Exercise Phys Exam 2 (with lecture notes)

Chapter 5: Energy Expenditure

Total Energy Expenditure (TEE)

1. What are the components of total energy expenditure (TEE), and which is the most variable between individuals?

   • TEE includes basal metabolic rate (BMR), activity energy expenditure (AEE), and energy used for digestion.

   • Activity energy expenditure (AEE) is the most variable because it depends on a person's movement, exercise, and daily activities.

2. How does fat-free mass influence basal metabolic rate (BMR)?

   • Fat-free mass (muscles, organs, bones, and fluids) is the primary determinant of BMR since metabolically active tissues require more energy.

3. What is metabolic scope, and how does it relate to physical activity level (PAL)?

   • Metabolic scope is TEE/BMR and is synonymous with physical activity level (PAL). It reflects how much energy a person uses beyond their resting metabolic rate.

Measuring Energy Expenditure

4. What two principles are the methods of measuring energy expenditure based on?

   • Oxygen consumption and CO₂ production are linked to energy production (used in indirect calorimetry and doubly labeled water).

   • Whole-body heat production is directly linked to energy production (used in direct calorimetry).

5. How does direct calorimetry work, and what are its pros and cons?

   • Measures heat production in a sealed chamber.

   • Pros: Accurate over time, useful for measuring resting metabolism.

   • Cons: Expensive, slow, not practical for exercise, and can be influenced by heat from exercise equipment and sweat.

6. How does indirect calorimetry estimate energy expenditure, and what are its limitations?

   • Measures oxygen consumption (V˙O2V̇O_2) and carbon dioxide production (V˙CO2V̇CO_2) to estimate ATP production.

   • Limitations: Only valid during steady-state exercise; inaccurate for non-oxidative energy production (e.g., anaerobic metabolism).

7. What is doubly labeled water, and how does it measure total energy expenditure (TEE)?

   • Ingested water contains isotopes (2H^2H and 18O^18O), which are tracked in urine.

   • The difference in elimination rates estimates CO₂ production, which correlates with TEE.

   • Pros: Accurate for long-term measurement.

   • Cons: Very expensive (~$700/test).

8. How does the respiratory exchange ratio (RER) indicate fuel utilization?

   • RER = V˙CO2/V˙O2V̇CO_2 / V̇O_2.

   • RER = 1.0 → primarily carbohydrate metabolism.

   • RER = 0.70 → primarily fat metabolism.

9. Why does RER increase with exercise intensity, and what physiological changes explain this?

   • As intensity increases, the body relies more on carbohydrates since they provide ATP faster than fat oxidation, leading to a higher RER.

Energy Expenditure During Exercise

10. How does metabolic rate change with exercise intensity?

    • Metabolic rate increases proportionally with intensity.

11. How can we estimate energy expenditure using V˙O2V̇O_2 and V˙CO2V̇CO_2, and why do we need both?

    • The Modified Weir Equation estimates kcal/min using both V˙O2V̇O_2 and V˙CO2V̇CO_2 because different fuel sources yield different ATP outputs.

12. What is the slow component of V˙O2V̇O_2 uptake, and why does it occur at high exercise intensities?

    • It refers to the gradual increase in oxygen uptake over time due to increased reliance on Type II fibers, which are less efficient.

13. What factors contribute to a higher V˙O2V̇O_2 max, and why is it a strong predictor of endurance performance?

    • High oxygen uptake, cardiac output, mitochondrial density, and oxygen delivery. It predicts performance for 8-30 min events.

14. How does excess post-exercise oxygen consumption (EPOC) occur, and what does it reflect?

    • Oxygen remains elevated after exercise to restore ATP, remove CO₂, repair muscles, and replenish glycogen.

15. How does economy of effort impact endurance performance?

    • More efficient athletes use less energy at the same pace, improving endurance.

New Ideas in Exercise Metabolism

16. What is metabolic compensation to physical activity, and what are its implications for energy balance?

    • Increased activity energy expenditure leads to reduced energy spent elsewhere, limiting weight loss.

17. What is "maximal sustained metabolic scope," and what limits its upper boundary?

    • It refers to the highest sustainable TEE/BMR ratio (~2.5x BMR), limited by nutrient availability and energy balance.

Chapter 6: Fatigue

Definitions and Causes of Fatigue

18. What are the two definitions of fatigue?

    • Muscle fatigue model: Inability to sustain power output.

    • Perceived exertion model: Sensation of tiredness.

19. What are the five major causes of fatigue, and how might they interact?

    • Inadequate energy delivery, metabolic byproducts, contractile failure, neural control issues, and central fatigue.

20. What is the difference between peripheral and central fatigue?

    • Peripheral fatigue: Occurs in muscles due to energy depletion or byproducts.

    • Central fatigue: Occurs in the brain and nervous system.

Peripheral Causes of Fatigue

21. How does glycogen depletion contribute to fatigue?

    • Depleted glycogen forces greater reliance on fats, which provide ATP more slowly.

22. Why does glycogen depletion affect different muscle fibers at different rates?

    • The fibers recruited first (Type I, then IIa, then IIx) fatigue earlier.

23. How does the accumulation of inorganic phosphate (Pi) impair muscle contraction?

    • It reduces calcium release and inhibits ATP breakdown.

24. How does an increase in hydrogen ion (H+) concentration affect muscle function?

    • It lowers pH, inhibiting glycolytic enzymes and reducing ATP production.

25. How does bicarbonate act as a buffer to maintain pH balance?

    • Bicarbonate binds H+ ions to prevent excessive acidity.

26. How might sodium bicarbonate supplementation enhance performance, and what are its side effects?

    • It helps buffer H+, delaying fatigue. Side effects: GI distress.

Central Causes of Fatigue

27. What is the Central Governor Model, and how does it challenge traditional views on fatigue?

    • The brain limits effort to prevent damage, rather than fatigue being caused purely by energy depletion.

28. What evidence does the Noakes article present against the catastrophic model of fatigue?

    • Athletes still have residual strength even when they feel exhausted.

29. How does the brain regulate fatigue according to the Psychobiological Model?

    • Motivation and perception influence endurance.

30. What were the key findings of the Marcora study on central fatigue?

    • Mental fatigue reduces endurance performance even when muscles are fresh.

31. Why does the brain impose limits on physical effort?

    • To protect against injury and metabolic failure.

Integrating Peripheral and Central Fatigue

32. How should we think about the relationship between central and peripheral fatigue?

    • Both interact—the brain processes muscle signals and adjusts exertion accordingly.

33. Why is fatigue considered a combination of physiological and psychological factors?

    • Physical stressors trigger fatigue, but brain perception determines its severity.

Fatigue Causes and Hypotheses ⚠

We've discussed three potential causes of fatigue, some of which are hypotheses and some of which have been proven. These include energy delivery issues, accumulation of metabolic byproducts, and failure to form actin-myosin cross-bridges, which is rare and usually requires significant muscle damage. Peripheral and central fatigue are additional hypotheses that are harder to prove.

Peripheral vs. Central Fatigue 💪🧠

Peripheral Fatigue: This type of fatigue originates within the muscle itself and has a biological, not psychological, cause.

Central Fatigue: This involves the brain becoming less efficient at recruiting muscles.

Hydrogen Ions and pH 🧪

During intense exercise, hydrogen ions build up, decreasing pH and potentially limiting muscle performance. In extreme cases, this can even make muscles temporarily unable to contract.

Glycolysis and Oxygen Availability 🏃‍♀

Glycolysis: The process of breaking glucose in half to produce ATP and pyruvate/lactate.

If enough oxygen is available, muscles can continue to function efficiently.

Lactate Formation 🏋‍♀️

When lifting heavy weights, lactic acid cannot be oxidized and instead breaks down into lactate. Lactic acid molecules dissolved in water release hydrogen ions.

If there isn't enough oxygen getting to the muscles, hydrogen ions and lactate build up.

pH Levels and Muscle Function 📊

Resting Muscle pH: Approximately 7.05-7.1

pH After Intense Exercise (30 seconds): Can drop to under 6.6 or even 6.5.

The pH scale is logarithmic, so this is a substantial change.

It takes a while for pH to return to normal after hydrogen ion buildup, which explains the recovery time needed after intense exercise.

Impact of Hydrogen Ions on Muscle Contraction 🚫

High levels of hydrogen ions can inhibit muscle contraction by interfering with glycolytic enzymes.

Glycolysis Enzymes: There are 11 enzymes involved in glycolysis.

If the muscle pH gets too low, glycolysis and the oxidative system may be impaired.

Bicarbonate as a Buffer 🛡

Bicarbonate, which is essentially baking soda without the sodium, is a molecule present in blood, interstitial, and intracellular fluids.

It buffers or grabs onto hydrogen ions, preventing them from causing as much damage or lowering pH significantly.

This prevents muscle pH from dropping below 6.4, which would be low enough to damage the muscle.

Sodium Bicarbonate as a Performance-Enhancing Drug? 💊

Consuming sodium bicarbonate (baking soda) has been explored as a way to enhance athletic performance by buffering hydrogen ions.

🧪 Baking Soda as a Performance-Enhancing Drug

How it Works

When performing glycolysis, your muscles break down into lactate.

Under aerobic conditions, lactate can be taken into the mitochondria and metabolized.

During intense activity, lactate builds up and hydrogen ions accumulate, leading to a drop in pH.

Hydrogen ions build up in the muscle, interstitial fluid, and bloodstream, causing pH to drop, which is problematic.

🩸 The Role of Bicarbonate

Bicarbonate (

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O) and carbon dioxide (

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).

Bicarbonate's buffering action helps in managing hydrogen ions, which are produced during intense exercise, thus preventing a drastic drop in pH levels.

🥄 Sodium Bicarbonate (Baking Soda)

Sodium bicarbonate (

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), commonly known as baking soda, can be ingested to increase bicarbonate levels in the blood.

Endurance athletes, especially those doing shorter, more intense activities, have experimented with baking soda as a performance-enhancing supplement.

It is legal and not banned by most doping agencies.

🚴 Cycling Channel Explanation

As exercise intensity increases, muscles produce more lactate, which enters the bloodstream.

The body can reuse lactate as fuel, but this process leaves hydrogen as a byproduct.

Hydrogen increases acidity in the blood, impairing muscle activity.

Sodium bicarbonate absorbs hydrogen, reducing acidity in the bloodstream and potentially reducing muscle burn, allowing athletes to perform harder for longer.

🤢 Ingestion and Side Effects

Traditional ingestion involves roughly 300mg of baking soda per kilogram of body weight, one to two hours before exercise.

Common side effects include thirst, bloating, nausea, diarrhea, and vomiting.

📊 Study Results

A 2021 study by Sebastian Dahl and colleagues found that bicarbonate lowered blood acidity and improved power output in a 90-second sprint by 3% in trained male cyclists.

There was no statistically significant difference in the Rate of Perceived Exertion (RPE), even though power and heart rate increased.

🔎 Systematic Review Findings

A 2019 systematic review by Hodzik and colleagues, analyzing 35 studies, showed that only half of them reported performance improvement with bicarbonate use.

Some individuals were non-responders or adverse responders, experiencing reduced performance.

🧪 Factors to Consider

Sodium bicarbonate is more likely to improve performance when paired with high-intensity exercise.

It may not provide significant benefits for longer, less intense events.

💊 Sodium Bicarbonate: Benefits and Usage

Current research suggests that the benefits of sodium bicarbonate may not extend to the end of long endurance events, especially those with high-intensity efforts early on.

Performance Improvement

When sodium bicarbonate has improved performance, it's been in the range of 2-3%, indicating a marginal gain rather than a significant advantage.

Usage Recommendations

If you're considering using sodium bicarbonate, here's what the research suggests:

Dosage: Take 200-300 mg per kg of body weight.

Example: If you weigh 68 kg (150 lbs) and use 650mg sodium bicarbonate pills, you would need to take approximately 31 pills.

Timing: Consume the dosage 1.5 hours before exercise.

Experimentation: It's advisable to test different ingestion methods on low-intensity training days to avoid gastrointestinal distress during important training sessions.

Cost Considerations

Option Cost per Serving Potential Gut Distress Monthly Cost (2 intense training days + 1 race)

Bicarbonate Pills $5 Likely $60

Mar10 (Expensive Option) $19 Possibly Less $230

🧠 Central Fatigue: The Central Governor Model

Transitioning to the topic of central fatigue, we'll explore recent research related to the central governor model.

The central governor model posits that the brain regulates physical exertion to prevent potential harm to the body.

🏃‍♀ Understanding Fatigue

Defining Fatigue

Decrements in muscular performance with continued effort, accompanied by sensations of tiredness.

Inability to maintain a required power output to continue muscular work at a given intensity.

It's important to distinguish fatigue from muscle damage. Fatigue is reversible by rest (a day or two), whereas muscle damage can take days or weeks to recover from.

Factors Influencing Fatigue During Exercise

Several factors can influence fatigue during exercise:

Type and intensity of exercise: The harder you go, the sooner you'll fatigue.

Muscle fiber type: Type II muscle fibers fatigue more easily.

Training state: Recovery from previous workouts affects fatigue.

Diet: Adequate fuel and hydration are crucial. Carbohydrates are especially important.

⚙ Causes of Fatigue Identified by Exercise Physiologists

Exercise physiologists have identified five primary causes of fatigue:

Inadequate energy delivery/metabolism

Accumulation of metabolic byproducts

Failure of muscle contractile mechanism

Altered neural control of muscle contraction

Psychological factors

We will delve into the first two in detail and briefly touch on the others.

⚡ Inadequate Energy Delivery/Metabolism

If the body can't produce ATP fast enough to meet the demands of exercise, performance will decline, and fatigue will set in.

🔥 Accumulation of Metabolic Byproducts

We will revisit lactate and the associated hydrogen ions, which have negative consequences on performance.## 💪 Causes of Fatigue

There are several potential causes of fatigue during exercise:

Energy Metabolism

Lactic Acid Accumulation

Electrolyte Depletion

Central Nervous System

Psychobiological Factors/Central Governor Theory

The first three causes pertain to what is happening in the muscles, which is known as peripheral fatigue. The last two are considered central fatigue and relate to changes occurring in the brain.

⛽ Energy Metabolism and Fatigue

Glycogen Depletion

As glycogen stores decrease, fatigue increases. A study was conducted where a subject exercised at 70% VO2 max for three hours on a treadmill, and muscle biopsies were taken every 30 minutes to measure muscle glycogen levels. The subject was also asked to rate their perceived exertion (RPE).

RPE: Rating of perceived exertion, a subjective measure of how hard an exercise feels.

The study showed that perceived exertion increased as glycogen stores were depleted, particularly in the last 30 minutes of the run.

It appears that the brain is sensing glycogen levels, but not the rate at which glycogen is being used. It's like a car's gas tank: the "low fuel" light comes on when the tank is near empty, regardless of how quickly fuel is being burned.

The Shift to Fat Metabolism

When glycogen is low, the body relies more on fat as an energy source.

Fat is not as efficient as glycogen at moderate to high-intensity exercise because it requires more oxygen to burn.

When glycogen runs low, there's a shift to burning more fat, which is less efficient and requires more oxygen, making it harder to meet ATP demand. This is perceived as muscular fatigue.

Hitting the Wall

Marathon runners sometimes "hit the wall" around mile 22, which is related to glycogen depletion. At this point, the body relies more on fat for fuel, which is less efficient and requires more oxygen. The brain tries to get you to slow down.

Unequal Glycogen Depletion

Muscle fibers that are used first during exercise will be depleted of glycogen first.

Muscle Fiber Types and Glycogen Usage 🏃‍♀

Fiber Identification and Glycogen Content

Muscle fibers can be identified as fast-twitch (light) or slow-twitch (dark) after staining. Slow-twitch fibers appear darker due to a higher myoglobin content.

After an 18.5-mile run, staining reveals glycogen content:

Slow-twitch fibers (type 1) appear clear, indicating glycogen depletion.

Type 2A fibers still contain some glycogen.

This suggests that the runner did not exert enough effort to require the use of type 2A fibers during the run.

Continued Exercise and Fiber Recruitment 💪

If the runner continues:

Type 1 fibers, depleted of glycogen, can utilize fat and liver glycogen.

The rate of ATP production decreases, reducing force output.

Type 2A fibers are recruited despite the runner not increasing intensity because they provide an alternative for muscle contraction even if they aren't ideal for endurance.

This demonstrates that type 2A fibers can be recruited for endurance when type 1 fibers are depleted, even though they are typically used for strength and high-intensity activities.

Fatigue can result from glycogen depletion in specific fiber types.

Varying Exercise Types and Glycogen Usage 🏋‍♀️

Different types of exercise impact glycogen usage differently. For exercises relying heavily on type 2A fibers:

Type 2A fibers would be depleted of glycogen first, while slow-twitch fibers would still have glycogen.

Examples include intervals, high-intensity exercises for a few minutes, and sports involving big movements with rest periods.

The type of exercise dictates where glycogen is used and in which fiber types.

Muscle Groups and Fatigue 🦵

Fatigue can also occur due to glycogen depletion in different muscle groups.

Running primarily uses the lower body, with minimal upper body involvement.

Glycogen consumption is highest in the gastrocnemius (calf muscles), especially during uphill running.

Glycogen depletion in specific muscle groups leads to uneven fatigue.

Personal Anecdote: The Boston Marathon 😥

An experience from the Boston Marathon in 2007 illustrates muscle-specific fatigue:

Quad muscles fatigued due to running into a headwind, leading to walking in the last two miles.

Other muscle groups (calves, hamstrings) were not as fatigued.

Limiting Factor: Performance is limited by the first muscle group to fatigue or break down, not necessarily overall muscle condition.

Exercise training should focus on identifying and strengthening the most used muscles in a sport to prevent them from becoming the limiting factor.

Fat Depletion vs. Glycogen Depletion 🍎

Fat depletion is not a major cause of fatigue because the body stores a significant amount of fat. Glycogen depletion occurs much earlier and leads to fatigue before fat becomes a limiting factor.

Byproducts and Fatigue 🧪

Accumulation of Byproducts

Instead of depletion, fatigue can also be related to the accumulation of byproducts created from the energy pathways.

Heavy or Extreme Exercise

Heavy or extreme exercises like sprinting, throwing, or heavy lifting relies mostly on the ATP-PCR system.

ATP-PCR: Used for big powerful movements, recruiting all muscle fibers.

ATP-PCR System and Inorganic Phosphate ⚡

When engaging in activities that demand quick bursts of energy, such as heavy weightlifting or sprinting, the body relies heavily on the ATP-PCR system. This system involves breaking down creatine phosphate to regenerate ATP rapidly.

Here's how it works:

Initial ATP Breakdown: The readily available ATP in muscles is utilized first, providing energy for about 10 seconds.

Creatine Phosphate Conversion: Creatine phosphate is broken down into creatine and a phosphate group.

ATP Regeneration: The phosphate group is transferred to ADP, regenerating ATP.

During intense exercise, ATP is broken down at a high rate due to numerous active myosin cross-bridges forming and breaking rapidly. The breakdown of ATP results in ADP and inorganic phosphate (Pi).

The Role of Inorganic Phosphate 🚫

Inorganic phosphate is not just a byproduct; it directly impairs muscle contraction in several ways:

Inhibition of Cross-Bridges: High concentrations of inorganic phosphate inhibit active myosin cross-bridges.

Reduction of Calcium Release: It reduces calcium release from the sarcoplasmic reticulum, which is essential for forming actin-myosin cross-bridges.

Negative Feedback Loop: High levels of inorganic phosphate and ADP create a negative feedback loop, slowing ATP breakdown.

The buildup of inorganic phosphate during intense activities limits the muscle's ability to contract, contributing to fatigue.

Lactic Acid and Hydrogen Ions 🧪

Lactate as a Fuel Source 💪

Lactate, often known as lactic acid, is primarily beneficial as a fuel source. Type 1 muscle fibers readily use lactate, oxidizing it in the electron transport chain similar to pyruvate.

The Downside of Lactic Acid 📉

During intense exercise, type 2A fibers produce a significant amount of lactic acid. When oxygen supply is insufficient, the lactate cannot be oxidized quickly enough. This leads to the dissociation of lactic acid into lactate and hydrogen ions (H+).

The Impact of Hydrogen Ions on pH 🌡

Hydrogen ions directly influence the pH levels in muscles and blood.

pH is a measure of hydrogen ion concentration; a high concentration indicates acidity and low pH.

The accumulation of hydrogen ions decreases the pH, making the environment more acidic.

Real-World Example: pH Drop During Sprinting 🏃‍♂

In a study where individuals sprinted as hard as possible for about 30 seconds, muscle pH dropped from a neutral 7.1 to about 6.5. It took approximately 40 minutes for the pH to recover to normal levels. This explains why repeated high-intensity efforts require significant rest periods.

Bicarbonate as a Buffer 🛡

The body has mechanisms to counteract the effects of increased acidity. Bicarbonate, present in blood, interstitial fluids, and muscle cells, acts as a buffer.

Bicarbonate, similar to baking soda without sodium, is an alkaline substance that neutralizes hydrogen ions, preventing drastic pH drops.

Bicarbonate helps maintain muscle pH above 6.4 during intense exercise, preventing it from dropping to dangerously low levels.

Glycolysis and Enzyme Sensitivity ⚙

Glycolysis, the process of breaking down glucose into pyruvate or lactate, involves 11 enzymes. These enzymes are pH-sensitive, with many not functioning optimally at low pH levels.

The graph of enzyme function at different pH's shows that most enzymes in glycolysis perform best in a mildly basic to neutral environment. However, performance diminishes greatly when pH is low (acidic).

Specifically, the first enzyme in the glycolytic pathway is very sensitive to low pH, with function stopping completely at pH levels of 6.3 or lower. Because of this, any amount of hydrogen ion buildup is expected to affect the enzyme's function.

Impact on Muscle Function and Fatigue 😩

As lactate production increases during intense exercise, hydrogen ions accumulate, leading to a decrease in muscle pH. Although bicarbonate buffers these ions, any downward shift in pH affects the enzymes involved in glycolysis. When these enzymes do not work, you will feel intense fatigue.

Enzymes and Substrates 🧪

Enzymes are proteins that act as catalysts to speed up chemical reactions within cells.

Enzymes work by binding with substrates (smaller molecules), weakening their bonds, and facilitating a reaction.

Each enzyme has a specific shape that matches only certain molecules, like a lock and key.

pH and high temperature can alter the shape of enzymes, a process called denaturation, rendering them useless.

Denaturation: A process in which proteins or nucleic acids lose their structure due to some external stressor or compound, such as a strong acid or base, concentrated inorganic salt, organic solvent, or heat.

The body needs a specific range of pH and temperature to maintain enzyme function.

Lactate, Hydrogen Ions, and Muscle Fatigue 😥

Lactate can be used as fuel in the presence of oxygen.

During intense exercise, when oxygen is limited, hydrogen ions from lactate buildup reduce muscle pH.

Reduced pH impairs the ability to form cross-bridges and perform glycolysis, hindering muscle function.

Intense exercise can cause muscle acidity, reducing the ability to perform muscle exercises.

Reactive Oxygen Species (Oxidative Stress) ☢

Reactive oxygen species are metabolic byproducts that can accumulate in muscles and limit their ability to contract, causing fatigue. This is also known as oxidative stress.

Electron Transport Chain and Mitochondrial Function 💪

The electron transport chain is located in the inner membrane of the mitochondria.

The electron transport chain uses a series of proteins to facilitate the production of ATP via the oxidative system

The process involves pumping hydrogen ions to create a concentration gradient.

Hydrogen ions flow through a protein (like a turbine) that assembles ATP molecules.

Uncoupled Proteins and Heat Generation 🔥

In some tissues, including muscles, uncoupled proteins (UCP) allow hydrogen ions to flow back through the mitochondrial membrane without producing ATP.

This process wastes energy, converting it into heat.

Every cell has these proteins, contributing to body temperature regulation.

Reptiles like Banjo cannot generate their own body heat through this process and rely on external sources.

ATP Production ⚡

ATP is created when hydrogen ions flow through a protein that assembles ATP molecules. This can be shown with the following equation:

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→ATP Where ADP is adenosine diphosphate,

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is inorganic phosphate, and ATP is adenosine triphosphate.

VO2 Max and Its Influences 🫁

Genetic and Trainable Aspects of VO2 Max 💪

VO2 max is partly trainable and partly genetic. While genetic predispositions play a role, consistent training can significantly improve an individual's VO2 max. For instance, even without the genetic gifts of elite athletes, dedicated interval training can potentially boost VO2 max by 10 to 20 points.

Sex Differences in VO2 Max 🚻

On average, VO2 max tends to be higher in men than in women, with untrained young men (18-30 years) typically having a VO2 max in the upper 40s, while women average around 40. However, these are just averages, and there is substantial overlap between the sexes. The differences are often attributed to:

Higher fat-free mass (muscle): Men tend to have more muscle mass per unit of body weight, which contributes to higher oxygen utilization.

Higher hemoglobin levels: Men generally produce more red blood cells due to the influence of testosterone, enhancing oxygen-carrying capacity.

Treadmill Tests and VO2 Max Measurement 🏃‍♀

VO2 max can be measured using various exercises such as running or walking on a treadmill, cycling on an exercise bike, or even cross-country skiing on a specialized machine. The best approach is to measure VO2 max during the activity the athlete is accustomed to. For instance, a runner should ideally be tested on a treadmill, while a cyclist may be tested on a bike.

VO2 Max in Trained vs. Untrained Individuals 🏋‍♀️

A graded exercise test is used to measure VO2 max, where the effort (speed, power) is increased every few minutes until exhaustion. In a VO2 max test, we look for the stage where the individual is able to continue to the next level, but their VO2 does not increase, showing that all additional ATP is being created with the glycolytic system.

A trained person will achieve a higher VO2 max compared to an untrained person. An example from the lecture transcript, explains the concept:

At 8 miles per hour on a treadmill, the test subject's VO2 max reached 32 milliliters per kilogram body weight per minute. The subject continued to the next stage, but their oxygen consumption did not increase.

This indicates that their type 1 muscle fibers had reached capacity, and additional ATP was being generated from the glycolytic system with their type 2 fibers.

The Importance of Aerobic Exercise 🚴‍♀

VO2 max is generally higher in individuals who engage in regular exercise, particularly endurance activities. However, even sports with intermittent aerobic components, such as hockey, can improve VO2 max due to the oxidative system being used during rest periods to rebuild ATP supplies.

Economy in Sports 💰

Definition of Economy 💡

Economy refers to using less energy for a given pace or amount of work. It can be measured in terms of speed, power output, or any relevant metric for the activity.

This concept is analogous to fuel economy, where the goal is to maximize the distance or work achieved per unit of energy (e.g., miles per gallon).

Relevance of Economy in Sports 🏆

As athletes become more skilled, they tend to use less energy for a given power output due to improved motor control and technique. This applies to various sports, including golf, running, cycling, and swimming:

Golf: Better swing economy allows for more efficient transfer of muscular power to the ball.

Running/Cycling: Limiting unnecessary motions in the stride or cycling style reduces energy wastage.

Swimming: Economy is crucial due to the high drag in water, where efficient technique significantly reduces energy expenditure.

Economy matters in competition because athletes who use less energy for a task can sustain higher performance levels for longer durations.

Illustrative Example: Oxygen Requirements in Runners 🏃‍♂

Consider two distance runners, A (red) and B (blue), undergoing a treadmill test with increasing speeds. The test measures their oxygen consumption relative to body mass at different speeds.

Speed Runner A Oxygen Cost (ml/kg/min) Runner B Oxygen Cost (ml/kg/min)

Incremental Increases Various measurements taken, plotted in red Various measurements taken, plotted in blue

Both runners were able to complete the test, but it's uncertain who would win a race based solely on this data. The test doesn't measure VO2 max, but rather the oxygen cost at submaximal speeds. Runner B may win because they can use energy more economically.

🏃 Biological Attributes of Endurance Athletes

When comparing endurance athletes with similar VO2max and mental fortitude, running economy becomes a crucial factor.

Runner B, who uses less oxygen per kilogram of body mass, is more economical.

If all other factors are equal, the more economical athlete is likely to perform better in endurance sports.

Differences in economy often stem from form, which improves with practice, leading to greater efficiency.

The biological traits that make a good endurance athlete include:

High VO2max: Indicates the efficiency of the body's oxygen utilization for ATP production.

High Lactate Threshold: Can go closer to their VO2max limit before hitting lactate threshold.

High Economy of Effort: Efficient energy usage at a given power output.

High Percentage of Type 1 Muscle Fibers: Facilitates oxidative metabolism and lactate processing.

Lactate Threshold as a Percentage of VO2 Max

Having a high lactate threshold as a percentage of VO2 max is beneficial to endurance athletes. Two runners can have similar VO2 max levels, but the runner who is able to run at a higher percentage of their VO2 max before accumulating blood lactate will likely perform better.

When an athlete can perform at a higher percentage of their VO2 max without accumulating lactate, it indicates that their muscles are highly efficient at utilizing lactate as fuel. The muscles are able to process lactate effectively due to well-trained mitochondria, delaying discomfort and energy disruption associated with high lactate levels.

📊 EPOC: Elevated Post-Exercise Oxygen Consumption

Elevated Post-Exercise Oxygen Consumption (EPOC) refers to an elevated metabolic rate after exercise.

To understand EPOC, consider a graph with oxygen consumption on the y-axis and time on the x-axis.

At rest, oxygen consumption is low (around 5-6 mL/kg/min).

During exercise, oxygen consumption increases dramatically.

Oxygen Debt

In the initial phase of exercise (the first minute or so), energy primarily comes from the ATP-creatine phosphate system, creating an oxygen deficit or oxygen debt.

Oxygen Debt: The ATP and creatine phosphate that are used up when starting to exercise.

This debt must be repaid, either during exercise (if intensity allows) or after exercise, by using extra oxygen, carbs, and fats to replenish ATP and creatine phosphate. The oxidative system rebuilds ATP and PCR stores.

Excess Post-Exercise Oxygen Consumption (EPOC) 🫁

After exercise, even when resting, oxygen consumption remains elevated for a period ranging from minutes to potentially 24 hours.

Repaying Oxygen Debt

During exercise, the body may accumulate an oxygen debt, especially if the exercise intensity is high. This debt is partly repaid during exercise if the oxidative system is engaged and is fully addressed post-exercise.

Factors Contributing to Elevated Oxygen Consumption After Exercise

Replenishing ATP and PCr stores: Immediately after exercise, the body works to regenerate ATP and phosphocreatine (PCr) pools.

Clearing Carbon Dioxide: Elevated cardiac output is maintained to remove carbon dioxide from tissues.

Elevated Body Temperature: A warmer body temperature increases the overall metabolic rate, enhancing cell repair and other metabolic processes.

Muscle Repair and Glycogen Replenishment: The body engages in enhanced muscle repair, rebuilding glycogen stores, resynthesizing used enzymes, and repairing damaged muscle tissue, all requiring energy.

Impact on BMR Measurement

In very active individuals, measuring Basal Metabolic Rate (BMR) can be challenging because it might always be elevated due to recent exercise. A true BMR measurement might only be obtained after several days of rest.

Energy Expenditure and Exercise 🏋‍♀️

The Old vs. New Understanding

The old idea of how exercise affects Total Energy Expenditure (TEE) assumed a linear relationship: as physical activity increases, TEE increases proportionally.

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TotalEnergyExpenditure=BasalMetabolism+PhysicalActivity

Example (Old Idea):

Basal metabolism and daily activities: 2,000 calories/day

Exercise (Calories/Day) Total Energy Expenditure (Calories/Day)

0 2,000

500 2,500

1,000 3,000

However, recent evidence suggests this isn't entirely accurate. As exercise increases, the body compensates in two primary ways:

Economy of Movement: The body becomes more efficient, burning fewer calories than initially expected during exercise.

Energy Conservation Elsewhere: The body reduces energy expenditure in other areas, such as basal metabolism or daily activities.

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TotalEnergyExpenditure=BasalMetabolism+PhysicalActivity

Compensatory Mechanisms

The body seems to reduce energy expenditure in other areas to compensate for the energy burned during exercise. This might involve lowering the BMR.

Basal Metabolic Rate (BMR): The rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O2/min or joule per hour per kg body mass J/(h·kg). Proper measurement requires a strict set of criteria to be met. These criteria include being in a thermally neutral environment, in a post-absorptive state (meaning that the digestive system is inactive, which requires about 12 hours of fasting in humans), and being in a state of quiet wakefulness.

Example (New Understanding):

If someone builds up to burning 1,000 calories a day through exercise, the actual increase in total energy expenditure may be less than 1,000 calories because the body is compensating.

Open Questions

The exact mechanisms and areas where the body saves energy are still open questions. One potential area is BMR reduction.

🏃 Constrained Energy Expenditure

Subconscious Reduction of Muscular Movement

After intense physical activity like a big game or a strenuous workout, individuals tend to subconsciously reduce their muscular movement to conserve energy. This reduction can accumulate to a couple of hundred calories a day. People often move less during the rest of the day when they're on a diet or do lots of exercise.

Constrained Energy Expenditure Concept

The idea of constrained energy expenditure suggests that the more active you get, the more your body compensates by saving calories elsewhere.

Hunter-Gatherer Study

Research on hunter-gatherers in Tanzania revealed that despite walking long distances and engaging in muscular work, they don't burn significantly more calories than the average American.

🔬 Energy Expenditure in Runners: A Case Study

The Trans-America Footrace

Researchers measured the energy expenditure of six runners participating in a supported race across the United States, where they ran a marathon each day.

Baseline Energy Expenditure

Before the race, the runners' energy expenditure was broken down as follows:

Resting: Energy spent on resting activities.

Other Physical Activity: Energy spent on daily activities like walking around.

Thermic Effect of Food:

The energy spent on digestion.

BMR (Basal Metabolic Rate): Around 1,700 calories per day.

The amount of energy your body needs to function at rest. Measured at around 1,700 calories per day for the runners.

Total daily energy expenditure averaged around 3,000 calories.

Week One: Initial Energy Expenditure During the Race

After the first week of running a marathon a day, the observed energy expenditure was approximately 6,500 calories per day, which closely matched the predicted value. About 3,500 calories were spent on running alone. BMR and the thermic effect of food remained relatively stable.

Final Week: Observed Changes in Energy Expenditure

In the final week of the race, the runners' total energy expenditure decreased compared to the first week, despite still running 26 miles a day. They were spending slightly less calories on their running than they did at first because their running economy improved.

Key Findings

Total Energy Expenditure: Decreased over time.

Running Economy: Improved as the race progressed.

BMR: Did not change significantly.

Other Movement: Reduced, indicating that the runners were subconsciously resting more.

Summary of Energy Expenditure Changes

Category Before Race (Calories) Week 1 (Calories) Final Week (Calories)

Running Minimal 3,500 Slightly Less

BMR 1,700 No Change No Change

Other Physical Activity Significant N/A Almost None

Thermic Effect of Food N/A No Change No Change

Total Energy Expenditure 3,000 6,500 Decreased

Implications

The study provides evidence that when engaging in extensive exercise, the body compensates by reducing energy expenditure in other areas, leading to a lower-than-expected calorie burn.