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6 Measuring Human Energy Expenditure, Work and Power

Black-and-white archival image from June 1980 showing athletes at the starting line of a 100-yard dash. The runners are beginning their flight, after the starting signal with shoulders extending and flexing to gain momentum.
Athletes preparing to run the 100-yard dash-THE START. June 1980. The Century Magazine.

Learning Objectives

  • Describe the key principles of energy expenditure and its importance in exercise physiology.
  • Differentiate between direct and indirect calorimetry and explain how each method measures energy expenditure.
  • Calculate energy expenditure using the respiratory exchange ratio (RER) and understand its limitations.
  • Explain the significance of measuring RMR in clinical and research settings.
  • Describe the methods for measuring VO₂, including closed and open circuit spirometry.
  • Define VO₂max and explain its importance in assessing cardiovascular fitness.
  • Identify the criteria for achieving VO₂max and understand the protocols for incremental exercise testing.
  • Perform calculations to determine work and power output in humans.
  • Explain the factors that influence exercise efficiency and the concept of running economy.
  • Measure and compare the O₂ cost of different activities to assess exercise efficiency.
  • Use the 2011 Compendium of Physical Activity to estimate energy expenditure for various activities.

 

Introduction

        One cannot grasp the fundamentals of exercise physiology without first understanding the key principles of energy expenditure. In Chapter 4, we explored several metabolic pathways involved in the formation of adenosine triphosphate (ATP), the primary chemical energy source for our bodies. ATP can be synthesized using substrates such as sugars, fats, or proteins and is essential for cellular activities, including muscle contraction. The ability to expend energy, particularly during exercise, depends on metabolic function, rate, and the availability of substrates. This chapter will delve into how energy expenditure varies between rest and exercise and how the duration and intensity of exercise influence the amount of energy utilized.

Measuring Energy Expenditure

        Energy utilized by contracting skeletal muscles cannot be directly measured. However, several indirect laboratory methods can calculate whole-body energy expenditure. Energy expenditure (EE), measured in kilocalories (kcal) per minute, reflects the body’s rate of heat production. A calorie, the System International (SI) unit of heat, is the amount of heat required to raise 1 gram of water by 1°C. Given the small size of a calorie, energy content is typically expressed in kilocalories (1 kcal = 1,000 calories). Estimating or measuring energy expenditure is valuable for individuals using activities like walking, running, or swimming for fitness or performance improvement. Additionally, understanding energy expenditure is crucial for weight-loss programs. There are two primary techniques for measuring human energy expenditure: direct calorimetry and indirect calorimetry.

Direct Calorimetry

        Direct calorimetry measures the heat produced by the body during rest or exercise. This technique is based on the principle that energy expenditure results in heat production. The rate of heat production is directly proportional to metabolic rate, making heat measurement a direct indicator of energy expenditure. A calorimeter, an insulated chamber allowing free exchange of O₂ and CO₂, measures body heat. The person’s body heat raises the temperature of the water or insulation surrounding the chamber, and the temperature difference over time indicates the amount of heat produced. Although scientists have used this technique since the eighteenth century, direct calorimetry is costly due to the size and maintenance of the chamber. While it provides accurate measures of resting metabolism, it is not commonly used by exercise physiologists. Figure 6.1 illustrates a human calorimeter at the School of Human Kinetics in Ottawa, Canada.
The principles of direct calorimetry can be demonstrated by the following relationship:
Foodstuff + O2 → ATP + Heat → Cell Work → Heat
Image of the Snellen direct calorimetry chamber at the School of Human Kinetics, University of Ottawa. The chamber is a sealed, climate-controlled room designed for precise measurement of human metabolic heat production. It features insulated walls, integrated sensors for temperature and humidity, and instrumentation for monitoring energy expenditure during physical activity or rest.
Figure 6.1 The Snellen direct calorimetry chamber, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada.

Indirect Calorimetry

        Unlike direct calorimetry, indirect calorimetry does not measure heat production directly. Instead, it involves measuring whole-body oxygen consumption (VO₂) and carbon dioxide production (VCO₂) from expired gases. The principle behind indirect calorimetry is the direct relationship between oxygen consumption and heat production. By measuring an ioxygen consumption (VO₂)ndividual’s oxygen consumption, we can indirectly estimate their heat production and, consequently, their energy expenditure.
        To convert the amount of oxygen consumed into heat equivalents, it is essential to know the type of nutrient being metabolized—carbohydrates, fats, or proteins. The energy released when fat is the sole metabolized nutrient is approximately 4.7 kcal per liter of oxygen consumed (kcal/LO2). When only carbohydrates are metabolized, the energy release is about 5.05 kcal/LO2. For practical purposes, the caloric expenditure of exercise is often estimated at around 5 kcal/LO2. Therefore, an individual exercising at an oxygen consumption rate of 3.0 LO2/min would expend approximately 15 kcal of energy per minute.
3.0 LO2/min x 5 kcal/LO2 = 15 kcal/min
The principle of indirect calorimetry can be explained by the following relationship:
Foodstuff + VO2 → CO2 + Heat + H2O
  • VO₂ is the volume of oxygen consumed (in liters per minute).
Energy Equivalent per Liter of Oxygen varies depending on the nutrient being metabolized:
  • For fats: approximately 4.7 kcal/LO₂
  • For carbohydrates: approximately 5.05 kcal/LO₂
  • For mixed substrates: often estimated at 5 kcal/LO₂        
        This relationship allows us to estimate the total energy expenditure based on the measured oxygen consumption during various activities. 
        There are two primary methods used to measure VO₂ in humans: closed circuit spirometry and open circuit spirometry.
  1. Closed Circuit Spirometry. In closed circuit spirometry, all the air breathed in and out by the subject is contained within a chamber. Historically, the subject would wear a nose clip to prevent nasal breathing and use a respiratory valve that allowed room air to be inhaled while the exhaled gas was collected in a Douglas bag. Figures 6.2 and 6.3 illustrate this setup. The collected air in the bag was later analyzed for gas volume and the percentages of O₂ and CO₂. Although this technique did not allow for breath-by-breath measurements, it was useful in the early stages of studying exercise energy expenditure.
Image of a Tissot spirometer setup, a historical modification of the Douglas bag method used to measure respiratory gas exchange. A person wearing black has a rubber bag on their back and a tube that runs to their mouth.
Figure 6.2 The Tissot spirometer, which is a modification of the Douglas bag method for determining the respiratory exchange. The barometric pressure, temperature, and volume of air are important for measurement of the air. The composition of air was determined by the Haldane gas analysis apparatus. The Douglas bag was made of a rubber-lined cloth, and was capable of holding from 50 to 100 liters. It was considered useful for investigations during exercise, since it was fitted with straps so that the bag could be fastened to the shoulders.
Photograph from 1945 showing a British naval officer participating in a physiological experiment on the effects of heat on human efficiency. The subject is inside a high-temperature environmental chamber, wearing a Douglas bag apparatus to measure oxygen consumption. The bag is strapped to the subject’s back, with a breathing tube connected to a mouthpiece. A medical officer, visible on the right, monitors the subject’s condition and data.
​Figure 6.3 A British naval officer takes part in tests to investigate the effect of temperature on efficiency in 1945. The subject is working in a high temperature room and wearing a Douglas oxygen consumption bag. The doctor can be seen on the right-hand side, checking on progress.

2. Open Circuit Spirometry. Open circuit spirometry, on the other hand, involves the subject breathing in ambient air and exhaling into a collection system. This method allows for continuous, breath-by-breath analysis of the expired gases, providing more detailed and immediate data on VO₂ and VCO₂. Open circuit spirometry is more commonly used in modern exercise physiology due to its accuracy and practicality.

The most common technique used to measure oxygen consumption today is open-circuit spirometry. In this method, the subject breathes in environmental air, and the exhaled air is analyzed for CO₂, O₂, and N₂. Modern open-circuit spirometry utilizes advanced computer technology to measure the volume of exhaled gas on a breath-by-breath basis. This exhaled gas is then directed to a mixing chamber where samples are analyzed. Figure 6.4 shows a modern indirect spirometer being used to measure metabolic rate.
Photograph showing a U.S. Navy sailor at Pearl Harbor undergoing a metabolic assessment using an indirect calorimeter during a Wellness Vehicle visit. The sailor is seated and connected to the calorimeter via a mouthpiece and tubing.
​Figure 6.4 A sailor stationed at Pearl Harbor uses indirect calorimeter to check his metabolic rate during a Wellness Vehicle visit.

Resting Metabolic Rate

The rate at which the body utilizes energy is known as the metabolic rate. Indirect calorimetry is frequently used to estimate energy expenditure both at rest and during exercise. Under resting conditions, an average person consumes about 0.3 liters of O₂ per minute, which translates to 18 liters of O₂ per hour or 432 liters of O₂ per day. Knowing a person’s VO₂ allows for the calculation of their caloric expenditure.
This figure illustrates the physiological relationship between food, oxygen consumption, ATP, heat, and carbon dioxide production in the measurement of human energy expenditure. The image includes a visual representation of metabolic pathways, such as aerobic respiration, showing how inhaled oxygen is used in cellular metabolism and how carbon dioxide is produced as a byproduct.
​Figure 6.5 How oxygen and carbon dioxide relate to the measure of human energy expenditure.
        For example, a resting Respiratory Exchange Ratio (RER) value of approximately 0.80 is typical for most individuals on a mixed diet. The caloric equivalent associated with an RER value of 0.80 is 4.80 kcal per liter of O₂ consumed. Energy expenditure can then be calculated using the following formula:
kcal/day = LO2 consumed/day x kcal/LO2
so, kcal/day = 432 LO2 /day x 4.80 kcal/LO2
= 2,074 kcal/day
Measurement of resting metabolic rate (RMR) is utilized in both clinical and research settings to provide invaluable information regarding energy requirements and the types of fuels being oxidized at rest. RMR, measured by indirect calorimetry under standard conditions, provides data on oxygen consumption (VO₂), carbon dioxide production (VCO₂), and the respiratory exchange ratio (RER)[1]. Figure 6.5 illustrates the relationship between oxygen and carbon dioxide in measuring resting metabolic rate.
        RMR represents the energy required to maintain essential physiological processes in a relaxed, awake, and reclined state. It is a significant component of total daily energy expenditure (TEE), accounting for 65% to 75% of the total daily energy demands in adults[2]. Practically, RMR indicates the number of calories a person needs per day before considering the calories expended through physical activity. RMR is expressed in kilocalories per day and is also used to calculate physical activity levels (PAL), where PAL = TEE/RMR[3].
        RMR can be measured using indirect calorimetry or estimated through prediction equations. One of the most accurate equations for estimating RMR is the Mifflin-St Jeor equation, developed in 1990 to estimate the caloric needs of men and women[4]. The equation is as follows:
Men: 9.99 x weight (kg) + 6.26 x height (cm) – 4.92 x age +5
Women: 9.99 x weight (kg) + 6.26 x height (cm) – 4.92 x age – 161
        When measuring RMR, it is crucial that subjects are fasted for at least 5 hours prior to testing. Indirect calorimetry measurements typically occur over approximately 30 minutes while the subject lies supine in a relaxed, awake state. Data is usually collected for 5 minutes once the subject reaches a steady state of oxygen consumption, which is then analyzed to determine the RMR[5].

Metabolic Rate During Exercise

        Research has shown that exercise significantly increases energy requirements beyond the resting metabolic rate (RMR). Studies on energy expenditure and oxygen cost (O₂ cost = VO₂ at steady state) during exercise have demonstrated that it is possible to estimate energy expenditure with reasonable precision. Detailed studies have been conducted on various exercise types such as walking, running, and cycling[6][7][8]. Indirect calorimetry is used to determine VO₂ at different exercise intensities, which is then used to calculate the metabolic rate of that exercise.
        VO₂ can be expressed as an absolute value (absolute VO₂ = L O₂/min) or the relative value, relative to body mass (relative VO₂ = ml O₂/kg*min). Expressing VO₂ relative to body mass is appropriate when comparing the O₂ cost between individuals or when describing the O₂ cost of weight-bearing activities such as walking, running, or climbing steps. The energy cost of horizontal treadmill walking or running, as well as the O₂ requirement, increases linearly, as shown in Figure 6.6[9]. A similar relationship exists for cycling, up to a power output of about 200 W[10].
Graph illustrating the linear relationship between oxygen cost (VO₂) and speed during steady-state walking and running. The x-axis represents speed (e.g., in meters per minute), and the y-axis represents oxygen consumption (VO₂, typically in mL/kg/min). Two distinct linear trends are shown: one for walking and one for running, both indicating that as speed increases, VO₂ increases proportionally. This relationship supports the use of VO₂ as a reliable indicator of energy expenditure during submaximal aerobic activity.
​Figure 6.6 The relationship between oxygen cost (VO2) and speed for both walking and running at steady state. Note that the relationship between speed and VO2 in these types of exercise is linear.
        Accurately measuring energy expenditure is limited to the time spent in steady state during exercise. This limitation makes it challenging to measure the energy costs of activities other than running, walking, and cycling. Consequently, the energy costs of various activities are often expressed as metabolic equivalents of a task, or METs.
     The metabolic equivalent of a task (MET) represents the energy expended during resting metabolism, with one MET conventionally equal to 3.5 ml O₂/kg/min. The energy cost of activities can be expressed in multiples of the MET unit. METs can also be used to express the number of calories expended per kilogram of body weight per hour.
        For example, if a subject is working at 12 METs, or 42 ml O₂/kg/min, and exercises for 60 minutes, the total oxygen consumption would be 2,520 ml/kg/hr. If the person is using a mixture of carbohydrates and fats for fuel, the VO₂ is multiplied by 4.85 kcal per liter of O₂ (the average between 4.7 and 5.05 kcal/LO₂) the energy expenditure would be 12.22 kcal/kg/hr. This method provides a practical way to estimate the energy expenditure of various activities based on MET values and body weight.
12 MET x 3.5 ml/kg/min = 42 ml/kg/min
42 ml/kg/min x 60 min/hr = 2,520 ml/kg/hr 
2,520 ml/kg/hr = 2.52 L/kg/hr
= 2.52 L/kg/hr x 4.85 kcal/LO2
= 12.22 kcal/kg/hr
       
 The following steps outline how to convert metabolic equivalents to the number of calories expended per hour based on body mass:
  1. Determine the MET value of the activity.
  2. Multiply the MET value by 3.5 to convert to ml O₂/kg/min.
  3. Multiply by the duration of the activity in minutes to get ml O₂/kg/hr.
  4. Convert to liters by dividing by 1,000.
  5. Multiply by the caloric equivalent (4.85 kcal/LO₂) to get kcal/kg/hr.
  6. Multiply by body weight in kilograms to get total kcal/hr.
 The 2024 Compendium of Physical Activity website is designed to provide an updated resource and MET codes for physical activities used in research[11][12]. It serves as a valuable reference for the energy expenditure associated with various sports, household activities, and other categories. For example, the energy expended for one hour of a basketball game is 8.0 METs, while one hour of competitive volleyball equals 6.0 METs. The Compendium was not intended to determine the precise energy cost of physical activities for individuals but rather to standardize MET intensities. It does not account for differences in body mass, adiposity, age, sex, exercise efficiency, or environmental conditions. Therefore, individual differences in energy expenditure may vary from the MET levels presented in the Compendium.

Calculating Rates of Gas Exchange

        Using indirect calorimetry, exercise physiologists can measure the three variables needed to calculate the volume of oxygen consumed (VO₂) and the volume of carbon dioxide produced (VCO₂). The calculation of the rate of gas exchange involves subtracting the amount of expired gas from the amount of inspired gas. Specifically, VO₂ is equal to the volume of O₂ inspired minus the volume of O₂ expired. To calculate the volume of O₂ inspired, we multiply the volume of air inspired by the fraction of that air composed of O₂ (FIO₂). The volume of O₂ expired is calculated by multiplying the volume of air expired by the fraction of the expired air composed of O₂ (FEO₂). The oxygen consumption, in liters of oxygen consumed per minute, can then be calculated as follows:
VO2 = (VI x FIO2) – (VE x VEO2)
 Carbon dioxide production (VCO₂) is calculated in a manner similar to oxygen consumption. To determine the volume of CO₂ produced, we need to account for the inspired and expired fractions of CO₂. Specifically, VCO₂ is equal to the volume of CO₂ inspired minus the volume of CO₂ expired. The volume of CO₂ inspired is calculated by multiplying the volume of air inspired by the fraction inspired of air composed of CO₂ (FICO₂). The volume of CO₂ expired is calculated by multiplying the volume of air expired by the fraction of the expired air composed of CO₂ (FECO₂). The carbon dioxide production, in liters of CO₂ produced per minute, can then be calculated as follows:
VCO2 = (VI x FICO2) – (VE x VECO2)
        These equations provide reasonably good estimations of gas exchange; however, there are limitations. The equations assume that there are no changes in gases stored within the body and that the volume of O₂ consumed equals the volume of CO₂ produced. During exercise, it is known that the volumes of CO₂ increase due to increases in metabolic rate.
        More accurate equations have been derived for exercise based on the fact that a third important gas, nitrogen, is also inhaled and exhaled. The volumes of nitrogen inspired (VIN₂) and the volume of nitrogen expired (VEN₂) should also be considered. The following is called the Haldane transformation, and it is used by exercise physiologists to compute the volume of oxygen (VO₂):
VO2 = (VE) x {[1 – (FEO2 x FECO2) x (0.265)] – (FEO2)}
Where:
  • VI is the volume of air inspired.
  • VE is the volume of air expired.
  • FIO₂ is the fraction of inspired oxygen.
  • FEO₂ is the fraction of expired oxygen.
  • FECO₂ is the fraction of expired carbon dioxide.
        The Haldane transformation accounts for the constant volume of nitrogen in inspired and expired air, providing a more accurate measure of VO₂ during exercise. 
        Computers now calculate VO₂ automatically and correct the expired air concentrations because body temperature (BT), ambient pressure (P), and water vapor saturation (S) can influence the accuracy of the measurements. Therefore, every gas volume is routinely converted by correction equations to its standard temperature (ST: 0°C or 273 K) and pressure (P: 760 mmHg), dry equivalent (D) or STPD[13].

Respiratory Exchange Ratio (RER)

        The amount of oxygen used during metabolism depends on the type of substrate or fuel being oxidized. Generally, the amount of oxygen needed to completely oxidize a molecule of fat or carbohydrate is proportional to the amount of carbon in that fuel. By evaluating the amount of CO₂ released compared with the amount of O₂ consumed, we can estimate the type of fuel being utilized. The ratio between the rate of CO₂ released (VCO₂) and oxygen consumed (VO₂) is termed the respiratory exchange ratio (RER).
RER = VCO2/VO2
        The RER is measured by indirect calorimetry. The theoretical RER limits range from 0.70 to 1.00. An RER of 1.00 indicates that 100% of the energy produced in metabolism is derived from carbohydrates, with no contribution from fats. For example, glucose (C₆H₁₂O₆) contains six carbon atoms. During the combustion of glucose, six molecules of oxygen are used to produce six molecules of CO₂, six molecules of H₂O, and 30 ATP molecules:
6 O2 + C6H12O6 → 6 CO2 + 6 H2O + 30 ATP
        This reaction illustrates the complete oxidation of glucose, where the ratio of CO₂ produced to O₂ consumed is 1:1, resulting in an RER of 1.00.
RER = VCO2/VO2 = 6 CO2/6 O2 = 1.0
        Inversely, an RER of 0.70 indicates that 100% of the energy produced in metabolism is derived from fat, with no contribution from carbohydrates. Fats have considerably more carbon and hydrogen atoms but less oxygen than glucose. Consider palmitic acid (C₁₆H₃₂O₂). To completely oxidize palmitic acid, 23 molecules of oxygen are needed:
C16H32O2 + 23 O2 → 16 CO2 + 16 H2O + 129 ATP
        This reaction shows that the ratio of CO₂ produced to O₂ consumed is lower than 1:1, resulting in an RER of 0.70. This lower RER reflects the greater oxygen requirement for the oxidation of fats compared to carbohydrates. Thus, the RER of palmitic acid is 0.70.
RER = VCO2/VO2 = 16 CO2/23 O2 = 0.70
        Combustion of fat requires significantly more oxygen than a carbohydrate molecule. This results in a substantially lower RER value for fat (e.g., 0.70) compared to carbohydrates (e.g., 1.00). The respiratory exchange ratio chart, shown in Table 6.1, varies with the type of fuels being used for energy. Once the RER value has been determined, the chart can be used to identify the food mixture being oxidized and calculate the amount of energy being expended.
        For example, if the RER value is 1.00, the cells are using only glucose or glycogen, and each liter of oxygen consumed generates 5.05 kcal/L of O₂. Therefore, if the muscles are using only glucose and the body is consuming 3 L of O₂ per minute, the rate of energy production would be:
3L/min*5.05 kcal/L = 15.15 kcal/min
      Each substrate has a specific energy yield[14]:
  • The oxidation of pure fat yields 4.69 kcal/L of O₂ consumed.
  • The oxidation of only protein yields 4.46 kcal/L of O₂ consumed.
      This information allows for precise calculations of energy expenditure based on the type of substrate being metabolized.
Table 6.1 Percentage of fat and carbohydrate metabolized as determined by a non-protein respiratory exchange ratio (RER).
 RER
 FAT %
 CHO%
 1.00
 0
 100
 .98
 6
94 
 .96
 12
88 
 .94
19 
81 
 .92
26 
74 
 .90
32 
68 
 .88
38 
62 
 .86
47 
53 
 .84
53 
47 
 .82
62 
38 
 .80
68 
32 
 .78
74 
26 
 .76
81 
19 
 .74
88 
12 
 .72
94 
6 
 .70
100 
0 
 The measurement of RER has limitations mainly due to the assumption that the body’s O₂ content remains constant and that CO₂ exchange in the lungs is proportional to CO₂ release from the cells. Therefore, calculations of carbohydrate and fat usage based on indirect calorimetry are valid only at rest or during steady-state exercise. Additionally, protein cannot be completely oxidized in the body because nitrogen is not oxidizable, making it impossible to calculate the body’s protein use from the RER. As a result, the RER is sometimes referred to as non-protein RER because it ignores protein oxidation. Despite its shortcomings, indirect calorimetry still provides the best estimate of energy expenditure at rest and during steady-state (aerobic) exercise.

Maximal Exercise Testing

        The most valid measurement of cardiovascular fitness is the maximal capacity to transport and utilize oxygen during exercise. Maximal oxygen uptake (VO₂max) is defined as the maximum rate of VO₂ (ml⋅min⁻¹⋅kg⁻¹) obtained by working to exhaustion. Simply put, VO₂max is the body’s maximal capacity to consume, distribute, and utilize oxygen during an incremental exercise test. VO₂max is also known as the aerobic capacity or maximum physical work capacity[15].
        Typically, changes in oxygen uptake are measured during an incremental (graded) exercise test conducted on a treadmill or a cycle ergometer. An incremental exercise test usually begins with a 5-minute warm-up at 60-70% of VO₂max, followed by a brief rest. The protocol then starts with an initial load set at about 60-70% VO₂max and includes a series of planned progressions that increase the work rate at each stage. Each stage can last from 1 to 3 minutes, and the test continues until the subject cannot maintain the desired power output. Excluding the warm-up, subjects should reach the limit of tolerance within 8-12 minutes to ensure that aerobic metabolism is functioning at full capacity. On a treadmill, increasing the grade (incline) or speed are methods used to increase the work rate. On a cycle ergometer, resistance is applied to the flywheel as the subject tries to maintain the required cadence. Fatigue is determined when a subject (under verbal encouragement from the experimenters) can no longer sustain a pedaling cadence of at least 60 rpm or terminates a running test at their own volition. Another type of test progression used by exercise physiologists is called an incremental ramp protocol, where the work rate is rapidly incremented as a “smooth” function of time[16].
        Research has shown that oxygen uptake increases linearly with the work rate until VO₂max is reached. When VO₂max is reached, an increase in work rate or power output does not result in an increase in oxygen uptake; thus, VO₂max represents a “physiological ceiling” for the oxygen transport system to deliver O₂ to working muscles[17]. Classically, VO₂max levels were thought to exhibit a “plateau” of oxygen consumption at the end of the incremental test. This value is still considered a criterion for achieving VO₂max; however, not all individuals demonstrate this plateau at the end of an incremental test. Perhaps it is because the subject simply cannot complete one more stage beyond the one at which VO₂max was achieved. Nevertheless, other criteria recognized by exercise scientists can also demonstrate that the highest value reached is VO₂max[18]. The major criteria include:
  1. A maximal heart rate within 10 beats per minute of the subject’s predicted maximum heart rate (predicted max-HR = 220 – age).
  2. A rating of perceived exertion (RPE) greater than 17 on the Borg RPE scale (shown in Table 6.2).
  3. A respiratory exchange ratio (RER) greater than 1.1.
  4. A “plateau” of oxygen consumption less than or equal to 150 ml O₂/min.
        If the subject reaches 3 out of the 4 aforementioned criteria, it is said that the highest value of oxygen consumed is the VO₂max. Additionally, some researchers consider high levels of blood lactate to be an additional criterion for achieving VO₂max[19].
        Clinically, incremental exercise tests (also called stress testing) are often employed by physicians to examine patients for possible heart disease. Along with an echocardiogram (ECG) assessment of the stress test, a physician can determine various pathophysiological conditions. Often with these patients, a “system-limited” peak oxygen uptake (VO₂peak) value, rather than a VO₂max value, is reported. A VO₂peak is reported when a subject or patient only achieves 2 out of the 4 criteria for VO₂max. It is still not known if failure to reach a plateau in VO₂ is due to “insufficient effort” or if plateaus of VO₂ are rarely attained despite “good effort” from the subjects[20]. Knowing an athlete’s VO₂max can be valuable for coaches who need to know the maximum speed of athletes under training or of competitors in various sports and games where endurance is a good criterion for selection, training, and improvement. It is also known that greater cardiorespiratory fitness correlates with greater speed in a mile run[21].
Table 6.2 The Borg ratings of perceived exertion scale (RPE).
Rating
Perceived Exertion
6
No exertion
7
Very, very light
8
9
Very light
10
11
Fairly light
12
13
Somewhat hard
14
 
15
Hard
16
 
17
Very hard
18
 
19
Very, very hard
20
Maximum exertion

Calculating Work and Power in Humans

        An ongoing problem in exercise science is the failure to standardize units of measurement in presenting research data. In the United States, the English system of measurement remains in common use despite the standard system of measurement for scientists. The metric system is used in most other countries and is the standard system of measurement. Almost all scientific journals use the metric system, with the basic units of length, volume, energy, and mass being the meter, the liter, the joule, and the gram, respectively. Because of this, a uniform system of reporting scientific measures has been developed. This system, developed through international cooperation, is called System International units, or SI units. SI units have been endorsed by almost all exercise and sports medicine journals for the publication of data to make comparison of published values easy and are summarized in Table 6.3.
        It is important to understand the terms work and power to compute human work output and exercise efficiencies. Work is defined as the product of force and the distance through which the force acts: 
Work = force x distance
        Human work is quantified in joules (J) and is a function of force expressed in newtons (N) and distance in meters (m). The following example demonstrates how to calculate work for a person lifting a 20 kg weight upward over a distance of 2 meters.
Example: Lifting a 20 kg weight upward over a distance of 2 meters in 60 seconds
        
The kilopond is a unit of force that represents the effect of gravity on a mass of 1 kilogram. Thus, is important to realize that at the earth’s surface, a mass of 1 kg exerts a force of 9.81 N due to gravity. Therefore, the mass (kg) of the weight must first be converted to a force (N).
Step 1: Convert kg to Newtons (N), where 1 kg = 9.81 N
20 kg x 9.81 m/s2 = 196.2 N
Once the mass is converted to a force, the force can be multiplied by the distance to find the work done, expressed in joules (J).
Step 2: Multiply force (N) by distance (m) to find work
Work = 196.2 N x 2 m
Work = 392.4 Newton-meters or 392.4 joules (J)
        An additional step is required to calculate power for the same example. Power is expressed in watts (W) and measures the rate at which work is completed. Power describes how much work is accomplished per unit of time.
        Then, to calculate power, work must be calculated from weight and distance as in Step 1 and Step 2. Then, power (W) can be determined by dividing work by time in seconds (s):
Power = work/time
Step 3: Divide joules by seconds to find power
Power = 196.2 N x 2 m
Power = 6.54 watts (W)
        In some situations, traditional units are used to express both work and energy. Table 6.4 contains a list of terms commonly used today to express work, power, and energy and their conversions to SI units. Note that both work and energy use joules (J). The energy content of commercial food products is often listed on the label in kilocalories (kcals) or Calories (kcals) with a capital letter “C.” However, the SI unit for energy content and expenditure is joules, where 1 kilocalorie is equal to 4,186 joules (J) or 4.186 kilojoules (kJ). In the UK and other European countries, energy is expressed as both kilojoules (kJ) and kilocalories (kcals) on food labels. In the United States, food labels often represent energy as Calories (kcals), which may be misleading because the capital “C” represents kilocalories and not calories, which are 1,000 times smaller than one kilocalorie. 
Table 6.4 Common units and conversion factors used to express work, power, or energy expended in humans
Unit                
SI Unit
Mass
Kilogram (kg)
Distance
Meter (m)
Time
Seconds (s)
Force
Newton (N)
Work, Energy
Joule (J)
Power
Watt (W)
Velocity
Meters per second (m/s)
Torque
Newton-meter (Nm)
        The measurement of work output is termed ergometry. Several devices measure specific types of work or power in humans. These apparatuses are called ergometers and are used in exercise physiology laboratories. One of the earliest ergometers to measure work in humans was the bench step, where the total work performed was a function of body mass, step cadence, and the height of the step. Other commonly used devices to measure work or power include the cycle ergometer, rowing ergometers, motor-driven treadmills, and arm crank ergometers. Figure 6.7 shows examples of ergometers commonly used to measure human work and power.
Figure 6.7 displays three types of ergometers commonly used to measure human work and power output in exercise physiology. A) A bench step ergometer, consisting of a raised platform used to assess aerobic capacity through step tests. B) A cycle ergometer, resembling a stationary bike, used to measure power output and cardiovascular response during controlled pedaling. C) A rowing ergometer, designed to simulate rowing motion, used to evaluate both aerobic and anaerobic performance.
​Figure 6.7 Examples of ergometers used in the measurement of human work and power output. A) A bench step ergometer, B) a cycle ergometer, and C) a rowing ergometer.

Exercise Efficiency

        Exercise efficiency is the capacity to convert energy expenditure (EE) into work, with some energy inevitably lost as heat. Net efficiency is a function of work output and energy expended.
Net efficiency = (Work output/Energy Expended) x 100
        Factors influencing efficiency include the percentage of slow muscle fibers, which display greater efficiency. Slow fibers require less ATP per unit of work. Subjects with increased efficiency generate greater power output at any given EE rate. Horizontal running efficiency cannot be calculated directly. Instead, the O₂ cost of running at any speed is measured to make comparisons. The O₂ cost is defined as VO₂ at steady state. A runner with poor running economy would require a higher VO₂ at any given running speed. Running economy is the relationship between oxygen consumption (VO₂) and the velocity (v) of running, or the aerobic demands of running[22].

Chapter Summary

        In this chapter, we explored the fundamental principles of energy expenditure and its critical role in exercise physiology. We began by examining the metabolic pathways involved in ATP formation and how energy expenditure varies between rest and exercise. We discussed the methods for measuring energy expenditure, including direct and indirect calorimetry, and highlighted the importance of understanding the respiratory exchange ratio (RER) in determining the type of fuel being utilized.
        We also delved into the measurement of resting metabolic rate (RMR) and its significance in both clinical and research settings. The chapter covered the methods for measuring VO₂, including closed and open circuit spirometry, and the use of the Haldane transformation for more accurate calculations during exercise. Furthermore, we examined the concept of maximal exercise testing, the criteria for achieving VO₂max, and the practical applications of knowing an athlete’s VO₂max. The importance of standardized units of measurement in exercise science was emphasized, along with the calculation of work and power in humans.
        Finally, we discussed exercise efficiency, the factors influencing it, and the concept of running economy. By understanding these principles, we can better appreciate the complexities of energy expenditure and its implications for exercise performance and overall health.

Scholarly Questions

  1. What is calorimetry?
  2. Is VO2 a direct or indirect measure of energy expenditure?
  3. What does MET stand for? How would you calculate a MET from a given VO2? How many METs is a person working at if VO2 is 35 ml/kg/min? 60 ml O2/kg/min?
  4. What is work?
  5. Calculate the work a person does if they lift 40 kg weight upward over a distance of 4 meters.
  6. What is power?
  7. Calculate the power a person does if they lift 40 kg weight upward over a distance of 4 meters in 30 seconds.
  8. What is an ergometer? Can you name an example of an ergometer that is used in the lab?
  9. What factors can affect exercise efficiency?
  10. What does RER stand for? How is it expressed mathematically? Can you calculate RER if VO2 is 12 ml/kg/min and VCO2 is 10 ml/kg/min?
  11. What is the difference between the RQ (Respiratory Quotient) and RER?
  12. What are the theoretical limits of RER?
  13. What is a VO2 max? What are the 4 criteria to know it has been achieve?
  14. What is the average improvement seen with VO2max from endurance training?
  15. What is a VO2 peak? What is the difference between VO2max and VO2 peak?
  16. What are the two ways for expressing VO2? Which is better for comparison and why?
  17. If the caloric expenditure of exercise is 5 kcal/LO2, how many kcals is a person burning if they are exercising at 25 LO2/min?
  18. Usually, what type of exercise mode (cycle, treadmill, stepper) will yield the highest VO2max values? Why (hint: which mode utilizes the most muscle mass)? 
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