Muscle Bio Week 2: Exercise intensity, duration and fuel use

Adaptations to Endurance Training

• Enhanced fat utilization as fuel conserves limited carbohydrate stores.
• Using fat improves overall endurance performance.

Measuring Fuel Utilization

• Cellular respiration involves oxidizing carbohydrates and lipids using atmospheric oxygen.
• By-products of this process include carbon dioxide (exhaled) and ATP.
• Respiratory exchange ratio (RER) = CO2 produced : O2 consumed.
• Normal RER values on a Western diet range from 0.8 to 0.85.
• An RER of 1 indicates carbohydrate metabolism, while an RER of 0.7 signifies fat metabolism.

Effects of Exercise Intensity and Duration on Fuel Use

• Energy production must match energy demands during exercise.
• Fuel source selection directly impacts exercise intensity capability.
• Fat provides more energy per gram than carbohydrates but is utilized slower.
• Fats dominate energy provision during low-intensity activities; carbohydrates are preferred for quick energy needs.

Energy Systems of the Body

• Phosphagen System: Improves maximum power output for short bursts (e.g., 100m sprint).
• Anaerobic Glycolysis: Fuels activities lasting 1-2 minutes (e.g., set of squats, 400m sprint); relies on glucose and glycogen; lactate accumulation limits performance.
• Oxidative Phosphorylation (Carbohydrates): Sustains less intense activities over longer durations (e.g., 10km race) with low power output.
• Oxidative Phosphorylation (Lipids): Supports extensive, prolonged activities, but not suited for maximum power output.

Additional Notes

• Red blood cells (RBCs) lack mitochondria, relying entirely on anaerobic metabolism.
• High-intensity exercise depletes glycogen stores within roughly 90 minutes, resulting in fatigue known as "hitting a wall."
• Transitioning to fat metabolism is slower and less effective during intense workouts.

Red Blood Cells and Metabolism

• RBCs lack mitochondria, relying solely on anaerobic metabolism for energy.

Exercise Duration and Power Output

• Maximum power output cannot be sustained beyond a few seconds; fat cannot be used for peak power production.
• Glycogen reserves fuel about 90 minutes of high-intensity exercise before depletion, leading to "hitting the wall."
• Transitioning to fat as a fuel source during high-intensity efforts is slow and inefficient, requiring oxygen.

Energy Production and Fuel Selection

• The rate of energy production matches energy requirements during exercise.
• Fuel source selection is based on its ability to meet energy demands; switching fuels affects performance.
• Exercise intensity and duration dictate the rate and amount of energy required.
• Carbohydrates are the preferred fuel for rapid energy needs despite lower energy yield per gram compared to fats.

Low vs. High-Intensity Exercise

• Fats are mainly utilized during low-intensity exercise due to lower energy demand.
• Carbohydrates are essential for high-intensity efforts, providing quick energy but with lower energy yield per gram.
• Absolute intensity measures total fuel used, while relative intensity assesses fuel proportions based on body weight.

Endurance Training Adaptations

• Improved fat utilization is a key adaptation from endurance training, preserving carbohydrates for later use, enhancing overall endurance.

Measuring Fuel Utilization

• Oxygen is used to oxidize carbohydrates and lipids, generating energy and carbon dioxide.
• The Respiratory Exchange Ratio (RER) indicates the fuel being burned: RER = CO2 produced : O2 consumed.
• Resting RER for individuals on a typical Western diet ranges from 0.8 to 0.85, suggesting a mix of fats and carbohydrates for energy.

Implications of RER

• An RER of 1 indicates exclusive carbohydrate metabolism, while an RER of 0.7 points to fat metabolism.
• Higher oxygen demand for fat oxidation is due to the need for beta-oxidation.

High-Intensity Exercise Considerations

• At around 65% of VO2max, lipolysis decreases due to increased blood glucose inhibiting triglyceride breakdown.
• Above 85% of VO2max, fat oxidation declines due to reduced blood flow and availability of free fatty acids (FFAs).
• Carbohydrates are favored during high-intensity workouts, with muscle glycogen becoming critical (65-110% of VO2max).

Low-Intensity Exercise Fuel Selection

• Fuel selection remains consistent during prolonged low-intensity workouts lasting 1-2 hours, relying predominantly on fats.

This quiz covers key concepts related to adaptations in endurance training, focusing on the utilization of fats as fuel and how exercise intensity affects energy production. Understand the respiratory exchange ratio and the differences in energy systems during various activities.