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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 ENERGY CONSUMPTION Energy Units Energy (work) – joules (J) or calories (cal) Power – watts(W) 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 Glucose and glycogen are the 1st energy sources Fat is the largest energy resource, but the last one used ENERGY CONSUMPTION 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 CONSUMPTION 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 ENERGY CONSUMPTION 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 ENERGY CONSUMPTION 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. 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 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 ENERGY CONSUMPTION 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 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 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 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 HEART RATE AND WORK DEMANDS 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 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 LIMITS OF HUMAN 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 LIMITS OF HUMAN LABOR CAPACITY 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 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 DESIGNING HEAVY HUMAN 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 DESIGNING HEAVY HUMAN WORK Design work to fit the human Avoid exhausting work 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 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