Measurement of Oxygen Uptake, Carbon Dioxide Production, Energy Expenditure, and Mechanical Efficiency

Introduction

Metabolism encompasses all the processes occurring within a living organism (Brooks et al.

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, 2005). The metabolic rate of an organism is defined as the rate at which heat is produced. Accurate and efficient methods for measuring metabolic gas exchange in humans are crucial for both clinical research and sports and exercise science. In physiology labs, the Douglas Bag method is commonly employed to obtain gas samples, providing valuable information and measurements on individuals (Rosdahl et al., 2010).

Estimating metabolism can be achieved in two ways: through direct determinations of heat production and through determinations of oxygen consumption.

During exercise, direct calorimetry is impractical, necessitating the use of indirect calorimetry. Although not suitable for high-intensity exercise in harsh conditions, indirect calorimetry is appropriate for experiments conducted at 65% intensity, as in this study. Indirect calorimetry measures both oxygen consumption (VO₂) and carbon dioxide production (VCO₂), which can then be used to calculate resting energy expenditure (EE) and respiratory quotient (RER). It is considered the gold standard for measuring metabolic rate due to its reliability and validity (Brooks et al., 2005).

This data, in turn, allows us to calculate energy expenditure (EE), defined as an individual's capacity to perform internal and external work (Matarese, 1997). Energy exists in various forms and is interchangeable, such as the conversion of chemical energy into mechanical energy for muscle contractions and thermal energy for body temperature regulation. As expected, an increase in minute ventilation (Ve) places a greater demand on the respiratory system and muscles. Studies have shown that circulatory and metabolic costs of breathing at maximal levels can reach up to 16% in trained individuals and 10% in untrained individuals (Guenette & Sheel, 2007).

Furthermore, research indicates that dietary carbohydrates and fats both serve as substrates for human metabolism during exercise, with the influence of each substrate depending on factors like training status, diet, exercise intensity, and hormonal conditions (Goedecke et al., 2000). Variability in these factors exists among untrained and trained individuals, possibly related to differences in skeletal muscle characteristics.

Experimental Aims

The primary objectives of this experiment are:

1. To directly measure the rate of oxygen uptake and carbon dioxide production to compare the rate of energy expenditure at rest and at the same absolute exercise intensity between the two subjects.
2. To calculate and compare the efficiency of converting metabolic energy to mechanical work during cycling.

Methods

The participants in this study were volunteers from the class group who willingly participated in the experiment and provided verbal consent. To ensure accurate measurements, each volunteer followed specific guidelines, including fasting overnight for early morning labs and fasting for at least three hours before the experiment in other cases. Prior to commencing the experiment, various data points were collected from the volunteers, including height, weight, age, and gender. Additionally, each participant was equipped with monitoring devices.

The following measurements were recorded for each volunteer:

After ensuring the participants were comfortably seated on the exercise bike, they were fitted with sterile equipment, including a mouthpiece connected to the Douglas Bag via a tube and a nose clip. It was recommended to moisten the area of the polar monitor placed on the sternum to enhance heart rate detection.

The experiment began with a 5-minute resting period during which an expired gas sample was collected using the Douglas Bag technique. The collected air sample was then analyzed, and the resting heart rate was recorded. Air analysis was performed by sealing the valves of the bag and connecting it to the AMIS 2001 automated metabolic cart, which measured oxygen and carbon dioxide levels. The volume of the expired air was measured using the Harvard Dry Gas Meter.

The predicted steady-state heart rate equivalent to 65% of each subject's VO₂ max was calculated using the Karvonen formula, a common method for determining exercise intensity based on maximum heart rate (She et al., 2015). The formula is as follows:

Maximum Heart Rate = 220 - Age of Subject

Target Heart Rate = ((Maximum Heart Rate - Resting Heart Rate) x % Intensity) + Resting Heart Rate

Each subject was assigned a specific load based on individual characteristics, for example, Subject 1 had a load of 2kg, and Subject 2 had a load of 1.5kg. With the target heart rate determined, the participants began cycling until they reached the target heart rate and maintained this pace for 2 minutes before initiating a 5-minute recording of their heart rate. During the final minute of this 5-minute cycle, an expired gas sample was collected, and the AMIS 2001 Automated Metabolic Cart was used to analyze the sample. At the end of the experiment, the used mouthpieces were disposed of properly to prevent reuse by others.

Results

Table 2a displays the resting expired gas samples for subjects 1 and 2. Detailed calculations can be found in the appendix.

Table 2b presents the rest expired gas samples for subjects 1 and 2, with detailed calculations available in the appendix.

Table 3a displays the exercising expired gas samples for subject 1 and 2, with calculations available in the appendix.

Table 3b presents the exercising expired gas samples for subjects 1 and 2, with detailed calculations available in the appendix.

The data and calculations for the experiment are summarized in tables 2a, 2b, 3a, and 3b. Detailed formulae used for these calculations can be found in the appendix.

Table 4 presents the calculated Target Heart Rates (HR) for subjects 1 and 2, determined using Karvonen's formula:

The target heart rate was calculated as follows:

• For Subject 1: 220 - 20 = 200 and ((200 - 86) x 65%) + 86 = 160.1
• For Subject 2: 220 - 19 = 201 and ((201 - 65) x 65%) + 65 = 92.65

Table 5 displays the heart rate data calculations for subjects 1 and 2, including measures of central tendency and dispersion:

Table 6 provides key identifying factors for the abbreviations used in Table 5:

Table 7 presents the calculations for Fat Oxidation for Subjects 1 and 2, with detailed formulae available in the appendix:

Table 8 displays the calculations for Carb Oxidation for Subjects 1 and 2, with detailed formulae available in the appendix:

Efficiency calculations are shown in Table 9:

It is important to note that the mechanical efficiency for cycling typically falls within the range of 20-25% (Perrault Ph.D, 2006), which is consistent with various studies and literature. In some instances, elite cyclists have been found to have a mechanical efficiency closer to 25%, with potential variations depending on output (Etemma & Loras, 2009).

Discussion

Oxygen uptake refers to the rate at which the body's demand for oxygen increases to match the energy demands placed on the muscles during high-intensity exercise (Draper & Marshall, 2014). Maximal oxygen uptake, on the other hand, is the point at which oxygen uptake plateaus despite increases in work rate and intensity (Baba, et al., 1996).

In this experiment, Subject 1 recorded higher measurements compared to Subject 2. This difference could be attributed to gender, with Subject 1 being male and Subject 2 being female. The MET for Subject 1 at rest and during exercise was 2.2506 units and 13.18 units, respectively, while Subject 2 had 1.2364 units at rest and 7.23 units during exercise. Additionally, absolute VO2 (not accounting for subject weight) for Subject 1 at rest and during exercise was 0.64 l/min and 3.67 l/min, while Subject 2 had 0.33 l/min at rest and 1.89 l/min during exercise. Relative VO2 (accounting for subject weight) for Subject 1 at rest and exercise was 7.9 ml/kg/min and 45.7 ml/kg/min, whereas Subject 2 had 4.3 ml/kg/min at rest and 25.2 ml/kg/min during exercise. These considerable differences in parameters between the two subjects are noteworthy.

Mechanical efficiency provides insight into the proportion of total energy expended that can produce external work. In muscle physiology, efficiency reflects how skeletal muscles transform biochemical energy into external work required for movement (Perrault Ph.D, 2006). Both gross efficiency and net efficiency contribute to this concept. Gross efficiency, the simpler method of calculation, directly considers work output and the subject's speed, utilizing all energy expended by the subject's skeletal muscles while in motion (Perrault Ph.D, 2006). Net efficiency, however, is more complex as it accounts for the subject's resting energy expenditure in the calculation, subtracting it from the total energy expended during exercise. Delta efficiency, another formula, is defined as the work accomplished divided by the delta in energy expended. This calculation considers the energy expended during two steady-state exercise bouts in which the internal work of movement is technically the same (Perrault Ph.D, 2006).

As previously mentioned, Subject 1 outperformed Subject 2 in all recorded parameters, and gender was identified as a potential explanation. Studies have shown that maximum VO2 readings for males exceed those for females by 15-30%, even among trained individuals. This difference becomes even more pronounced when considering absolute VO2 max instead of relative VO2 max. Physiological factors and body composition contribute significantly to this gender disparity. Lung structure and airway dimensions differ between males and females, with women generally having smaller lung capacity and decreased lung diffusion capacity compared to men (Richards, et al., 2004). Women also exhibit lower maximal expiratory air rates due to smaller alveoli and airway diameters. Body composition, especially in the untrained population, plays a role as well. Untrained females typically have approximately 26% body fat on average, while their male counterparts have an average of 15% body fat (McArdle , et al., 1986). Even in the trained population, females tend to have a higher percentage of body fat compared to trained males. This lower body fat percentage in males allows them to generate more aerobic energy due to their larger muscle mass. Hemoglobin, crucial for oxygen-carrying capacity, also varies between genders, with males having higher hemoglobin concentrations (approximately 10-14%). This higher hemoglobin concentration enables males to circulate more oxygen during intense exercise, contributing to their greater aerobic capacity. For example, males aged 20 were found to have an average hemoglobin concentration of approximately 14.8 g/ml, while females had a concentration of 13.5 g/ml (Hawkins, et al., 1954), although this research may be considered dated, its underlying scientific principles remain valid today.

Ventilation (Ve) during steady-state exercise, such as cycling in this experiment, increases linearly with oxygen and carbon dioxide consumption. This increase in ventilation is mainly achieved by augmenting tidal volume. During steady-state exercise, FeO2 leads to an increase in the activity of internal intercostal and abdominal muscles, working in conjunction with lung elasticity, resulting in more rapid and forceful exhalation (Nalbandian, et al., 2017). Ve/VO2 decreases from rest to exercise because, as exercise intensity rises, there is an increased oxidation of oxygen.

Limitations

Several limitations were encountered during the course of this experiment:

• The sample size used was small, consisting of only two subjects. While the results aligned with expectations, a larger sample size would have provided a more representative view of the two populations.
• There was no prior contact with the subjects before the experiment, making it impossible to verify if they adhered to the fasting requirement of at least three hours before the experiment or if they engaged in any form of exercise prior to participation. The accuracy of this information solely relies on the subjects' statements.
• The equipment used, including the polar monitor and paired watch, proved to be challenging to use and may not have been entirely accurate in all instances.

Conclusions

The analysis of expired gas samples revealed an increase from rest to exercise in both subjects. As expected, there were significant differences in the recorded values between the two subjects, primarily due to distinct physiological characteristics and body compositions associated with their genders.

Appendix: Formulae Used

Work output = Force x Distance

Energy Expenditure = (15.8 x VO2) + (4.86 x VCO2)

Gross Efficiency = (work output/energy expenditure) x 100

Net Efficiency = (work output/energy expended) x 100

CHO Oxidation = (4.55 x VO2) – (3.21 x VCO2)

Fat Oxidation = (1.67 x VO2) – (1.67 x VCO2)

MET = EE x 60/BM/4.18 kg/min

REE = Male – (66.47 + 13.75(weight, kg) + 5 (height, cm) – 6.76 (age, year))

Female – (655.1 + 9.65 (weight, kg) + 1.84 (height, cm) – 4.68 (age, year))

Rest Calculations:

Energy Expenditure = (15.8 x VO2) + (4.86 x VCO2)

Subject 1: 12.5906

Subject 2: 6.4776

MET:

Subject 1: 2.2506

Subject 2: 1.2364

Ve/VO2:

Subject 1: 38.09375

Subject 2: 28.3

Ve/VCO2:

Subject 1: 47.86

Subject 2: 35.97

Steady-State Calculations:

Energy Expenditure = (15.8 x VO2) + (4.86 x VCO2)

Subject 1: 73.7324

Subject 2: 37.881

MET:

Subject 1: 13.18

Subject 2: 7.23

Ve/VO2:

Subject 1: 31.45

Subject 2: 25.62

Ve/VCO2:

Subject 1: 35.62

Subject 2: 29.35

REE:

Subject 1: 767.16 kCal

Subject 2: 641.5 kCal

References

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