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Concepts in Human Factors Engineering (Part 8 of 11)

Concepts in Human Factors Engineering (8): Human Body Energy

by Dennis R Andrews PhD, PSP, CECD

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The body needs energy and takes in nutrients in the form of food it stores this energy for future use. Metabolism is a term used to define the use of stored energy and the chemical process. The human body can be considered or thought of as a machine. A machine receives its energy from either a liquid fuel or electricity. Either of these sources must be created such as crude oil into usable fuel and generating electricity. The human body does the same basic function; it turns the intake of food into usable energy. We previously equated the human body with cylinders, pistons or gears. Metabolism is a chemical process for creating usable energy in our bodies. The human body must balance the intake and output of energy so that the body is not excessively depleted, which can be dangerous. Muscles can be considered as the power for levers, which actually perform the desired work.

Concepts in Human Factors Engineering is a series containing eleven articles:
  1. An Overview of Anthropomorphic Data Gathering
  2. The Fragile Skeletal System
  3. The Energy Force of Our Frame
  4. The Message Delivery System
  5. The Body as a Machine
  6. The Oxygen Machine
  7. The Body’s Transportation System
  8. Human Body Energy
  9. A Hot Workplace
  10. The Rhythm of Working
  11. The Bionic Worker of the Future
Energy efficiency is the ratio between the work and energy input, in other words it is the percentage of work obtained from one unit of energy input. It is estimated that this efficiency is extremely low such as five percent for work relating to objects outside the body. This five percent can be increased maybe five times for a professional athlete. The efficiency has a lot to do with respiratory and circulatory values. Controlling the internal temperature of the body is crucial since inappropriate temperatures will rob the body of work energy efficiency. The body receives most of its energy from carbohydrates, fats and proteins. Unfortunately carbohydrates and sugar create fats, which can become unhealthy if in excess. The balance between food intake and exercise or work is crucial otherwise the body will begin to store unwanted fat can become unhealthy.

After the food passes the mouth it enters the digestive system where the nutrients are extracted and distributed to the needed locations. Obviously this is a slow process and the process itself requires energy. Most of the energy is withdrawn from the food in the small intestine the final extraction is done in the large intestine. As with any intake waste is created and must be disposed of. Waste of the human body is disposed of through urine and human feces. The food is moved along the digestive tract by muscle contraction. Units for measuring energy are joules (J) or calories (cal). 4.2 joules equal 1 cal. A kilocalorie (kcal) equals 1000 cal. One unit of work (W) is equal to one joule per second. Proteins are vital for energy and are made up of amino acids. Some of these acids are distributed in the body in order to build new proteins. Enzymes are used to break down the foodstuffs so a chemical process can take place during digestion.

Foodstuffs are assimilated and absorbed by the human body to create the needed energy. Energy in the body is in the form of glucose, neutral fat and proteins. Fat stores most of the energy a young male has about 16 percent of body fat, and 22 percent by middle age. Young females have 22 percent of body fat rising to 34 percent by middle age. Naturally athletes have much lower body fat percentages. When beginning to exercise or work quick energy is used for about 10 seconds. After this time the amount of energy must be maintained and if not the performance will be reduced. Oxygen must be available during the time of heavy exercise or work if not the muscles will be starved of oxygen. If an insufficient amount of oxygen is present lactic acid will develop in the muscle. During short bursts of energy the energy is consumed faster than during short aerobic periods but more energy is expanded during long aerobic periods than long anaerobic periods. Energy from the food we eat is mostly transformed into heat.

Trying to match a persons work capacity with a job is difficult. One needs to know the energy capacity and the job demands. As everyone knows the ability of people to perform tasks varies greatly. Assessing this ability takes diligence and patients. The body must be in equilibrium meaning the amount of energy expanded must be equal to or better yet slightly below energy intake. This is not something that can be determined readily it must be observed over a long period of time and under different conditions. If the body takes in more calories then it uses it will store the potential energy in the form of fat. Fat can be measured directly using instruments such as calipers or may be determined by weighing a person in water. The fat is more buoyant then muscle and consequently the difference between a dry weight and the wet weight would be the fat value. Some of these measures are more accurate than others and studies or experiments should be utilized to determine the probability of error and accuracy.

In an effort to determine any errors or deviations a baseline or reference point must be used. It must be completely understood that any experiments whether done in a laboratory or not is not totally comparable with real world activities or measurements outside of the laboratory. A good researcher will attempt to determine any built-in biases and quantify them so that adjustments to any findings will be more meaningful. It is important to determine oxygen consumption for analyzing energy needed to perform work. Measuring oxygen intake and carbon dioxide exhaled is quite straightforward. Correlation can be determined although it may not be linear between oxygen intake and fat content. Obviously more oxygen will be required for heavy work; there is a linear correlation between the periods of light work and heavy work.

Basal metabolism is measured without expenditure of energy by the body such as moving for heavy digestion. Energy expended to do work is much like pushing an object where at the beginning more energy is needed to overcome the inertia of the object then energy output stabilizes once the object is in movement. Such is similar to the expenditure of energy during work periods between any before and after rest periods. We can calculate the energy required for a specific task by simply knowing the time spent for the task and the tasks metabolic cost per time unit. Knowing this information it would be a simple matter of multiplying the time unit by time spent. There are tables estimating energy consumption and energy cost per day in various jobs but caution using these tables is advised. Some tables are quite old and consequently many of the listed tasks are either no longer performed or the energy cost may be dramatically changed given today's technology.

It is agreed and understood that heavy work requires more energy then light work. The problem is in defining the two categories and any categories in between. Heavy work for one person could be actually medium work and medium work could be light work. Whenever calculations are being used specific definitions should be developed and adhered to. If a table is developed using various anthropomorphic data chances are the energy expenditure would be more accurate than trying to extrapolate into generalized categories. Normally light work is defined as 10,000 joules per minute and 90 beats per minute, medium work 20,000 joules per minute and 100 beats per minute and heavy work 30,000 joules per minute and 120 beats per minute. Naturally the beats per minute can vary between these categories more than the joules value would for different persons.

Fatigue is a stage where the worker becomes more prone to injury the deeper they enter into the fatigue stage. Fatigue is caused by the decrease of oxygen capacity and blood flow to the muscles. The longer a person stays at this level the greater the risk of injury and the more fatigue the person becomes until total fatigue break down occurs, which could include a serious injury or fatality. Fatigue is dealt with in two ways one is to stop the work altogether and the other is to reduce the workload. Over a long period of time more work would be accomplished and safer if workers were given strategically spaced breaks. Breaks not only serve to regain strength but can temporarily give a mental break as well.

In summary the measurement of the caloric function is crucial in determining a workload and length or hours of the workday. The specific measurements must be determined either through reference tabular information or calculations. Assessing workers ability and limits is necessary in filling employment with the best available workers. Each job requires complete knowledge of the physical requirements for a position. Certainly a task requiring reaching it would not be advisable to hire a person of shorter or borderline stature since the task would not be performed efficiently or safely.

  • Click here for the next article in the Concepts in Human Factors Engineering series

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