Cornell University Ergonomics Web
Cornell University Ergonomics Web
DEA 3250/6510 CLASS NOTES
Some Contemporary Job Analysis Systems
Several techniques exist for studying and quantifying
how stressful a job is on the person.
1. Timed Activity Analyses (TAA) - Techniques focus on analyzing
the work pattern, i.e. what the worker is doing and how this is being done
in time.
a. Components -
1) Work job activities
2) Tasks - Discrete activities within a work period.
3) Cycles -a) Work Cycle - Time to complete an action on the product or equipment.
b) Recovery Cycle - Time between work cycles (may include light work).
4) Motion - Individual motions that make up a job such as: moving (M); placing (P); and fastening (F).
b. Work Task Analyses - A chronological diary of job tasks and recovery periods can show the ratio of physically demanding work to light work or rest. The higher the work/rest ratio, the greater the chance for fatigue, for errors, and accidents. This analysis identifies the likely problem work periods, tasks, and equipment.
c. Analysis of time-shared motion - In time-shared work the worker must do tasks simultaneously or intermittently. Since cycle times may be very short, this type of analysis identifies time pressure stress. Work may be machine-paced.
d. Motion Analysis - can identify problem movements by recording the length and type of each movement. Methods-time measurement (MTM) can help identify problem muscle groups that will be stressed, and inefficient and awkward movements.
e. Work Cycle Analysis - Work cycles within tasks can be identified. Variability in cycle time is a useful indicator of job demands, amount of operator control, and detecting of any fatigue that may affect performance.
2. Job Postural Evaluation Method - (Postural Targeting) - Job analysis
observes a worker at random times in the workday and for each observation
records the angular configuration of various body segments using a "body
diagram."
a. Angular Data - are recorded by checking an X on each postural target any deviations from erect anatomical position. The pattern of "X's" can then be counted/plotted and used to represent the postural profile of a job. The type of activity is also recorded and it is possible to combine this information with load magnitudes as well.
1) Concentric Circles - represent 45, 90, 135, angular deviations of joints. These are shown with central arrows indicating the front of the body.
2) Radial Lines - indicate the amount of deviation from saggital plane when viewed from overhead.b. Upper Extremity Postural Analysis - The operator is filmed and the posture of the upper extremity is coded from each film frame using the procedure chart shown. Both limb positions and the type of grip used. Finally, the point of force loading on the hand is noted using 0 - palm contact; 1 - 5 - for the digits 1 through 5.
3. Physical Stress Checklists and Surveys - Jobs can be evaluated
by looking at the profile of component physical tasks. These surveys can
take many forms. An analyst can directly observe a job, noting frequency
and time of particular activities and any weights handled, as well as left
and right hand activities. This data can then be compared with clinical
assessment of a person's capabilities in terms of mobility and strength.
(e.g. NIOSH Analysis )
Job Energy Expenditures
1. Mechanical Properties of Muscle -
a. Type of Contraction -
i) Isometric - Contraction doesn't change muscle length. Produces muscle tension but no useful mechanical work.
ii) Isotonic - Contraction changes muscle length. Produces useful mechanical work.b. Muscle Contraction - when a muscle is stimulated by a single nerve action potential, a contraction called a twitch occurs. The duration of the periods during muscle contraction varies greatly between muscles. When several neural stimuli arrive at the muscle this continues summation of contraction until at a certain rate. The number of stimuli leads to tetanus, the maximal sustained contraction.
i) Compression - at the joint, force along the bone.
ii) Rotation - at the joint, force perpendicular to the bone.c. Muscle Movement - Force from a contracting muscle is transmitted by the tendon. When this occurs at a joint there is movement.
i) Agonists - Prime movers of muscles, e.g. biceps.
ii) Antagonists - Those muscles having an opposite effect, e.g. triceps.
iii) Synergists - Muscle groups acting in synchrony to produce controlled movement involving a series of joints. Muscles also prevent the joint from dislocating by providing stability.
2. Muscle Mechanics - Forces developed by muscle are results of muscles' contractile and connective tissue, and arrangements of muscles on the skeleton. Contractile an connective tissue affect the length-tension characteristics of muscles.a. Contractile Elements - are the individual myofibrils which change length to produce tension.
b. Connective Tissue - determines the stiffness of the muscle by acting as a parallel elastic component (like an elastic band). This component is slack at the resting length of a muscle so the length-tension curve is like that for the contractile elements alone. As the muscle is lengthened the non-linearity of the connective tissue is increased and the tissues increase in tension.i) More Connective Tissue - means that muscles are stiffer and have higher passive tensions, e.g. postural muscles which must continuously act against gravity.
ii) Less Connective Tissue - means that muscles have lower passive tensions, e.g. biceps.
iii) Total Tension of Muscle - is the sum of the contractile tension and the parallel elastic tension.3. Muscular Contraction and Movement - all mechanical work involves muscular contraction.
a. Effort - is influenced by the duration of contraction, i.e. the continuous period of muscle contraction or total amount of time per minute muscles are active, and frequency, i.e. the number of work cycles per minute, hour, or shift which defines the pattern of work, and intensity, i.e. heavy, moderate, or light work according to oxygen demands or strength requirements.
b. Energy Requirements - Energy consumption is greater with more intense contractions, longer strenuous effort and more frequent repetitions. Carbohydrates (sugars and starches) and fats (free fatty acids) are the two nutrients most involved in providing energy for muscular work. Nutrients are burned to provide energy for muscular work via two processes.
c. Anaerobic Processes - muscular processes which don't need oxygen. There are two ways muscles can contract without oxygen:
i) Anaerobic Glycolysis - Breakdown of glucose leads to lactic acid. If glycolysis is the only energy source then work cannot be sustained for very long because of lactic acid accumulation leads to cramps. Energy produced is stored in adenosine triphosphate (ATP).
ii) High Energy Phosphates - Creatinine phosphate (CP), and ATP stored in muscles can release energy for work. As CP and ATP levels decline it leads to rapid fatigue of muscles. CP is a storage system for ATP and it replenishes ATP supply as this is used.
d. Aerobic Processes - muscular processes which use oxygen. Small amounts of oxygen are stored bound in muscle protein, myoglobin, which can be utilized in short, intense periods of work. All other oxygen is supplied via the circulatory system. It can take up to one minute for blood flow to respond to sudden increases in workload so there may not be sufficient oxygen at the start of work.
i) Aerobic Glycolysis - After CP and ATP levels decline, aerobic glycolysis takes over and by measuring the amount of oxygen consumed by a person during work we can determine the amount of aerobic metabolism taking place for that work. Aerobic glycolysis generates almost 20 times as much ATP as anaerobic glycolysis.
ii) Oxygen Debt - is the amount of oxygen required by muscles after the beginning of work over and above that which is supplied to them by the circulatory system during their activity. Following work, there is a recovery period during which the oxygen debt must be repaid.
iii) Muscle Fatigue - occurs because of decreased energy stores in muscle along with the accumulation of lactate (waste metabolic product).
iv) Endurance Time - Time it takes for a muscle to fatigue and this is a function of contraction force.
v) Stretch Reflex - As strain on muscle increases so the reflex contraction increases. This stretch reflex is maintained until the load is removed. The reflex controls posture a muscles respond to being strained by gravity.
vi) Muscle Tone - is the result of the stretch reflex and the elastic properties of the muscle AS WELL AS the ability to resist passive movement.
e. Energy Expenditure - is measured in kilocalories (kcal) per minute. [70 kcal per minute = 1 watt]. Energy expenditure is about five times oxygen use (in liters per minute). Work typically ranges between 1.6 - 16.2 kcal/minute. The pool of high-energy compounds stored in the body is typically less than five kcal which is enough to support moderate work for one minute. Energy costs for most types of work vary considerably with the way in which this work is done.
i) Postural Effects - Postural changes can increase energy consumption by almost 100% even when picking up light objects from the ground.
ii) Pace of Activity - Optimum pace of activity seems to vary between individuals and between ages. Since this pace varies for each person, externally paced work can be problems for some workers and not others.
iii) Energy Costs of Grades of Work - for adult males, basal metabolic requirement is about 2300 kcal per day. Estimated maximum energy output is about 4800 kcal per day. Maximum continuous work output is about 2500 kcal per day or 5 kcal per minute. About 4 kcal per minute has been suggested as maintainable work output per day.
iv) Work and Rest - Since many work activities exceed 5 kcal per minute, we need rest periods to compensate for this excess. These can be calculated as follows:
see diagram r equals t times k-s over k-1.5 R= rest required in minutes
T= total working time (minutes)
K= average kcal per minute of work
S = kcal per minute chosen as desirable standard
1.5 = approximate resting level in kcal per minute.
Roughly, one needs about 30 minutes of rest for every 30 minutes worked. Total rest requirements for different types of work can be estimated.
Other Issues
COMPENSATION DISABILITY
IMPAIRMENT
INJURY
ACHES AND PAINS
EARLY WARNING DISCOMFORT
Sitting and doing work the body is more static.
INJURY RATES
Nurses 431 Have to move poorly designed
Electricians loads
Construction workers
Telecommunication workers
Ex. Nurses bathe patients, move them, etc., often at wrong height
Weight of the object
NIOSH & European standards - weight at 50 lbs.
Packaging in Europe is marked with:
- weight
- handles
- center of gravity
Great the height of lift, greater the compressive force.
Heavy loads have enormous effect on back injuries
Variables which contribute to back injury: too heavy of a load and poor
posture
Lifting up is more risky than lifting down off shelf.
Almost half of all injuries occur at moment of initial lifting. Carrying
distance
Static muscle strength decreases quickly
2/3 of all injuries occur within first minute
Frequency of lifting
Postural Issue
15% 58% 25% Position of back at time of injury
Movement at Time of Injury
- Bending (Two most risky)
-Twisting
- Standing
- Sudden change in position
Odds of Prolapsed Lumbar Disc
Controls Lifting while twisting Lifting while twisting
w/bent knees with knees straight
TYPE OF ACTIVITY
Maximal Isometric Muscle Strength (MVC)
Comparison of healthy males, stronger back and abdominal muscles, less risk
of injury
Abdominal exercise prior to injury
Employees programs that encourage abdominal exercises.
Occupational Factors That Contribute to Lower Back Pain
1. Job Design Size, weight, distance to move the load
2. Change work methods
3. Lack of adequate strength, stamina, or flexibility
4. Lack of appropriate training
5. Inadequate tools
ex. suitcases with stiff arms and wheels
ex. Back safety belt - no evidence that they do anything
To reduce spinal stress - MINIMIZE: - bending
- twisitng
- reaching
- static work loads & postures
WORK EVALUATION (cont.)
Muscle Mechanics
contractile forces - contractile elements myofibrils (long stringy
things)
(muscle tone)
connective tissue - parallel elastic component (like a rubber band)
muscle with lots of connective tissue are stiffer and have a higher passive
tension
- skeletal muscles controlling body posture
ex. biceps - have relatively low passive tension
How do you determine the optimum amount of time to work in relationship
to rest?
Total muscle tension = contractile elements + parallel elastic tension (of
that muscle group)
People with more connective tissue have a higher basal tone.
ENERGY EXPENDITURE
Kcal (kilocalories)
(70 Kcal per minute = 1 watt)
person who weighs ~ 165 lb. ~ 44 lbs of muscle
anaerobic aerobic (1 Kcal)
(3.5 Kcal) creatinine phosphate (CP) Adenosine Triphosphate (ATP)
ex. sleeping = 1.3 Kcal
sitting = 1.6
standing = 2.25
Calories expended carrying loads:
Double pack Head Rucksack Rice Bag Yoke Hands
100 144
Picking up load:
Bending arm on thigh
no arm support - least efficient and most risky
squatting - most efficient posture
HOW QUICKLY DO YOU WORK?
- want to place worker in optimum efficiency zone
.