

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). MPL is considered the weight limit at which most people would experience a high risk of back injury (for those lift conditions).

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

NIOSH Lifting Guide (Revised 1991) NIOSH lifting equation is the ratio of load lifted to RWL LI = L / RWL LI = L / RWL (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 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

NIOSH Lifting Guide (Revised 1991) compute the RWL –RWL= LC x HM x VM x DM x AM x FM x CM LC=load constant –Max. recommended weight under optimal conditions eg. Symmetric lift, occasional lift, no torso twist, good coupling, <25cm vertical distance of lift HM=horizontal 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

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.

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.

NIOSH Lifting Guide (Revised 1991) compute 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 ComponentsMetric US LC=load constant 23kg 51 lb. HM=horizontal multiplier 25/H 10/H VM= vertical distance multiplier (1-.003(V-75) (V-30) DM= distance multiplier /D /D AM= asymmetric multiplier A A FM= frequency multiplier see table 11.2 CM= coupling multiplier see table 11.3

Figure for NIOSH Lifting Analysis (consider QOTD 3. Compute the RWL) Outgoing J-conveyor Incoming conveyor 16” 62” 8” 36”

H=16”V=44”D=18”A=80degrees F=3 lifts/minute C=Good coupling Job duration: 8 hrs/day Wt. Lifted: 15 lbs.

Six multipliers that can be calculated to get Recommended Weight Limit (RWL): HM= 10/H VM= x(V-30)=DM= /D=.AM= xA= FM= (from table) CM=( from table) RWL=51xHMxVMxDMxAMxFMxCM

Six multipliers that can be calculated: HM= 10/H = 10/16=.625 VM= x(V-30)= x(44-30) =.895 =.895DM= /D= /18=.92AM= xA= x80=.744 FM=.55 (from table at 3lifts per min. V>30”) CM=1.0 (good coupling; from table) RWL=51xHMxVMxDMxAMxFMxCM = =

Six multipliers that can be calculated: HM= 10/H = 10/16=.625 VM= x(V-30)= x(44-30) =.895 =.895DM= /D= /18=.92AM= xA= x80=.744 FM=.55(from table at 3lifts per min. V>30”) CM=1.0 (good coupling; from table) RWL=51xHMxVMxDMxAMxFMxCM =51x.625x.895x.92x.744x.55x1.0 =51x.625x.895x.92x.744x.55x1.0 =10.74 (lbs) =10.74 (lbs)

Lift index LI = L/RWL = 15/10.74=1.4 some workers would experience an increase in risk of back injury because the lift index is >1.0. some precautions should be taken to minimize the risk of injury, and the job may need to be redesigned to lower the LI.