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The term ergometry refers to the measurement of work output. The word ergometer refers to the apparatus or device used to measure a specific type of work. It is important to point out that the determination of work and power requires that a force is moved against some resistance. This could be a person‘s body weight (a force) moving up a step against gravity (a resistance) or pedaling a stationary bike when a resistance has been applied to the wheel. Many types of ergometers are in use today in exercise physiology laboratories (Fig. 1.1). A brief introduction to commonly used ergometers follows.
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One of the earliest ergometers used to measure work capacity in humans was the bench step. This ergometer is still in use today and simply involves the subject stepping up and down on a bench at a specified rate. Calculation of the work performed during bench stepping is very easy. Suppose a 70-kg man steps up and down on a 30-centimeter (0.3-meter) bench for 10 minutes (min) at a rate of 30 steps per minute. The amount of work performed during this 10-minute task can be computed as follows:
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Force = 686.7 N (i.e., 70 kg × 9.81 N/kg)
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$Distance=0.3m\xb7step\u22121\xd730steps\xb7min\u22121\xd710min=90m$
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Therefore, the total work performed is:
$686.7N\xd790m=61,803joulesor61.8kilojoules(kJ)(roundedtonearest0.1)$
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The power output during this 10 minutes (600 seconds) of exercise can be calculated as:
$Power=61,803J/600seconds103J\xb7s\u22121or103watts(W)$
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Using a more traditional unit of work, the kilopond-meter (kpm), power can be calculated as follows:
$Work=70kp\xd70.3m\xd730steps\xb7min\u22121\xd710min=6300kpmPower=6300kpm\xf710min=630kpm\xb7min\u22121or103watts(SeeTable1.4forconversions)$
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The cycle ergometer was developed more than 100 years ago and remains a popular ergometer in exercise physiology laboratories today (see “A Look Back—Important People in Science”). This type of ergometer is a stationary exercise cycle that permits accurate measurement of the work performed. A common type of cycle ergometer is the Monark friction-braked cycle, which incorporates a belt wrapped around the wheel (called a flywheel) (Fig. 1.1(b)). The belt can be loosened or tightened to provide a change in resistance. The distance the wheel travels can be determined by computing the distance covered per revolution of the pedals (6 meters per revolution on a standard Monark cycle) times the number of pedal revolutions. Consider the following example for the computation of work and power using the cycle ergometer. Calculate work given:
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A LOOK BACK—IMPORTANT PEOPLE IN SCIENCE
August Krogh: Nobel Prize Winner and Inventor
As mentioned in the previous chapter, August Krogh (1872–1949) received the Nobel Prize in Physiology or Medicine in 1920 for his research on regulation of blood flow through capillaries in skeletal muscle. Krogh was born in Grenaa, Denmark in 1874. After entering the University of Copenhagen in 1893, he began to study medicine, but developed a strong interest in research and decided to leave his medical studies to devote himself to the study of physiology. Krogh began his research career at the University of Copenhagen in the medical physiology laboratory of the famous Danish physiologist Christian Bohr.
Krogh completed his Ph.D. studies in 1903 and two years later married Marie Jørgensen, a renowned physiologist. August Krogh was a dedicated physiologist with extreme curiosity. He devoted his entire life to understanding physiology, and he worked both day and night in his laboratory to achieve his research goals. In fact, Krogh even performed physiology experiments on his wedding day!
During his distinguished research career, Dr. Krogh made many important contributions to physiology. For example, Dr. Krogh’s work greatly advanced our understanding of respiratory gas exchange in both mammals and insects. Further, he studied water and electrolyte homeostasis in animals and published an important book titled Osmotic Regulation in 1939. Nonetheless, Krogh is best known for his work on the regulation of blood flow in the capillaries of skeletal muscle. He was the first physiologist to describe the changes in blood flow to muscle in accordance with the metabolic demands of the tissue. Indeed, his research demonstrated that the increase in muscle blood flow during contractions was achieved by the opening of arterioles and capillaries. This is the work that earned him the Nobel Prize.
In addition to physiology research, August Krogh invented many important scientific instruments. For instance, he developed both the spirometer (a device used to measure pulmonary volumes) and an apparatus for measuring metabolic rate (an instrument used to measure oxygen consumption). Although Krogh did not invent the first cycle ergometer, he is credited with developing an automatically controlled cycle ergometer in 1913. This ergometer was a large improvement in the cycle ergometers of the day and permitted Krogh and his colleagues to accurately measure the amount of work performed during exercise physiology experiments. Cycle ergometers similar to the one developed by August Krogh are still in use today in exercise physiology laboratories.
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Duration of exercise = 10 min
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Resistance against flywheel = 1.5 kp or 14.7 N
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Distance traveled per pedal revolution = 6 m
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Pedaling speed = 60 rev · min^{−1}
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Therefore, the total revolutions in 10 min
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= 10 min × 60 rev · min^{−1} = 600 rev
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Hence, $totalwork=14.7N\xd7(6m\xb7rev\u22121\xd7600rev)=52,920Jor52.9kJ$
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The power output in this example is computed by dividing the total work performed by time:
$Power=52,920joules\xf7600seconds=88.2watts$
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The following steps show how to do the calculations using alternative units:
$Work=1.5kp\xd76m\xb7rev\u22121\xd760rev\xb7min\u22121\xd710min=5400kpmPower=5400kpm\xf710min=540kpm\xb7min\u22121=88.2watts(SeeTable1.4forconversions)$
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When a person walks or runs on a treadmill at 0% grade, the center of mass (imagine a point at the hip) of that person rises and falls with each stride, more for running than walking since there is a period of flight during each running stride. However, it is very difficult to measure that small vertical displacement of the center of mass, and consequently “work” cannot be easily determined when walking or running on a horizontal treadmill. In contrast, it is very easy to measure work when a person is walking or running up a slope (grade). The incline of the treadmill is expressed in units called “percent grade.” Percent grade is defined as the amount of vertical rise per 100 units of belt travel. For instance, a subject walking on a treadmill at a 10% grade travels 10 meters vertically for every 100 meters of the belt travel. Percent grade is calculated by multiplying the sine of the treadmill angle by 100. In practice, the treadmill angle (expressed in degrees) can be determined by simple trigonometric computations (Fig. 1.2) or by using a measurement device called an inclinometer (9).
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To calculate the work output during treadmill exercise, you must know both the subject’s body weight and the distance traveled vertically. Vertical travel can be computed by multiplying the distance the belt travels by the percent grade. This can be written as:
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Vertical displacement = % Grade × Distance
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where percent grade is expressed as a fraction and the total distance traveled is calculated by multiplying the treadmill speed (m · min^{−1}) by the total minutes of exercise. Consider the following sample calculation of work output during treadmill exercise:
$Bodyweight=60kg(force=60kpor588.6N)Treadmillspeed=200m\xb7min\u22121Treadmillangle=7.5%grade(7.5\xf7100=0.075)asfractionalgradeExercisetime=10min$
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Total vertical distance traveled
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= 200 m · min^{−1} × 0.075 × 10 min = 150 m
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Therefore, $totalworkperformed=588.6N\xd7150m=88,290Jor88.3kJ$
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Using more traditional units, the following calculations can be made:
$Work=60kp\xd70.075\xd7200m\xb7min\u22121\xd710min=9000kpmor88,290J(88.3kJ)$
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IN SUMMARY
An understanding of the terms work and power is necessary to compute human work output and the associated exercise efficiency.
Work is defined as the product of force times distance:
$Work=Force\xd7Distance$
Power is defined as work divided by time:
$Power=Work\xf7Time$