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