Energy Cost of Work

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Transcript Energy Cost of Work

Muscular Activity
Supplies the energy needed to slide the actin filaments over the myosin
filaments. It is a chemical process of converting food into mechanical
work and heat.
Some mechanical work is consumed by the body while other is consumed by
physical activity
Basic source of energy for contraction of the muscle is glycogen or glucose
which is abundant in the blood
Sources of energy (next time)
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Metabolism - Sources of energy (see Figure 8-2 Sanders &
McCormick (7th ed)
First 3-5 secs
adenosine triphosphate (ATP)-a high energy phosphate compound is
mobilized. It breaks down to adenosine diphosphate (ADP) which
releases energy.
ATP  ADP + P (phosphate radical) + free energy
ATP Regenerated
To continue muscular activity, ATP must be regenerated
creatine phosphate + ADP  creatine + ATP
creatine phosphate is high energy existing in small amounts in muscles
Depletion of creatine phosphate occurs in about 15 sec
Blood glucose or glycogen is mobilized. Glucose is a blood sugar
which is converted by various stages first into pyruvic acid.
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Metabolism – further breakdown may be
Anaerobic work – if O2 is not supplied to the muscle, pyruvic acid is
converted into lactic acid while ATP is regenerated. Lactic acid accumulation
causes muscle fatigue and pain
glucose + 2 phosphate + 2 ADP  2 lactate + 2 ATP
Aerobic work – if O2 is supplied, pyruvic acid is broken down into water and
carbon dioxide, releasing large amounts of ATP
glucose + 38 phosphate + 38 ADP + 6 O2  6 CO2 + 44 H2O + 38 ATP
Oxidation of pyruvic acid in aerobic work involves enzymes, co-enzymes, and
fatty acids (Krebs cycle, figure 3.4 – Pulat)
O2 is key to efficient work. Its supply requires more blood be pumped to
muscle per unit time as well as heavier breathing to oxygenate blood
Kilocalorie (kcal) – most common measure of energy requirement for
physical activity
Resting energy  0.3 kcal per minute for man of about 154 lbs
Resting male (laying down and no digestive activity)  1700 kcal/day
Resting female (laying down and no digestive activity)  1400 kcal/day
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Basal metabolic rate (BMR) – amount of energy needed per unit of
time to sustain life
Total metabolism equals sum of:
Basal metabolism
Activity metabolism
Digestive metabolism (10 % sum of basal + activity)
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Supporting Systems
Respiratory Response
Nose and mouth, pharynx, larynx, trachea, lungs (consisting of
bronchi and bronchioles, alveolar ducts, alveolar sacs, and alveoli)
Primary function is to expose a large volume of water-saturated air
to a large volume of blood to facilitate exchange of gases between
air and inhaled air
Gas exchange 70-90 m2 for average adult (surface area)
At rest, 250 ml of O2 is absorbed and 200 ml of CO2 expelled
Lung capacity
7-8 L for tall athletic young male
Women – 10% less
Untrained – 60-80 % of volume
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Supporting Systems
Heart acts as double pump: blood vessels outside the heart can be
categorized into 2 classes
Pulmonary vessels – transport blood from the right ventricle of the heart
through the lungs and back to the left atrium of the heart
Systemic vessels – transport blood from the left ventricle of the heart to all the
other parts of the body including the head and lower body back to the right
Pulmonary and systemic  peripheral circulation
Kidneys – remove waste (toxic) from the blood
Liver, lungs, skin & intestines also eliminate waste
Arteries – carry blood away from the heart
Veins – carry blood to the heart
Ingestion, digestion, absorption, and utilization of nutrient substances
Provides necessary fuel and chemicals for physical activity
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Muscular Activity
Dynamic activity – characterized by rhythemic contraction
and relaxation of the muscles involved
Example: turning a handwheel to open a valve
Alternating tension and relaxation allows more blood to circulate
Static activity – characterized by a prolonged state of
contraction, which restricts blood flow to muscles
Example: holding a box in static posture
Since no glucose or oxygen are being received – activity won’t last long
Design Element – Dynamic vs. Static
Compared to dynamic effort, static effort will require longer rest periods
Static effort could result in employee complaints and turnover
Designers should minimize job elements in a process with static loading
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Energy Cost of Work
Energy demands increase with the onset of physical work.
Increase depends on:
Physical conditioning
Intensity of activity
Body weight
Cardiovascular response
At rest
Heart rate (HR) – 60-85 beats per minute
Normal resting blood leaves lungs 97 % saturated –  breathing harder will
not increase O2 in muscles alone
At work
Increase cardiac output – to increase O2 to muscles, more blood must flow
thereby increasing cardiac output
Heart rate increases - # pumping actions
Stroke volume – volume per beat (rest 5 L/min; hard work – 25 L/min or
At 40% of a person’s max capacity stroke volume stabilizes while HR increases
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Energy Cost of Work
Cardiovascular response
Oxygen lag
Fig 3.5, p. 34, Pulat
O2 and heart rate are linear under submaximal work (fig 3.6, 3.7, &
3.8 – Pulat)
Violated when:
Arms over head
Hot environments
Increased blood pressure (BP) – consequence of increased cardiac
Adrenal glands release catecholamines (hormones) that strength HR
and increase BP
BP is necessary to fill heart (note: @ 120 beats/min, heart has ½ sec
to fill between strokes)
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Energy Cost of Work
Redistribution of blood (Sanders & McCormick, p. 231)
Blood Flow Distribution (%)
Part of body
Heavy Work
Digestive System
Heart muscle
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Energy Cost of Work
Physical work capacity – worker’s capacity for energy
output function of:
Food, oxygen, sum of energy provided by aerobic and anaerobic process
Working at 30-40% of one’s maximum aerobic power in 8 hour shift
causes notable muscular fatigue
Energy cost of specific activities
O2  energy consumption have good relationship
Indirect measure of energy consumption
1 L of O2  4.8 kcal
Table 3.2 – different activities kcal
As rate of activity increases, energy cost increases
Body weight increase energy cost
Men  3000-3500 kcal/day
Women  2500-3000 kcal/day
At age 65  75 % calories expended compared to those at age 25
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Energy Cost of Work
Keeping energy cost of work at acceptable limits
For men
Maximum time-weighted average of 5 kcal/min due to activity energy
cost of work
Maximum time-weighted HR average of 100 beats/min
For women
4 kcal/min
90 beat/min
Table 3.3 is a classification of work
Factors affecting energy consumption
Method of work
Work posture
Work rate or pace
Tool design
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Energy Cost of Work
Manifested in slight tiredness to complete exhaustion
Produce lactate - which can be a good predictor of fatigue
One subjective measure is the Borg-RPE (rating of perceived
exertion) scale – similar to scale being used in MP 1
Scale of 6-20 are linearly related to the HR expected for that level of
Intended to rate exertion during dynamic work
Motivation – highly motivated underrate their level
Work-rest cycle
If workload cannot be maintained within recommended limits
(work standards) there must be rest
Some charts have been developed to determine amount of rest
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Rest Allowance
(Kodak, 1986)
Rest Allowance %
Rest Allowance
Someone working 10 minutes @
VH level get 80% rest or 8 minutes
Level of Work
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Rest Allowance
Murrell (1965)
T (W  S )
W  BM
R  rest required in minutes
T  total work time in minutes
W  average energy consumptio n of work in kcal/min
S  recommended average energy expenditure og kcal/min (4 - 5 kcal/min)
BM  Basal metabolism (1.4 - females, 1.7 - males, 1.5 - in general)
Solve Problem 3 in Pulat
Using Murrell and method in text book
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Strength and Endurance
Strength – the maximum force that one can exert voluntarily (kilograms using
Static – measured standing or sitting posture
Example: Pushing a button on a cell phone
Design so that 95% of population can exceed these forces
Dynamic – measured during work using dynamic or isokinetic strength testing
devices that control for speed of movement
Speed of movement affects lifting ability
Characteristics of strength
Strength peak by late 20s, gradual decline to 75% at age 65
Women on average have 2/3 strength of men
Exercise can increase by as much as 50%
Maximum grip @ 135 degrees of elbow flex
Endurance – ability to maintain activity over time
Research says that people can only maintain maximum effort only briefly
20% of peak static over time
30% of peak dynamic work
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Nervous System
Coordinates and regulates body activities
Responsible for the initiation and control of muscular
The basic element of the nervous system is the neuron.
Neuron is composed of a nerve cell along with a minimum
of two nerve fibers.
Dendrite brings messages to the nerve cell; multiple dendrites may be
associated with a cell therefore multiple paths may send signals
Axon takes messages to another cell; a neuron may not have more than
one axon.
Myelin coats the nerve fibers which serves to protect one nerve fiber's
messages from another; short-circuiting is prevented
Synapse is the gap between the dendrite of one neuron and the axon of
another neuron.
Chemical/electrical process allow an impulse of one neuron to be passed
to a succeeding on unaltered in 1/1000 of a second.
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Nervous System
Three major parts to the nervous system
Central Nervous system which includes the brain and spinal cord
Peripheral Nervous system which controls the voluntary activity
Automatic system which covers the balance of the nervous system
Automatic Nervous System
Controls involuntary body activity including function of glands, smooth
muscle tissue, and the heart. Includes the:
Sympathetic or thoracolumbar division stimulating these fibers creates small
amounts of thick saliva, depression of gastrointestinal activity, and increased
heart rate and occur in emergencies such as freight and consume energy.
parasympathetic or craniosacral division
For example when people are in a health threat, parasympathetic affect blood
pressure, constricting of the pupils, etc.
An axon in hand controls 3 -6 muscle fibers; An axon in leg controls 100 -200
muscle fibers
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Nervous System
Peripheral Nervous System
Controls the voluntary activity of the body
2 major elements are the sensory and motor systems
Sensory system is responsible for conveying information from your
senses to the central mechanisms; sensory neurons have one axon and
one dendrite each
Once a message is received and processed through the Central
nervous system, signals are passed down the pathways to the
appropriate muscles. Motor neurons lie in the central mechanisms,
and their axons travel in groups called motor nerves
Speed of transmission
Motor fibers 70-120 m/s
Others 12-70 m/s
Considering the distance between the sensory organs and motor
organs to the central mechanism, it is not uncommon to have reaction
times in the 300-500 millisecond range
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Nervous System
Central Nervous System
Two parts: spinal cord and brain
They are connected by the brain stem
Spinal cord is gray matter encased in a bony column called the
spinal column.
contain the reflex neurons
contains the neurons of the sensory system
The brain
medulla - connects the spinal cord with the higher centers of the
brain; nuclei for the neurons of the automatic nervous system
cerebral cortex is the part of the brain where information is processed;
where information is stored; most important with respect to
perception and processing of information
Thalamus - relay station for the brain; sorts out messages and directs
them to correct motor channels
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