Exercise Physiology Notes

In this chapter, we will explore the physiological adaptations that occur in response to training, with a particular emphasis on how these changes influence VO2max, athletic performance, and muscular strength. Understanding these mechanisms is crucial for optimizing training programs and achieving desired outcomes.

Principles of Training

• Challenge Principle: If a specific area of the body is challenged, it will respond to that challenge.

• Responses to Training:
    - Acute Responses: These are immediate or short-term changes resulting from exercise.
    - Chronic Responses: Long-term changes or adaptations from consistent training. These adaptations are largely due to the cumulative effect of acute responses over time.

Key Terms in Training

• Overload: The principle of overload refers to exercising a physiological system at a level beyond what it is normally accustomed to. This is achieved by modifying the "dose" of exercise (e.g., intensity, duration).

• Reversibility: This term indicates that gains in fitness are lost when the overload is removed, leading to a plateau in physical fitness levels.

• Specificity: The training effects are specific to multiple factors such as:
    - The type of muscle fibers recruited during exercise.
    - The energy systems utilized (e.g., aerobic vs. anaerobic systems).
    - The contraction velocity (i.e., shortening velocity, Vmax).
    - The type of muscle contraction (e.g., eccentric, concentric, isometric).

Part 1: Endurance (Aerobic) Training and Effects on VO2max

• Endurance Training Effects:
    - Proper exercise prescription (Ex Rx) leads to an increase in VO2 max.
    - Helps maintain internal environment during submaximal steady-state exercise.
    - Expected increase in VO2 max: 15–20% on average; 2–3% for those with high initial VO2 max.
    - High initial VO2 max requires an exercise intensity of >70% of VO2 max for noticeable changes, whereas those with low initial VO2 max may see improvements at 40-50% VO2 max.
    - Genetic predisposition plays a role: 21 identified genes influence changes in VO2 max with training.

Genetics and VO2max Response to Endurance Training

• Genetics significantly influence both VO2 max and training responses:
    - Untrained/Sedentary individuals: Genetics accounts for about 50% of their VO2 max.
    - Average improvement in VO2 max for sedentary individuals is estimated at 15–20%.
    - Heritability explains approximately 47% of training-induced changes in VO2 max.
    - Genetic predisposition is essential in conjunction with training for achieving a high VO2 max, often related to high cardiovascular capacity and an elevated percentage of slow-twitch muscle fibers.

Guidelines for Improving VO2max (FITT Model)

• Frequency: 3–5 times per week

• Intensity: 50–85% of VO2 max

• Time: 20–60 minutes

• Type (Mode): Dynamic activities involving large muscle groups; High-Intensity Interval Training (HIIT) as an appealing option.

Physiological Changes Associated with Aerobic Exercise Training (AET)

Improving VO2 max Through AET

• The Fick Equation describes the relationship governing VO2 max:
  $ext{VO2 max} = Q imes (a-v O2 ext{ difference})$
    - Where Q represents cardiac output.

• Factors contributing to improved VO2 max include:
    - Increased maximal stroke volume (SV max)
    - Increased absolute (a-v) O2 difference

Central Components of VO2 max Improvements

• Cardiac Output & Stroke Volume:
    - Improvements in stroke volume result from several physiological adaptations:
      1. Increased preload (end-diastolic volume)
      2. Decreased afterload (total peripheral resistance)
      3. Increased contractility

• Adaptations can develop quickly (within 6 days of training):
    - 11% increase in plasma volume
    - 10% increase in stroke volume
    - 7% increase in VO2 max

AET-Induced Changes in Maximal Stroke Volume (SV max)

• Mechanisms Leading to Increased SV max:
    1. Increased plasma volume
    2. Increased blood volume over short and long terms
    3. Enhanced venous return
    4. Increased ventricular volume occurring over months
    5. Decreased arterial constriction in trained muscles
    6. Greater muscle blood flow without a change in mean arterial pressure (MAP)

Peripheral Components of VO2 max Improvements

• Improved Muscle Blood Flow: Reduced sympathetic nervous system (SNS) vasoconstriction leads to better blood flow.

• Improved O2 Extraction in Trained Muscles:
    - Increased capillary density is critical, primarily through the secretion of Vascular Endothelial Growth Factor (VEGF) leading to angiogenesis.
    - Untrained individuals: 10–30% increase in capillary density over 6-8 weeks.
    - Trained individuals: Continuous increases correlated with training duration.
    - Slower blood flow through muscle reduces diffusion distance to mitochondria.
    - Increased number of mitochondria shifts muscle fiber types, enhancing O2 extraction from the blood.

Time-Course of VO2max Improvements with AET

• Initial Changes:
    - Central component improvements (50%) happen early, occurring as early as 6 days to around 4 months.
    - Peripheral improvements (also 50%) occur later (≥28 months), which are crucial for sustained performance and homeostasis during submaximal exercise.

Structural and Biochemical Changes in Skeletal Muscle

• Key Changes:

1. Increased capillary density (structural change)

2. Increased number of mitochondria (biochemical change) - termed "mitochondrial biogenesis"

3. Increased oxidative enzymes favor aerobic metabolism (related to the Krebs cycle & electron transport chain)

4. Increased reliance on fats as primary fuel (β-oxidation)

5. Improved NADH shuttling system from cytoplasm to mitochondria

6. Decreased lactate formation resulting from reduced reliance on glucose due to the fat metabolism shift.

Detraining After Endurance Training

• VO2 Max & Detraining:
    - Rapid decline in VO2 max: approximately 8% after 12 days, 20% after 84 days.
    - Stroke volume max decreases due to rapid loss of plasma volume.
    - Decreased maximal (a-v) O2 difference, mitochondrial reduction, and a decrease in oxidative capacity of muscle.
    - Shift in fiber composition from Type IIa fibers to Type IIx fibers.

Effects of Retraining

• Mitochondrial Adaptations:
    - Adaptations can double within 5 weeks of training.
    - Loss of adaptations occurs quickly: gains decrease by 50% within 1 week of detraining.
    - Majority of adaptations are lost within 2 weeks.
    - It takes 3–4 weeks of retraining to regain mitochondrial adaptations.

Dose-Response Relationship: Exercise Volume & Health

• A graphical representation of the dose-response relationship illustrates the maximum effect of physical activity on various health markers, including:
    - Insulin sensitivity
    - Blood pressure
    - Heart rate
    - VO2 max
    - HDL cholesterol levels

Calculation of VO2max

• The formula for VO2 max:
  $ext{VO2 max} = [HR_{max} imes SV_{max}] imes [(a-v)O_2 ext{ diff}_{max}]$
    - Where Q (cardiac output) is a critical determinant of VO2 max and is the product of heart rate (HR) and stroke volume (SV).

Conclusion

• Endurance training increases VO2 max through both central and peripheral mechanisms, emphasizing the importance of training volume and intensity for optimal health benefits and performance improvements.

Figures and Graphical Representations

• Mitochondrial Content Over Time: Shows increases and decreases in mitochondrial content during training and detraining.

• Physical Activity and Mortality Risk: A heat map illustrating the relationship between physical activity levels and risk of all-cause mortality, demonstrating the importance of maintaining activity levels while minimizing sedentary behavior.