Metabolic Differences in Children & Young Adults after Exercise, Study notes of Literature

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EXCESS POST-EXERCISE OXYGEN CONSUMPTION

AND SUBSTRATE UTILIZATION IN CHILDREN AND

ADULTS

By Justin Ross Bland A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Kinesiology

ABSTRACT

EXCESS POST-EXERCISE OXYGEN CONSUMPTION

AND SUBSTRATE UTILIZATION IN CHILDREN AND

ADULTS

By Justin Ross Bland INTRODUCTION: The majority of available literature on excess post-exercise oxygen consumption (EPOC) focuses on the adult population. No study has quantified EPOC in children after exercise lasting more just a few minutes. Previous research has shown fat oxidation, measured by respiratory exchange ratio (RER), to be greater in children than in adults at rest and during exercise. Few studies have investigated substrate utilization post- exercise in children. PURPOSE: To examine young adult-child differences in EPOC and substrate utilization following moderate and vigorous intensity exercise performed on the cycle ergometer. METHODS: 19 children (7 to 9 years old) and 22 young adults (20 to 23 years old) visited our laboratory on three separate occasions and completed an exercise trial each time: VO 2 max test, moderate exercise (MOD), and vigorous exercise (VIG). Maximum power output (MaxPO) during the VO 2 max test was used to determine workload for MOD (35% MaxPO) and VIG (70% MaxPO) exercise. MOD and VIG trials were randomized and counterbalanced. Participants rested for 30 minutes (min) prior to exercise; the last 20 min were used for baseline VO 2 measures. Tests were 2-min square-wave intervals lasting 1 hour for the MOD trial and 30 min for the VIG trial. Expired gases were captured 10 min prior to the cessation of exercise and for 20 minutes post-exercise. EPOC was examined using ANOVA and repeated measures ANCOVA to control for sex, fitness, body composition, and caloric intake. Substrate utilization was

Justin Ross Bland examined using multivariate analysis of variance (ANOVA) for RER at minutes 1, 5, 10, 15, and 20 post-exercise. Another ANOVA model statistically controlling for sex, fitness, body composition, and caloric intake was also performed. RESULTS: After MOD and VIG, children had significantly lower volume of EPOC when compared to young adults (0. 30 + 0.13 vs. 1.18 + 0.38 L; F = 19.609, p < 0.001, d = 0.31 for MOD and 0.71 + 0. vs. 2.46 + 0.95 L; F = 59.73, p < 0.001, d = 2.44 for VIG exercise). Children returned to baseline significantly faster than the young adults after both MOD (240 + 105 vs. 552 + 325 seconds, respectively; F = 16.413, p < 0.001, d = 1.30) and VIG (424 + 345 vs. 925 + 340 seconds, respectively; F = 21.912, p < 0.001, d = 1.50). Children’s RER was significantly lower than young adults at 1, 5, and 10 min (0.84 + 0.05 vs. 0.90 + 0.05; 0.93 + 0.05 vs. 1.00 + 0.08; and 0.86 + 0.03 vs. 0.91 + 0.05, respectively; all p < 0.01; effect sizes of 0.61, 0.26, and 0.29, post MOD respectively). After VIG, RER was similar in children and young adults at min 1 and 10; however, children’s RER in min 5 was significantly lower than adults (1.02 + 0.06 vs. 1.15 + 0.11, p = 0.001; effect size of 0.60). Children’s RER in min 15 and 20 was significantly higher than adults’ (0.83 + 0. vs. 0.77 + 0.06; 0.83 + 0.05 vs. 0.75 + 0.06, respectively; p ≤ 0.001; effect sizes of 1. and 2.12, respectively). CONCLUSIONS: Children had lower EPOC and recovered faster than young adults after both MOD and VIG. Children had lower RER than young adults after MOD, but higher RER 15 and 20 min after VIG. Children relied more on fat oxidation at rest and during exercise than young adults. This study observed children to utilize a greater proportion of carbohydrates than young adults after VIG. Funded by Michigan State University College of Education

iv To my loving and unbelievably supportive wife. “some day, you will get the best of me…”

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ACKNOWLEDGEMENTS

“The LORD is my rock, my fortress and my deliverer; my God is my rock, in whom I take refuge.” – Psalm 18: “Fear not, for I have redeemed you; I have summoned you by name; you are mine. When you pass through the waters, I will be with you…For I am the LORD, your God, the Holy One of Israel, your Savior.” – Isaiah 43:2a, 3a I wish to acknowledge my amazing advisor: Karin Pfeiffer, thank you for your patience as well as your display of grace and truth. I wish to acknowledge my family, thank you for your ceaseless prayers, love, and support. I wish to acknowledge a mentor: Joe Eisenmann, thank you for your wisdom and encouragement while “Running with Joe.” I wish to express thanks to my committee (Karin Pfeiffer, Jim Pivarnik, Lorraine Weatherspoon, and Kim Maier) for their hard work, dedication, and ease of access. I wish to acknowledge the funding from Michigan State University College of Education, which made this study possible. Finally, I wish to acknowledge all those who became my family in East Lansing, thank you for your love and support.

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TABLE OF CONTENTS

LIST OF TABLES .................................................................................................................ix

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ix

• CHAPTER LIST OF FIGURES ...............................................................................................................ix
• INTRODUCTION
  • Pre-exercise metabolism
  • Exercise metabolism
  • Post-exercise metabolism...........................................................................................
  • Substrate utilization
  • Problem statements
  • Research aims and hypotheses
• CHAPTER
• LITERATURE REVIEW
  • History and definition of EPOC.................................................................................
  • Review of literature....................................................................................................
    • Measurement of EPOC
    • Effects of intensity and duration on EPOC
    • EPOC in children and adolescents
    • Summary of studies involving children
    • EPOC in obese individuals
    • Adult-Child differences in EPOC
    • Substrate utilization post-exercise
    • Nutritional considerations for assessing substrate utilization and EPOC
  • Summary of the literature
• CHAPTER
• METHODS
  • Subjects
  • Measures
  • Procedures
  • Analysis......................................................................................................................
  • Significance of the results
• CHAPTER
• RESULTS
  • Physical characteristics
  • Nutrition – caloric consumption and macronutrient composition
  • Pre-exercise oxygen consumption
  • Exercise oxygen consumption
  • Excess post-exercise oxygen consumption
  • During exercise respiratory exchange ratio
  • Post-exercise respiratory exchange ratio vii
  • Substrate utilization one-hour post-exercise
• CHAPTER
• DISCUSSION
  • Major findings............................................................................................................
  • Interpretation of findings
  • Ventilatory threshold issues
• substrate utilization Effects of caloric consumption, and macronutrient composition on EPOC and
  • Limitations
  • Strengths
  • Future research
• REFERENCES
• Table 1. Combined physical characteristics........................................................................... LIST OF TABLES
• Table 2. Physical characteristics
• Table 3. Caloric consumption and macronutrient composition
• Table 4. Child: moderate vs vigorous
• Table 5. Young adult: moderate vs vigorous
• Table 6. Children versus young adults during moderate exercise
• Table 7. Children versus young adults during vigorous exercise
• Table 8. Pre-exercise vs. post-exercise VO 2 in children and young adults – total sample
• Table 9. Descriptive statistics of EPOC duration in seconds (data from all subjects)...........
• who reached baseline within the twenty minutes of continuous measurements) Table 10. Descriptive statistics of EPOC duration in seconds (data represent only those
• Table 11. Pre-exercise vs. post-moderate exercise VO 2 in children and young adults
• Table 12. Pre-exercise vs. post-vigorous exercise VO 2 in children and young adults
• Figure 1. Physical maturation and EPOC LIST OF FIGURES
• Figure 2. Time constant of VO 2 in children vs adults after different exercise intensities
• Figure 3. EPOC volume after moderate exercise: children and young adults (20 minutes).
• Figure 4. EPOC volume after moderate exercise: children and young adults (10 minutes)
• Figure 5. EPOC volume after vigorous exercise: children and young adults (20 minutes)...
• Figure 6. EPOC volume after vigorous exercise: children and young adults (10 minutes)...
• Figure 7: Children’s EPOC: comparison between moderate and vigorous exercise (
• minutes)..................................................................................................................................
• Figure 8: Young adult’s EPOC: comparison between moderate and vigorous exercise (
• minutes)..................................................................................................................................
• Figure 9: Comparison of RER after moderate exercise
• Figure 10: Comparison of RER after vigorous exercise
• Figure 11: Children’s RER after moderate and vigorous exercise
• Figure 12. Young adults RER after moderate and vigorous exercise.

Chapter 1 – Introduction Total daily energy expenditure (TDEE) results from a combination of factors: resting metabolic rate (RMR), thermic effect of food (TEF), and physical activity (PA). Each factor is further comprised of smaller elements. RMR is the energy expenditure due to basal metabolic rate (energy required for vital functions) and arousal metabolism (energy required for a general state of awareness). RMR is highly dependent on an individual’s fat free mass (FFM), sex, age, and hormonal balance^1. The TEF can contribute significantly to TDEE, accounting for 3 – 25 percent of energy consumed in a meal depending on nutrient composition. All other things being equal a high protein meal elicits the highest TEF, followed by a high CHO meal, with high fat meals being the lowest 2 - 4

. The high TEF from protein is due to the greater cost of metabolism and storage when compared to CHO and fat^5. Lastly, PA can significantly increase an individual’s metabolic rate above resting values, both during and after the activity. PA bouts provide a unique opportunity to study the metabolic adaptations of the human body; these adaptations provide insight into control of metabolism 6 , 7 . Pre-exercise metabolism Pre-exercise metabolic rate refers to the energy expenditure immediately prior to the exercise bout and has long been assumed to be slightly higher than RMR. The premise behind this assumption is that the human body, in anticipation and preparation of the work that is to come, increases sympathetic output, and as a result metabolic rate increases. One author, reviewing the literature of EPOC, suggests that pre-exercise measurements should be recorded with the understanding that RMR is not accurately

represented by pre-exercise metabolic rate and; therefore, if RMR is desired, measurements should be taken well in advance of the exercise bout^8. Other authors disagree. Thomas et al reported no significant differences between pre-exercise metabolic measurements and RMR during a control visit^9. Turley et al reported no significant difference between inpatient and outpatient RMR 10

. Against commonly held assumptions some research suggests that pre-exercise, inpatient metabolism is not significantly different from controls. Exercise metabolism During exercise, aerobic metabolism increases from commencement to completion of exercise, and there are subtle differences between the onset of exercise and the attainment of a steady state of exercise. As exercise commences, the fuel needed to produce power initially is supplied by stored ATP-PCr in the muscle and, moments later, the slightly slower metabolic process of glycolysis. Concurrently, oxygen consumption increases as the slower aerobic system is able to provide the majority of energy needed to perform sustained work. The beginning of exercise provides a unique insight into the kinetics of oxygen uptake and has been considered as an important window into the understanding of control of the aerobic energy system 6 , 11 . At the onset of exercise, the difference between the required energy to perform an activity and the produced energy by the aerobic system, as measured by oxygen consumption, has long been referred to as oxygen deficit 12 . The measure of the total volume of oxygen deficit, or accumulated oxygen deficit (AOD), has been used as a valid indication of anaerobic capacity^13.

Oxygen uptake kinetics studied at the onset of exercise are critical to the understanding of the control of aerobic metabolism^6 ,^11. As sustained exercise continues beyond a minute or two, oxygen consumption increases until the energy produced is able to meet the aerobic demand of the activity - this is considered attainment of “steady state.” While at metabolic steady state, an aerobically healthy individual can usually maintain the activity for extended periods of time. Steady state exercise allows for closer study of many variables, including the use of different substrates (CHO, fats, and protein) to fuel activity. Analyzing expired gases allows for the comparison of carbon dioxide production (VCO 2 ) and the oxygen consumption (VO 2 ), the ratio of which is known as the respiratory exchange ratio (RER). RER is calculated by VCO 2 /VO 2 and can be used to interchangeably as the non-protein respiratory quotient (RQ) to estimate the percentage of CHO and fats that the individual uses to fuel oxidative phosphorylation if the contribution of protein to metabolism in a healthy individual is negligible. It is generally accepted that protein metabolism is minimal during aerobic exercise, therefore RER can be an appropriate estimation of RQ 14

. As in all measurement techniques, there are limitations to assessing substrate utilization via RER^15 ; however, RER, as measured by indirect calorimetry, is widely accepted and especially practical since it is non-invasive 16 . As the ratio approaches 1. during steady state conditions, the reliance on CHO as the preferred substrate increases toward 100%. An RER of 0.7 is indicative of the sole reliance of fat to fuel metabolism. During heavy exercise the body relies preferentially on CHO as the desired fuel; therefore, RER drifts close to, or during intense exercise, greater than 1.0. Furthermore, RER exceeding 1.05 (or 1.1 in some laboratories) is used as a criterion measure for

maximal exertion in adults. Thus, observing steady state exercise has been important for the understanding of metabolism and in particular, fuel utilization. Post-exercise metabolism Recovery from exercise provides a view of the metabolic characteristics of an individual not seen during rest or exercise. Unlike rest, onset of exercise, or steady state exercise, the post-exercise period includes time of elevated metabolism with little skeletal muscle activity and was first published as a scientific observation in 1910 by A.V. Hill who later coined the phrase “oxygen debt” to describe the phenomenon^17 ,^18. The phrase “oxygen debt” was designed to imply excessive oxygen consumption after exercise was “making up” for the slower oxygen uptake kinetics at the onset of exercise. Thus, Hill believed that the oxygen debt was equal to the oxygen deficit at the onset of exercise. The implications of oxygen deficit and oxygen debt being equal have led some researchers to use oxygen debt as a measure of anaerobic capacity^19 -^21. However, the physiological differences between the onset of exercise and recovery from exercise led some researchers to reconsider the equality of oxygen deficit and oxygen debt. Thus, In 1984, Gaesser and Brooks addressed several faulty implications of the term “oxygen debt” and proposed a new term: excess post-exercise oxygen consumption (EPOC) citing that when compared to pre-exercise, recovery is characterized by: elevated body temperature, increased circulating catecholamines, increased uptake of calcium ions by the previously active muscles, continued liberation of fatty acids to fuel metabolism, and a host of other known and unknown factors which contribute to the uniquely elevated metabolic rate post-exercise 22

. Much of the literature concerning EPOC centers on the observation that

EPOC volume is dependent upon the intensity, duration, and mode of exercise 8 , 23 - 33

. In a review of the literature, EPOC is linearly correlated with the duration of the exercise and exponentially related with exercise intensity when exercising above 60% VO 2 max 8 . Controversy exists concerning the duration of EPOC. Some investigations have observed EPOC after aerobic exercise lasting 12 hours or more after cessation of the exercise bout 34 , 35 . Others have reported EPOC lasting between three and 10 hours 8 , 36 - 38 . Still others have observed the duration of EPOC to be less than an hour^33 ,^39 ,^40. Furthermore, at least one group of investigators found the duration of EPOC was as short as 13 minutes when exercising for 30-49 minutes at intensities below 60% of VO 2 max^41. Although controversy surrounds the duration of EPOC, intensity and duration of the exercise appear to be major contributors to the total volume of EPOC. Investigations into EPOC will contribute to the understanding of the body’s attempt to maintain homeostasis in the context of post-exercise. Furthermore, insights into the mechanisms of EPOC could provide information regarding metabolic control during recovery. The existence of EPOC has led some researchers to explore its potential impact on weight control in the adult population; however, the magnitude of the caloric expenditure from EPOC is not clear^26 ,^37 ,^42. Although the majority of the literature has concluded that EPOC contributes minimally to TDEE 8 , others have argued that more energy can be expended during EPOC than during the exercise itself^43. The accumulated effect of EPOC on energy expenditure with consistent exercise can contribute to weight balance, since even a seemingly negligible positive caloric balance, such as 25 kilocalories/day, can lead to obesity over time^44.

The exact mechanisms driving EPOC are still not fully understood 22 , 24 , 34 , 35 . Researchers have proposed that mitochondrial respiration is the keystone to the control of EPOC 22

. In turn, increased mitochondrial respiration can be affected by factors such as increased core body temperature, ionic disturbance, increased circulating catecholamines, and release of fatty acid 22 . Based on the available literature it is clear that more research is warranted in order to better understand the control of post-exercise oxygen consumption. The majority of the existing EPOC literature addresses the adult population. It is widely understood that the maturing child responds to exercise differently than the mature adult 45 . For example, healthy children have been observed, based on heart rate (HR), to reach a steady state faster and recover from exercise more quickly than adults 46 . Unfortunately, little is known about EPOC in children. The published research concerning oxygen consumption post-exercise in children is limited, scattered throughout different topics, and is rarely addressed as EPOC per se. Studies investigating recovery in children address issues in adult-child comparisons, assessment of anaerobic metabolism, effects of chronic exposure to altitude, and evaluation of children with various diseases 19 - 21 , 47 , (^48). Much research in children focuses on quantifying anaerobic metabolism by measuring oxygen debt 19 - 21 ; however, EPOC is not simply an indication of the anaerobic metabolism that takes place at the onset of exercise^22. The information presented in these various studies constitutes the bulk of what is known concerning EPOC in children. However, these data support no definite conclusions regarding EPOC in the pediatric population.

Substrate utilization The body preferentially uses different substrates (carbohydrate, fat, or protein) as fuel for metabolism depending on the needs of the individual. The body shifts from relying on carbohydrates (CHO) during moderate to vigorous exercise, to primarily fats oxidation during recovery in normal, healthy individuals 49

. Wong and associates observed that there were no differences in substrate utilization between the lean and obese men during a standard exercise bout; however, after exercise, obese adults showed a lower reliance on fat oxidation and a reduced EPOC when compared to their lean counterparts^49. It is of note that the degree of ability to utilize fats as a source of energy has been shown to be a significant component in the development of obesity 50 , 51 . Thus, it is possible that the suppressed fat oxidation in the obese individuals may only be observed during the post-exercise period of time. Substrate utilization, as estimated by RER, during rest and exercise is known to differ between children and adults^16. It is generally accepted that children have lower RER values than adults during rest and exercise, indicating that they rely more on fat oxidation for energy production 16 , 52 . Wong and Harber observed that after a 30-minute bout of cycling at ventilatory threshold RER was suppressed significantly below baseline in normal weight adults indicating a greater reliance on fat as a substrate for oxidative phosphorylation^49. Since skeletal muscle composition does not differ significantly between children and adults, it stands to reason that children would have a similar post- exercise response in RER. However, to our knowledge, no research studies have been published concerning the substrate utilization during recovery from submaximal exercise bouts in children when compared to adults. It is unknown if children utilize the same fuel

to recover from exercise as do adults. Therefore, more research is needed to investigate substrate utilization, particularly post-exercise, in children. Problem statements Investigations into exercise recovery contribute a unique view of metabolism and metabolic control in youth. However, little research is available concerning metabolism post-exercise in healthy children. Research on EPOC or “oxygen debt” available in the pediatric literature address exercise of differing intensity levels, acclimatization to altitude, and chronic diseases, making comparison across investigations difficult. In particular, it is unknown if EPOC differs as a function of moderate and vigorous intensity activities in which children are most likely to engage. Currently there is little research available to provide insight into a child’s metabolic profile, including oxygen consumption and substrate utilization, during recovery from moderate and vigorous exercise intensities. In addition, it is unknown if children continue to preferentially oxidize fats post-exercise when compared to adults. It is also unknown if children rely more on fat as a fuel after exercise than before exercise. It is important to first describe a normal response to exercise before researchers can identify anything deviates from normality. Once a normal response has been described, future research can begin to investigate different populations, specifically overweight and obese children.

Research aims and hypotheses The aims of this investigation were to:

1. Describe the metabolic profile (oxygen consumption pre-, during and post-exercise) of children and young adults.
2. Examine differences in EPOC between young adults and children after moderate and vigorous intensity exercise. Hypothesis A: Young adults would experience greater volume of EPOC than children after moderate intensity exercise. Hypothesis B: Young adults would experience greater volume of EPOC than children after vigorous intensity exercise. Hypothesis C: Young adults would experience longer duration of EPOC than children after moderate intensity exercise. Hypothesis D: Young adults would experience longer duration of EPOC than children after vigorous intensity exercise.
3. Examine differences in EPOC between moderate and vigorous intensity exercise in children and young adults. Hypothesis A: Vigorous exercise would elicit a greater volume of EPOC than moderate exercise in children and young adults. Hypothesis B: Vigorous exercise would elicit a longer duration of EPOC than moderate exercise in children and young adults.

4. Examine if differences exist between pre-exercise oxygen consumption and one-hour post-exercise (moderate and vigorous) in children and young adults. Hypothesis A: Children’s oxygen consumption would not be significantly different (<1.3ml/kg•min-^1 )^1 between pre-exercise and one-hour post-exercise for moderate or vigorous exercise. Hypothesis B: Young adults’ oxygen consumption would be significantly elevated (>1.3ml/kg•min-^1 )^53 one-hour post exercise when compared to pre-exercise values.
5. Examine if any differences exist in RER between young adults and children after moderate and vigorous intensity exercise at minutes 1, 5, 10, 15, and 20 post-exercise. Hypothesis A: Young adults would have higher RER than children after moderate exercise. 1 The author is unaware of any articles presenting VO 2 data at one-hour post exercise. Articles show the data graphically and display end results, but do not present the data at one hour. Thus, the author used existing data to calculate what would be considered significant at one hour post-exercise. Tarah et al investigated 679 young adults 53. Tahara Y, Moji K, Honda S, et al. Fat-free mass and excess post-exercise oxygen consumption in the 40 minutes after short-duration exhaustive exercise in young male Japanese athletes. J.Physiol Anthropol. 2 008;27(3):139-143. 53. Tahara Y, Moji K, Honda S, et al. Fat-free mass and excess post-exercise oxygen consumption in the 40 minutes after short-duration exhaustive exercise in young male Japanese athletes. J.Physiol Anthropol. 2008;27(3):139-143. 53. Tahara Y, Moji K, Honda S, et al. Fat-free mass and excess post-exercise oxygen consumption in the 40 minutes after short- duration exhaustive exercise in young male Japanese athletes. J.Physiol Anthropol. 2008;27(3):139-143.. The main outcome variable was EPOC measured for 40minutes after an exhaustive exercise bout (45-105 seconds on a treadmill set at 5 degrees). The article does not mention VO 2 at 40min, however the authors do give data that allows calculation of that value, although with not a much certainty. After 25 minutes post- exercise the participants had consumed 86.1% of total EPOC for 40min leaving 13.9% left to be consumed in the last 15 minutes of the rest period. Total EPOC consumption was 142.7 ml/kg for 40 minutes, 13.9% of which was over the last 15 minutes. 13.9% of 142.7 is 19.8353 ml/kg divided by 15 minutes = 1.322 ml/kg/min.

Hypothesis B: Young adults would have a higher RER than children after vigorous exercise.

6. Examine if any differences exist in RER between the post-exercise period of moderate and vigorous in children and young adults at minutes 1, 5, 10, 15, and 20. Hypothesis: The vigorous exercise would elicit a higher RER post-exercise when compared to moderate intensity exercise for children and young adults.
7. Examine if any differences exist in RER between pre-exercise and post-exercise (moderate and vigorous) in children and young adults. Hypothesis A: Children’s pre-exercise RER would be lower than RER from moderate and vigorous values at minutes 1, 5, 10, 15, and 20 post-exercise. Hypothesis B: Young adults’ baseline RER values would be lower than moderate and vigorous values at minutes 1, 5, 10, 15, and 20 post-exercise_. Hypothesis C: RER values one hour post-exercise would be similar to baseline in both children and young adults._

Chapter 2 – Literature review History and definition of EPOC In humans oxygen consumption is vital to cellular respiration at rest, submaximal exercise (also activities of daily living), and maximal exercise. Following an acute bout of physical activity or exercise, an individual’s metabolic rate remains elevated above resting levels then decays exponentially for a period of time depending on the intensity and duration of the workload. This phenomenon has been observed for a century^17 , and the mechanisms by which it operates have been under considerable debate 8 , 22 . The understanding of this elevated metabolic rate after the cessation of activity is rooted in classic work of A. V. Hill, who demonstrated not just an increase in heat produced after muscle contraction 17 , but that the heat that was produced after the cessation of contraction was equal to or greater than heat freed during the contraction itself 18

. These experiments were followed by the work of Hill and Lupton, who observed the elevated oxygen consumption after an exercise bout and labeled it “oxygen debt”^54. The proposed mechanism of this “oxygen debt” was due to the metabolism of a fraction of the lactic acid produced in the activity, specifically the conversion of lactic acid to glycogen in a 1:4 ratio, which was observed earlier by Meryehof in 1920^22. Margaria et al. 55 noted in 1933 that the delayed oxidation of lactic acid alone was an inadequate explanation for the phenomenon because previous work observed three key discrepancies between recovery O 2 and lactic acid: 1) much of the lactic acid produced, in the isolated muscle, occurs after the contraction is over, 2) at lower intensities the changes in lactic acid concentration are meager compared to the corresponding oxygen debt, and 3) the re- synthesis of glycogen from lactic acid was not the only oxidative process happening

during this recovery period. These discrepancies, in addition to the observation that a fast and slow component of the oxygen debt existed, led Margaria and associates to propose an amendment to the current oxygen debt theory 55

. They suggested two mechanisms for oxygen debt: the initial, rapid decline of oxygen post exercise where lactic acid is not contributing to the excessive oxygen consumption called “alactacid” and the prolonged elevation of oxygen post exercise in which lactic acid is responsible for the elevated metabolic rate which they called “lactacid”^22 ,^55. In 1984 Gaesser and Brooks 22 proposed that the oxygen debt hypothesis, as suggested by Margaria and associates, was “too simplistic” reporting that no causal relationship had been established between lactate metabolism and elevated oxygen consumption post-exercise. Furthermore, they stated that there are a host of other factors potentially “loosen the coupling between oxidation and phosphorylation” contributing to increased metabolism post-exercise. These other factors include, but are not limited to: elevated temperature, fatty acid mobilization, the commandeering of Ca+ by mitochondria 56 , and increased sympathetic drive 22 , 57 . Beyond these issues, lactic acid can fuel the recovery process by directly supplying the Krebs Cycle with substrate instead of using glucose or glycogen for recovery metabolism^57. Therefore, in order to avoid incorrectly implied mechanisms in the name “lactacid” and “alactacid” oxygen debt, Gaesser and Brooks suggested that the elevated level of oxygen consumption during recovery should be referred to as excess post-exercise oxygen consumption or EPOC. For the purposes of this review, EPOC is defined as “the VO 2 above resting requirements after the cessation of exercise”^8.

Review of the Literature Several review papers on EPOC have been published^8 ,^22 ,^24 ,^58 , but none have focused on children or adolescents. The purpose of this review is to compile the studies that have investigated EPOC in children and adolescents and identify where more research is needed. Given the limited information in youth, the following sections are included and reflect a blend of what is known from literature involving adults and children: measurement of EPOC using different protocols, influence of exercise intensity and duration on EPOC, age- and sex-associated variation in EPOC, adult-child differences in EPOC, EPOC in obese individuals, implications of EPOC in the context of TDEE, substrate utilization, and methodological considerations for investigating EPOC. Measurement of EPOC Although there is general agreement in the definition of EPOC, there is no consensus in literature on the most appropriate protocol to assess EPOC. The assessment of EPOC varies widely and includes several protocols, ranging from unloaded pedaling for 10 minutes after exercise 20 to time to 50% VO2peak 47

. Some of the methodologies are described in the following text. Adult literature concerning EPOC following aerobic exercise focuses on recovery from an exercise bout lasting several minutes or more. Children, on the other hand, rarely attain steady state during daily physical activity. In order to study pediatric oxygen kinetics in a more realistic environment, Zanconato et al. 20 used exercises lasting one minute. Oxygen kinetics, onset and recovery, was analyzed for five separate exercise intensities ranging from moderate to supramaximal. Prior to the exercise bout, subjects

cycled at an unloaded resistance for 3 to 5 minutes; the readers are left to assume that at this time “pre-exercise” measurements of oxygen consumption were recorded to compare to post-exercise measurements. After the one-minute of exercise, participants actively recovered for 10 minutes by unloaded cycling (~7 – 12 W) while expired gases and HR measurements were obtained. After a bout of supramaximal effort, unloaded cycling will aid in recovery from exercise; however, it also creates artifact when attempting to quantify EPOC such that the metabolism post-exercise is not simply a measure of the body’s requirement to recover, but also the requirement to move the lower limbs. In this study, EPOC was assessed while participants actively recovered from various workloads ranging from moderate to supramaximal, but it was justified by comparing post-exercise oxygen consumption to measured oxygen consumption of unloaded cycling pre- exercise^20. Children’s physical activity patterns must be considered when assessing EPOC, as children typically do not partake in exercise bouts lasting thirty minutes or more. Thus, shorter exercise bouts may be more appropriate when considering EPOC in children. Exercise recovery, specifically early oxygen consumption post-exercise, has been used to assess the disease severity in individuals with chronic chest diseases (CCD). Measuring early recovery is designed to aid diagnosing CCD, but it also captures the onset of EPOC. Unfortunately, measuring recovery for a short period of time is unable to fully answer the question of volume or duration of EPOC. Stevens et al. 47 studied 2 7 children with CCD and compared them to 27 healthy controls. A graded exercise test to exhaustion on an electronically braked cycle ergometer was used to establish the participants’ peak oxygen consumption. Expired gas measurements were recorded using

a 1 0 - second moving average throughout the pre-exercise, during exercise, and post- exercise time periods. Prior to the exercise bout, oxygen consumption was measured in the upright position. Immediately after the test the children were instructed to sit quietly on the cycle ergometer while oxygen consumption was measured. The recovery period was recorded as the time required for the participants to reach 50% of their VO2peak and was labeled “early recovery VO2.” Oxygen consumption was recorded for 10-minute post-exercise, but these results were not reported. These authors cited previous studies using the early recovery VO2 method in adults with COPD as a measure of functional capacity. Early oxygen uptake recovery, in this case time to reach 50% of exercise VO2, is an accepted method for measuring recovery 47

. The benefit of this assessment technique is that the authors’ measurement time post-exercise is short and the information that is gained is highly valuable for their specific research aim: to assess recovery time in CCD patients. However, by only assessing the initial fast phase of recovery they fail to assess the slow component of recovery and, therefore, the total volume and duration of EPOC is unknown in this population. The authors admitted that more appropriate measures need to be established to best represent recovery 47 . The majority of the investigations of EPOC include a comparison of the post- exercise period with a period of time that represents resting metabolic rate (RMR). Some investigators have used a control day to measure oxygen consumption throughout the time that the exercise trial and EPOC assessment would take place 23 , 59 - 62 . Conducting a control day increases the amount of time a participant must dedicate to the research study and is typically a drawback in terms of feasibility; however, a control day substantially increases the strength of the study. Others have used the convenience of the time period