exam 2 (ergonomics)

What is ergonomics?

Ergonomics is the process of designing or evaluating products, tasks, environments, or and systems involving people to improve performance and / or reduce risk of injury.

Human factors

cognitive, mental, sensors

Why do we care about this stuff?

• Poor ergonomics leads to more musculoskeletal disorders

• Poor ergonomics leads to higher cost

• Poor ergonomics leads to reduced worker and company performance

The overall ergonomics process

1. Characterize existing or potential problems

2. Perform Job Analysis

3. Implement controls

4. Evaluate effectiveness of controls

Guiding principle of ergonomics

D = task demand

C = human capacity

maintain D < C

The bigger the difference, the lower the risk of injury

6 types of interactions in work system

Human to machine

machine to human

human to environment

environment to human

machine to environment

environment to machine

Elasticity

A materials response to stress or strain depends on the applied stress or strain

Viscoelasticity

A materials response to stress or strain depends not only in the applied stress or strain, but also on time

• tissues respond over time based on movement

• ligament, tendon, cartilage, fascia, and to lesser extent, bone are all viscoelastic

• repeated loads or movements can accumulate over time to have larger effects than just one repitition

What are WMSDs?

• Work-related Musculoskeletal Disorders

• Injury to soft tissues of the body and joints (muscles, ligaments, tendons, cartilage, nerves)

Risk factor

A characteristic that increases the risk of injury

Three general types of risk factor for WMSDs

• Individual

• Task-related

• Psychosocial

Individual risk factors

• age

• sex

• obesity

• systemics diseases

• acute trauma

• congenital conditions

task related risk factors

• high force exertions

• high repetitive work or static work

• awkward / extreme postures

• vibration

• contact mechanical stress

• environmental stress

Psychosocial risk factors

psychological factors and social factors that influence the mental state of the individual

could be mental stress, overbearing supervisor, lack of emotional support, etc

Characterize existing or potential problems

Goals are to:

• quantify injuries or performance

• identify tasks that may have risk factors

Some examples

• injuries

• employee / supervisor concerns

• unnecessary steps in a work task

• production or quality problems

Perform Job analysis

Goal is to:

• identify and quantify task-related risk factors

Tools

• Measure work task characteristics

• biomechanical analysis

• psychophysical methods

• checklists

• measure energy expenditure

• employee interviews

Implement controls

Goal is to reduce exposure to task related risk factor

Most effective to least:

• Elimination

• Substitution

• Engineering controls

• Administrative controls

• PPE

Evaluate effectiveness of controls

Commonly involves repeating step 2

• Did you reduce the risk factors found in step 1

• Did you create any new risk factor

• Ask workers if they fell better or notice positive differences

• Provide employees with basic awareness of ergonomic principles

Awkward or Extreme Posture (Natural forces)

• Posture includes all your joint angles

• Neutral posture (most joint angles near middle of range of motion)

• Awkward posture - requires excessive effort against gravity

• Extreme posture - at or near joints limit of motion

• The problem is that it requires greater muscle effort which results in in greater force and stress on joints/ tissues

Awkward or Extreme Posture : Job analysis

Determine frequency, duration, and / or magnitude of awk/ext postures

High force exertions

Cause:

• Increased risk of muscle fatigue

• increased force and stress on joints/ tissues

• reduced / no circulation

• large tissue strain > creep

• increased risk of chronic muscle, tendon, and nerve disorders

High force exertions : Example - MMH

Manual Material Handling

• Lifting

• carrying

• pushing

• pulling

• grip

• pinch

Contact Mechanical Stress

Mechanical stress directly on blood vessels and nerves can reduce blood flow, exacerbate fatigue, or irritate nerves, any of which can increase the risk for WMSDs

Determine frequency, duration, and / or magnitude of contact force

Other task related risk factors

• Temperature

• Vibration

Anthropometry

The study of the physical dimensions of the human body

• used for design of dimensions of seating, furniture, tools, workspaces, and many spaces that humans occupy

• used in biomechanical models to predict human reach, force, and space requirements, and to calculate work demand

Anthropometric mismatch

Results when tools, furniture, workspaces, etc. do not fit well with the physical dimensions of the human body

General Design Strategies

• Design for Average

• Design for Extremes

• Design for Adjustability

Ratings of Perceived Exertion (RPE): The Borg Scales

Borg 6-20 scale

• Whole body assessment, based on heart rate

Borg 0-10 scale

• Localized Assessment, based on % of strength used

NIOSH Lifting equation (National Institute of Occupational Safety and Health

Was compiled by a panel of experts to provide a quantitative method for determining the amount of weight that can be lifted for specific conditions

• Limited to symmetric, two handed, lifts in the sagittal plane

• Four inputs included: horizontal position of object, vertical position of object at beginning of lift, vertical distance moved, and frequency of lifting

• Added inputs of asymmetry, and rating of coupling between load and hand

The NIOSH equation predicts the recommended weight limit (RWL) for lifting based on three injury risk criteria:

1. Biomechanical criterion: i.e. joint loading

2. Physiological criterion: i.e. energy requirements during repetitive lifting

3. Psychophysical criterion: i.e. maximum acceptable weight limit

Recommended Weight Limits and Lift Index

RWL = 23 kg x HM x VM x DM x AM x CM x FM

Lift index = (Actual load)/ RWL

Interpretation

• Lift Index < 1 OK

• Lift Index = 1 borderline

• Lift Index 1 to 3 may have increased risk

• Lift Index > 3 likely have increased risk

How is the lift equation enforced in the US?

There is no law requiring the use of the NIOSH LE, however

The General duty clause, Section 5(a)(1) of the Occupation Safety and Health Act of 1970, states employers are required to provide their employee with a place of employment that is “free from recognized hazards that are causing or likely to cause death or serious physical harm.”

• OSHA can levy fines against employers that violate the General Duty Clause

• OSHA can use the NIOSH LE to determine if employers are violating the General Duty Clause

What if our job analysis determines a MMH task has elevated risk?

• Elimination and substitution tend to be the most difficult to implement in an existing process (but most effective)

• Engineering controls are often lumped together with elimination and substitution because sometimes it is difficult to distinguish between these controls

Relative Measures of Injury Risk

1. Relative Risk (RR)

• A ration of two risk injury (or probabilities)

RR = (Risk of injury in group exposed to X) / (Risk of injury in group not exposed to X)

Where X can be a risk factor or control

Problems with high metabolic demands of work

• Elevated heart rate and respiration rate

- loss of fine motor control leading to more errors

-for individuals with heart problems, insufficient oxygen to the heart can increase the risk factor of heart attack

• General and localized muscle fatigue

-loss of metabolic energy can lead to fatigue

-insufficient oxygen > anaerobic metabolism > lactic acid > pain, cramping

-a fatigue worker is less efficient, less productive, more error-prone, and more injury-prone

-fatigue is a risk factor for WMSDs

What is metabolic energy?

• Adenosine Triphosphate (ATP)

-the body’s “molecule of energy”

-”Fuel” for all cellular functions including muscle contractions

- ATP > ADP + Pi + [energy for our body to live and work]

• ATP is stored locally in muscles, but only enough to provide 2-3 sec of energy supply. Therefore, constant regeneration of ATP is necessary

• Metabolism is the process of regenerating ATP from food -

-ADP + Pi + [energy from food] > ATP

Aerobic vs Anaerobic Metabolism

• Aerobic metabolism (requires O2)

-Slow production of ATP that can be sustained for long duration

-Always being used and is usually sufficient

-Efficient (produces 36 molecules of ATP per glucose molecule)

-By products: carbon dioxide

• Anaerobic metabolism (does not require O2)

-Quick production of ATP that can only be sustained for short duration (< 2 mins)

-Kicks in during high intensity tasks (> 85% max heart rate)

-Inefficient (produces 3 molecules of ATP per glucose molecule)

-By products: carbon dioxide, water, lactic acid (pain, cramps, tremors)

Aerobic Capacity

• Aerobic Capacity (AC) = maximum capacity to utilize oxygen

• aka VO2 max, maximum aerobic capacity, maximum oxygen consumption, maximum oxygen uptake

• AC measures the cardiorespiratory fitness of an individual

• Olympic level athletes

F: 70-80 (mL O2)/(kg*min)

M: 80-90 (mL O2)/(kg*min)

• Young adults

F: 30-40 (mL O2)/(kg*min)

M: 40-50 (mL O2)/(kg*min)

Physical work rate capacity or Recommended Work Rate

• Workers cannot work at their AC for longer than a few moments

• Physical work capacity (PWC) - the maximum rate your body can generate metabolic energy during continuous work over an extended time and not overexert itself

• Recommended work rate (RWR) = PWC

• Analogy: You can’t sprint a mile or you will fatigue too quickly and not be able to finish. Think of AC as your speed for sprinting 100 meters, and RWR as your best pace to run a mile.

Aerobic and Recommended work rate units

• Aerobic Capacity units = (mL O2)/(kg*min)

• Recommended work rate = Kcal/min where 5 kcal = 1 liter of O2

• 1 Kcal = 1 Calorie = 1 Calorie on a food label

We have two ways to calculate recommended work rate for continuous work

1) As a flat percentage of AC

• NIOSH (1981)

• 33% of AC for hours of continuous work

• For the US working population, 50th percentile AC is:

-15.0 kcal/min for males (33% in 5 kcal/min)

-10.5 kcal/min for females (33% in 3.5 kcal/min)

Comparison values

• resting = 1-1.5 kcal/min

• walking at 3 mph = 5-6 kcal/min

• physically demanding work = 8-12 kcal/min

We have two ways to calculate recommended work rate for continuous work

2) Predict based upon AC and working time

General Formula:

Recommended work rate = (log4400 - logt)(AC)/3.0

AC = aerobic capacity: kcal/min

t = working time (min)

log = log10

Energy Expenditure

• Energy expenditure (EE) is the metabolic or calorie demands of a task in kcals/min

• Usually quantified as a rate, even though the term does no indicate this

• Direct measurement such as during a VO2 max test is usually impractical

• Indirect methods for estimating energy expenditure (More complex, more accurate):

1. Average values reported for various tasks

2. Subjective ratings

3. Estimate from heart rate

4. Task analysis

4b. Energy Expenditure from task analysis

Metabolic Equivalent of Task or METs!

Using METS requires some understanding of our metabolism

Components of our Metabolism

• Basal Metabolism (BM) - EE required for the body to function (keep you alive)

• Activity Metabolism (AM) - EE required to perform specific activities

• Digestive Metabolism (DM) - EE required to digest foodstuffs (10% of the calories

• Total EE = BM + AM + DM

Metabolic Equivalent of Task (MET)

Describes the energy expenditure of an activity as multiple of your basal metabolic rate

1 MET = 3.5 ml O2/(kg*min)

1 MET = EE of sitting quietly (1 kcal/(kg*hr))

Fatigue is a risk factor

• Fatigue is a risk factor for human error , accidents, and non-WMSD injuries

• Fatigue is also a risk factor for WMSDs

• As a result, fatigue can also be an indicator for ergonomic intervention

• Later in this lecture, we will talk about methods to asses fatigue so that we can evaluate fatigue in steps 2 and 4 of the ergonomics process

Physical Fatigue

• Definition: Impaired ability to continue physical exertion at a typical level, or a decreased capacity to generate force or expend metabolic energy

• Symptoms & consequences (in workers):

-Feeling of physical discomfort

-Decrease muscle force/moment strength

-Decreased metabolic energy

-Loss of fine motor control

-Impaired decision making and possible mistakes

Types of Physical Fatigue

Physical fatigue can be broken down into two components

• Peripheral fatigue - metabolic changes in the muscle itself or nervous system to muscles

-accumulation of metabolic by products and depletion of energy sources in the muscle

• Central fatigue - central nervous system changes in the control of muscles

-reduced or changed muscle activation signals from the brain

Localized Muscle Fatigue

• A type of physical fatigue that can include central and peripheral fatigue, but is localized to a specific muscle or muscle group

• Common in physically demanding jobs

Results in:

• A decline in muscle force capacity

• localized discomfort and pain

• decreased motion control and accuracy

• negative effects in productivity and performance

Subjective fatigue evaluation methods

1. Borg Ratings of Perceived Exertion (RPE): (Exertion level is closely related to fatigue because high exertion levels lead to fatigue)

Strength vs Endurance

• Strength is the maximum joint moment or external force that can be generated (Shot term effort)

• Endurance is the time duration an exertion level can be sustained until exhaustion (aka time to exhaustion) (Longer term effect)

Fatigue, strength, and endurance are all related

• Fatigue is the process that causes a decline in capacity to generate a moment or force

• Endurance time (or time to exhaustion) is when capacity < demand

Endurance Time vs. Exertion Level

• Rhomert (endurance) curve

-Nonlinear relationship between endurance and static exertion level

• Endurance time is not indefinite for f<15% exertion level

• Large variability within and between individuals

Work Measurement

Def: is the systematic determination of the length of time it should take to complete a job

Work measurement can help:

• Determine the time required for job task or product production

• Determine number of workers and/or machines required

• Establish productivity standards or targets

• Maximize efficiency by eliminating lost or ineffective time

• Determine product cost

The main goal of work measurement is to determine standard time

Standard time (or time standard) is the time required for an average, qualified worker, working at a normal pace, to perform an operation and achieve a standard performance.

Work Measurement Techniques

Work measurement techniques >

1. Estimation > Historical Data > SWAG

2. Direct Observation > Time Study > Work Sampling

3. Predetermined Time systems > AFTWAYS > MTM > MOST

Direct observation methods for determining standard time

• Time study a method of developing a standard time by systematic, intensive observation of a task, and analyzing the timing of it. TS is the most widely used work measurement technique. Dates back to 1883!

• Work Sampling is a methods of analyzing work by taking a large number of observations at random intervals to analyze worker performance and establish standard time.

Time study procedure

1. Define and document standard method

2. Divide task into work elements

3. Time work elements to get observed time

4. Rate worker performance

5. Calculate normal time

6. Complete calculations on worksheet to determine average normal time

7. Calculate allowances

8. Calculate standard time

1. Define and document standard methods

• Define the standard methods to complete the task

-Standard method should be “best” method that balances safest, fastest, most productive, and least demanding methods to the worker

-Once standard method is defined, it should not be possible for operator to make further improvements

• Document the standard method

-Procedures (actions and motions)

-Tools used

-Machine settings

-Workplace layout, working conditions, work setup

-Irregular work elements

2. Divide task into work elements

• Work element

-A distinct elemental or fundamental motion (e.g., reach, grasp, place)

• Some tips

-Look for obvious “break points” between work elements (i.e. easily detectable, not ambiguous)

-Cannot be time gaps between elements

-Should not be too long (< 3 minutes)

-Should not be too short (>3 seconds)

-Irregular elements (those not performed every cycle of the task) should be identified & distinguished

4. Rate Worker Performance

• People do not work consistently from day to day or even from minute to minute

• As such, the analyst must account for this by rating worker performance

• Rating = 100% for standard (normal) performance by a qualified workers without over-exertion

• Rating is very important

-Can have a big effect on standard time

-Can depend on the experience, training, and judgment of analyst

Existing rating methods

• Speed rating: fastest and simplest method (so most valid results)

-Speed rating usually covers a range of 50% to 150%

-To effectively use speed rating (observer must have experience in the class of work performed)

• Other established rating methods exist that don’t focus solely on speed and include other factors such as motion data and task difficulty

Calculate normal time

NT = OT x rating/100

where:

NT = normal time

OT = observed time

• When speed rating is slower than 100% less NT is provided than observed (assumes worker can/should increase work rate)

• When speed rating is faster than 100% more NT is provided than observed (Assumes worker will/should slow work rate)

Example

• Calculate normal time if:

1. observed time = 10 seconds and rating = 80% (NT = 8 seconds)

2. observed time = 10 seconds and rating = 100% (NT = 10 seconds)

3. observed time = 10 seconds and rating = 120% (NT = 12 seconds)

Calculate allowances

• Allowances increase the normal time to account for losses in time due to various reasons

• Allowances are given as percentages of normal time

• Allowances are generally set by the company

• When we calculate the total allowance, we add all components of allowance i.e.,

-personal allowance allowance +

-fatigue/rest allowances +

-delay allowance

Calculate standard time

• Normal time (NT) = observed time (OT) x Rating / 100

• Standard time (ST) = NT (1 + allowance)

if NT = 10 seconds, calculate ST if:

1. Allowance = 0% (ST = 10 seconds)

2. Allowance = 5% (ST = 10.5 seconds)

3. Allowance = 10% (ST = 11 seconds)

• Sum the ST you have for each work element to determine the Total Standard Time

• The format of Time Study worksheets vary, but they all have these essential elements

Direct Observation methods for determining time standards

Work sampling is a methods of analyzing work by taking a large number of observation at random times to analyze worker performance and establish standard time

Work sampling is most commonly used to:

• Determine worker and machine utilization for setting staffing levels and machine needs

• Quantify the effects of worker performance on efficiency

• Collect information for allowance calculation

• Develop standard time and production standard

Work sampling vs. Time study

• Unlike a time study, work sampling does not require continuous observations by the analyst over time.

• Work sampling is better suited for jobs that have long cycle times or low repetition rates, such as those in maintenance, many office-type jobs, and sometimes material handling

• Total observation time to complete a work sampling study is typically shorter than a time study (therefore less costly)

• Time study provides a more detailed analysis of job elements than work sampling

Summary: Three types of work sampling studies

1. Elemental ration study - determine the percentage of total time that each element of a work task takes

2. Performance sampling study - determines the percentage of total time that each element of a work task takes, AND evaluates performance rating

3. Time standard development study - determines the percentage of total time that each element of a work task takes, AND evaluates performance rating, AND develops time standard.

Predetermined Time Systems

• PTS is another technique for determining time standards

• All work has been reduced to basic motions. Each basic motion has a time value associated with it

• Once all the motions for the task have been determined, the time values are totaled and the standard time for the operation is developed

• Compared to time studies and work sampling, PTS is a much more detailed and tedious technique that requires significant training

Example question:

How will you set time standards so you will know how many workers to hire, machines to buy or to build, and how much to charge for your product? PTS is the answer