Industrial Ergonomics Unit 3 Flashcards
Physical work capacity (PWC)
capacity to generate metabolic energy during continuous work over an extended time(i.e. max rate a worker can burn calories; aka max recommended work rate
Aerobic metabolism (requires O2)
- Slow production of ATP but long duration
- Used for lower intensity/most tasks, and can be sustained for long periods of time
- Efficient (produces 36 molecules of ATP per glucose molecule)
- By-products: carbon dioxide, water
Anaerobic metabolism (does not require O2)
- Quick production of ATP but short duration (< 2 minutes)
- Used for high-intensity tasks (>~85% max heart rate), and cannot be sustained for long
- Inefficient (produces 3 molecules of ATP per glucose molecule)
- By-products: carbon dioxide, water, lactic acid (pain, cramps, tremors)
Aerobic Capacity (AC)=
maximum capacity to utilize oxygen
We have two ways to determine the recommended work rate
1) As a flat percentage of AC
• 30-50% of AC can be maintained for 8 hours
2) Predict based upon AC and working time
General Formula:
Recommended work rate ~ [(log 4400 - log t)(AC)]/3.0
Energy Expenditure
Energy expenditure (EE) the metabolic demands of a task Usually quantified as a rate, even though the term does not indicate this This can be confirmed from the units of energy expenditure, which is typically kcal/min, watts (J/sec), or kJ/min
How do we determine the energy expenditure of a task?
Direct measurement of the O2consumption of a person performing the task. However, this can be impractical.
Indirect methods for estimating task demands:
- Average values reported for various tasks
- Subjective ratings
- Estimate from heart rate
- Task Analysis
Components of our Metabolism
BasalMetabolism (BM)—EE required for the body to function (keep you alive)
ActivityMetabolism (AM)—EE required to perform specific activities
DigestiveMetabolism (DM)—EE required to digest foodstuffs (10% of the calories you eat)
Total EE = BM + AM + DM
Metabolic Equivalent of Task (MET)
METs include both activity metabolism and basal metabolism
1 MET = 3.5 ml O2/(kgmin)
1 MET ~ EE of sitting quietly
1 MET ~ 1 kcal/(kghr)
Fatigue
is another workplace risk factor for WMSDs, similar to awk/ext postures, forceful exertions, repetitive exertions, and vibration.
Types of Fatigue Duration
Acute: short term, temporary state, occurs in response to work demands and/or stress
Chronic- long-term, often considered an illness, associated with persistent tiredness
Acute fatigue can progress into chronic fatigue if there is insufficient recovery from work demands or stress over an extended period.
Breaking Down Fatigue
Total Fatigue-> Mental and Physical Fatigue
Physical Fatigue-> Central and Peripheral Fatigue
Mental Fatigue
The psychological and physiological state resulting from the prolonged cognitive activity, or long boring lectures
Physical Fatigue
Inability to continue physical exertion at the level of one’s normal abilities, or a decreased capacity to generate force or expend metabolic energy
Types of Physical Fatigue
Peripheral fatigue – metabolic changes in the muscle itself or nerves from CNS to muscle
Central fatigue - changes in the control of muscles originating in the CNS
Two types of fatigue assessment
Subjective methods
Objective methods
Localized Muscle Fatigue
A type of physical fatigue that can include central and physical fatigue, but is localized to a specific muscle or muscle group
The decline in muscle force capacity
Localized pain and cramps
Decreased motion control and accuracy
Strength vs Endurance
Strength: Is the maximum joint moment or external force that can be generated. Short term effort
Endurance: is the time duration an exertion level can be sustained until exhaustion (aka time to exhaustion). Longer-term effort
Fatigue, strength, and endurance are all related
Fatigue is a process that causes a decline in capacity to generate a moment of 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
Worker Selection and Training
Three major ergonomic approaches for improving safety and efficiency in industry:
1) Job design
2) Worker selection
3) Worker training
The first fit the job to the person. The latter two fit the person to the job.
This law was changed by this case in the Ohio State Supreme Court (Village v. GM, 1984):
Michael Village with General Motors
“An injury which develops gradually over time as a result of the performance of the injured worker’s job-related duties is compensable [by worker compensation]”
What is Worker’s Compensation (WC)?
is a state-mandated insurance that provides wage replacement and medical benefits to employees injured in the course of employment. In exchange, the employee relinquishes the right to sue their employer for negligence.
What is in the OSHA Act of 1970?
- Established the Occupational Safety and Health Administration (OSHA) to set and enforce workplace health and safety standards
- Established theNational Institute for Occupational Safety and Health(NIOSH), an independent research institute
- General Duty Clause of the OSHA Act
- Recordkeeping – the OSHA act requires all places of employment to maintain substantial records of occupational injury and accidents and to maintain these records on-site for OSHA inspections
Ergonomic Standards Exist
Although there is no broad ergonomics standard, there are numerous industry-specific standards set by different agencies
US standards are not mandatory
Americans with Disability Act
More than 43 million Americans have one or more physical or mental disabilities.
Americans with Disabilities Act (July 1990)
Purpose – To provide a national mandate to eliminate discrimination against persons with disabilities. Provides clear, strong, consistent, enforceable standards addressing discrimination against the disabled.
Work Measurement
is the systematic determination of the length of time it should take to complete a job
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
Direct observation methods for determining time standards
Time Study is the most widely used work measurement technique.
Work Sampling is a method of analyzing work by taking a large number of observations at random intervals to establish standards and improve methods.
Rate Worker Performance
People do not work consistently from day to day or even from minute to minute, so the analyst must rate the worker performance
Rating is a very important step in time study, but also subject to criticism because depends on the experience, training, and judgment of the analyst
Standard (normal) performance = 100%
Calculate observed time and normal time
Observed time (OT) is the time to complete each element of the overall task. Normal time (NT) = OT x Rating/100
Allowances
Allowances increase the standard time to compensate for loses in time due to:
Personal needs (constant)
Fatigue (constant or variable)
Delays (constant)
Allowances are given as percentages of normal time
Calculate standard time
Standard time (ST) = NT(1 + allowance)
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.
Time study provides a more detailed analysis of job elements than work sampling.
Three types of work sampling studies
- Elemental ratio study– determine the percentage of the total time that each element of a work task takes.
- Performance sampling study– determines the percentage of the total time that each element of a work task takes, AND evaluates performance rating
- Time standard development study– determines the percentage of the total time that each element of a work task takes, AND evaluates performance rating, AND develops time standards.
Predetermined Time Systems
PTS is a technique of time standards development.
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.
What are the advantages of PTS?
- PTS can develop time standards before a job is started.
- Because it requires a complete analysis of all motion in a given job, it can find any inefficiencies, and standardize job procedures.
- Does not require a performance rating, so time standards from PTS are generally considered to be more accurate and less subjective than those from time study.
- PTS can be used to quantify learning time and performance ratings for worker evaluation.
What are the disadvantages of PTS?
- Significant amount of training time is needed before analysts can achieve consistent results.
- Difficult when studying machine-paced operations.
- Many PTS have been developed. Some more general and some more specific to a particular job. As such, not all PTS will give good standards for all work.
