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
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
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
Energy Units Energy (work) – joules (J) or calories (cal)
4.19 J = 1 cal Power – watts(W)
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
Glucose and glycogen are the 1st energy sources Fat is the largest energy resource, but the last one
used
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
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)
Prepackaged food labels break down energy contents
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 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
Work metabolism The increase from resting to working Used to assess the energy demands of work
Measuring heaviness of work Subjective: ask worker to rate the effort
difficulty Objective:
1. Observe the energy supplied to the body2. Measure heart rate at work3. Measure oxygen consumption at work
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
Oxygen consumption at work Average energy value of oxygen is 5kcal(21kJ)/LOxygen Therefore the volume of oxygen consumed
allows calculation of the energy converted by the body at work
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 during work Higher energy demands = more blood flow Heart must produce higher outputs BPM increase and pulse rate increases in
accordance with work demands
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 O2 uptake at work (fig 10.3) At work onset there is an immediate demand for
O2, but actual uptake lags behind 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
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
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 vs. grandchildren descriptions Figure 10.1 classifies work demands
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
Measuring people’s fitness to do heavy work 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
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 Neither test resembles work conditions
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
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
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
Design work to fit the human Work design must match individual
capabilities Avoid exhausting work
Daily energy consumption for moderately demanding work is 12,000-15,000kJ for men and 10,000-12,000kJ 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
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
Summary Figure 10.7 Human trait and conditions that
determine the amount of work an individual can do