 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 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 1 st 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 body 2. Measure heart rate at work 3. 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)/L Oxygen  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, ,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