Transcript Hard physical work

HARD PHYSICAL WORK
PHYSIOLOGICAL PRINCIPLES

Hard labor requires skeletal muscle to convert
chemical energy into work
From rest, muscle can increase its energy generation
50 fold
 Varied metabolic rate requires quick supplies of
oxygen/nutrients and removal of wastes
 Internal equilibrium depends on the proper functioning
of the respiratory and cardiovascular systems
 Body temperature control is important especially in hot
environments

PHYSIOLOGICAL PRINCIPLES

Assessing labor demands and worker capacity
 Heavy
work requires high energy consumption
 Measurements of the metabolic, cardiovascular
and respiratory functions are used to assess their
ability to perform heavy physical work
ENERGY CONSUMPTION

Skeletal muscles make the body work by moving
body segments
Mitochondria convert chemical energy into physical
energy to fuel contraction
 Figure10.1 diagram of energy flow within the body

food is broken down into nutrients by the digestive system
 Oxygen is brought into the lungs and enters the bloodstream
 Glucose and oxygen react to perform the metabolic processes,
supplying energy to the tissues
 Energy is consumed and wastes are removed asheat, CO2,
and water via the respiratory and cardiovascular systems and
the skin
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ENERGY CONSUMPTION
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Energy Units

Energy (work) – joules (J) or calories (cal)
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Power – watts(W)
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4.19 J = 1 cal
1 W=1 J/s and 1.163 W = 1 kcal/hr
Metabolism – chemical energy is converted into
mechanical energy

Nutrients consumed are:
Stored as energy
 Used for body growth and repair, given off as heat
 Broken down and used as energy

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Glucose and glycogen are the 1st energy sources
Fat is the largest energy resource, but the last one used
ENERGY CONSUMPTION
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Metabolic byproducts
Only part of the converted energy is used by the
muscles, the rest is used to build structures in the body
and the rest converts to heat
 Constant body heat of 37 degrees C, excess heat must
be dissipated
 Heat is removed via the bloodstream, lungs and skin
 Water is transported by the blood to the lungs and skin
 CO2 is removed by the lungs
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ENERGY CONSUMPTION
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Energy content of food and drink

Measurements of energy in food
1
kJ = 1000J
 1 Cal = 1 kcal = 1000cal
 4.19 J = 1 cal

Nutritionally useable energy per gram
 Alcohol
= 30 kJ (7 cal)
 Carbohydrates = 18 kJ (4.2 cal)
 Protein = 19 kJ (4.5 cal)
 Fat = 40 kJ (9.5 cal)
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Prepackaged food labels break down energy contents
ENERGY CONSUMPTION
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Basal Metabolism
Minimal amount of energy necessary to keep a body
functioning
 Depends on age, gender, height and weight
 Common value used= 1 kcal (4.2 kJ)/kg/hour or 4.9
kJ/min for a 70 kg person
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Resting metabolism
Difficult to measure, so metabolism taken in the
morning before work is often used
 Resting metabolism is about 10 – 15% greater than
basal metabolism
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ENERGY CONSUMPTION
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Work metabolism
 The
increase from resting to working
 Used to assess the energy demands of work
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Measuring heaviness of work
 Subjective:
ask worker to rate the effort difficulty
 Objective:
1.
2.
3.
Observe the energy supplied to the body
Measure heart rate at work
Measure oxygen consumption at work
ENERGY CONSUMPTION

Energy supply to the body
Observe what a person eats, drinks and weighs
 Subtract the basal metabolism and assume the rest is
used to perform work
 Inaccurate
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Oxygen consumption at work
Average energy value of oxygen is 5kcal(21kJ)/L
Oxygen
 Therefore the volume of oxygen consumed allows
calculation of the energy converted by the body at work
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ENERGY CONSUMPTION
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RQ (respiratory exchange quotient)
 More
detailed assessment of the type of nutrients
metabolized
 Compares the volume of CO2 expired to the O2
consumed
1
g Carb requires 0.83 L of O2 RQ = 1
 Protein RQ = 0.8
 Fat and alcohol RQ = 0.7
 Measuring
the CO2 and O2 volumes assesses
which energy source is being used
HEART RATE AND WORK DEMANDS
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Heart rate during work
 Higher
energy demands = more blood flow
 Heart must produce higher outputs
 BPM increase and pulse rate increases in
accordance with work demands
HEART RATE AND WORK DEMANDS
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Relation of heart rate and O2 measurements
 Close
connection between circulation and
metabolic functioning
 Heart rate (circulation) and O2 consumption
(metabolic conversion) have a linear relationship
 Therefore, heart rate measurement can replace O2
consumption measurement
 Good option because heart rate responds faster to
the changes in work demand and pulse is easier to
count than taking O2 measurements
HEART RATE AND WORK DEMANDS
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Heart rate and O2 uptake at work (fig 10.3)
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At work onset there is an immediate demand for O2,
but actual uptake lags behind
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the body incurs an oxygen deficit because it has to pull from
anaerobic sources
When work ends, the body must “repay” the oxygen
borrowed from the anaerobic stores as well as account
for the oxygen used during work; therefore the oxygen
debt is 2xs the original deficit
 The body repays the debt by maintaining an increased
heart rate and respiration rate after work has ended
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HEART RATE AND WORK DEMANDS
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Steady-state work
 When
the required work effort is below the maximal
capacity
 Blood flow, oxygen supply and respiratory rate can
maintain their normal levels
 Physically fit people can achieve this balance
between energy demand and supply at a higher
workload than an untrained person
HEART RATE AND WORK DEMANDS
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Classifying work demands
 Energy
expenditure and heart rate are objective
measurements of energy expenditures taken from
averages of fit and untrained workers
 Subjective descriptions can vary with
circumstances and experiences
 Grandparents
 Figure
vs. grandchildren descriptions
10.1 classifies work demands
LIMITS OF HUMAN LABOR CAPACITY
Maximal effort greatly increases energy
consumption, O2 uptake, cardiac action and
respiration (Table 10.2)
 Work can continue if the body is able to meet
the demands, but is forced to stop if demands
exceed the capabilities
 Physical fitness and skill play an important role
in individual labor capacity
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LIMITS OF HUMAN LABOR CAPACITY
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Measuring people’s fitness to do heavy work
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Bicycle tests
 Primarily
strains leg muscles
 Leg mass accounts for a large component of our body and so
puts a significant strain on the pulmonary, circulatory and
metabolic functions
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Treadmill Tests
 Also
stresses lower body, but is more realistic because legs
must support and propel the body
 Body is strained in a more complete manner than in bicycling
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Neither test resembles work conditions
LIMITS OF HUMAN LABOR CAPACITY
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Selecting persons fit for heavy work
 Important
to measure fitness to make sure an
employee can perform the work
 Ergonomically it is better to design tasks so they
impose low demands
 Workers
won’t be overtaxed
 More people can do the job
LIMITS OF HUMAN LABOR CAPACITY
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Static work
Requires continue muscle contracture
 If contraction > 15% of muscle strength, blood flow is
reduced, leading to fatigue
 Dynamic work encourages blood flow, acts as a muscle
pump
 Static work increases the pulse rate as the heart tries
to increase blood flow to the compressed tissue, but
metabolism is reduced since blood cannot reach the
working tissues
 Therefore, there is no linear relationship between HR
and energy consumption in static work
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DESIGNING HEAVY HUMAN WORK
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Human energy efficiency at work
 Assuming
energy storage in the body does not
change and the body does not gain or lose heat,
the energy balance can be represented as:
I (energy input) = H ( heat developed)+ W (work)
 Only 5% of energy coverts to work, the rest is lost
as heat
 Humans are such inefficient energy converters that
they are more productive running machinery than
performing physical work
DESIGNING HEAVY HUMAN WORK
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Design work to fit the human
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Avoid exhausting work
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Work design must match individual capabilities
Daily energy consumption for moderately demanding
work is 12,000-15,000kJ for men and 10,00012,000kJ for women
Provide rest and breaks
Physiological and psychological effects
 Multiple shorter breaks are more effective than fewer
long duration breaks

 Recovery
is steepest at the beginning of a break
DESIGNING HEAVY HUMAN WORK
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No static work
 Dynamic
activities = heart rate and energy
consumption are closely related
 Static activities = heart rate increases while energy
consumption does not
 Tiresome
but not productive
 Should be designed out of work procedures
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Summary
 Figure
10.7 Human trait and conditions that
determine the amount of work an individual can do