Primary fuel source for moderate to high-intensity exercise.
Stored as glycogen in muscles and liver.
Fats
Predominantly used during low-intensity, long-duration exercise.
Broken down into free fatty acids and oxidized in the mitochondria.
Proteins
Minimal contribution to energy production under normal conditions.
Used during prolonged exercise when glycogen is depleted.
Lactate and metabolic acidosis
Lactate production
Lactate is a byproduct of anaerobic glycolysis and is often misunderstood. During high-intensity exercise, pyruvate is converted into lactate to regenerate NAD+, which allows glycolysis to continue. This is a necessary process for sustaining ATP production during anaerobic conditions.
Lactate as an energy source
Contrary to popular belief, lactate is not the cause of fatigue. It serves as an important energy substrate, particularly for the heart and oxidative muscle fibers. Lactate can be transported to the liver via the Cori cycle and converted back into glucose for energy.
Metabolic acidosis
The accumulation of hydrogen ions (H⁺), rather than lactate, is the primary cause of metabolic acidosis. These hydrogen ions lower the pH within the muscle, impairing enzyme function and contributing to fatigue. Lactate acts as a buffer, helping to reduce the acidity by consuming H⁺ during its conversion back to pyruvate.
Lactate threshold and OBLA
The lactate threshold (LT) is the point during exercise at which blood lactate levels begin to rise exponentially. It is a critical marker of aerobic fitness and endurance capacity.
Training effects: Regular endurance and high-intensity training can delay the onset of LT, allowing athletes to perform at higher intensities before lactate accumulation limits performance.
OBLA refers to the point where blood lactate reaches a concentration of 4 mmol/L. It is often used as a marker of anaerobic capacity and correlates with exercise fatigue during high-intensity efforts.
Lactate threshold and OBLA
Excess postexercise oxygen consumption (EPOC)
EPOC refers to the elevated oxygen consumption following exercise. It represents the body’s effort to restore homeostasis and includes:
Replenishment of oxygen stores in muscles and blood.
ATP and CP resynthesis.
Lactate clearance and conversion to glucose.
Restoration of body temperature, circulation, and ventilation to pre-exercise levels.
Factors influencing EPOC
Exercise intensity: Higher intensity results in greater EPOC due to increased metabolic demands.
Exercise duration: Prolonged exercise increases EPOC as recovery processes are extended.
Training implications: High-intensity interval training (HIIT) maximizes EPOC, leading to greater calorie expenditure post-exercise and improved aerobic and anaerobic fitness.
EPOC
Recovery and resynthesis
Phosphagen system:
CP stores recover fully in 3-5 minutes with adequate rest.
Glycogen repletion:
Muscle glycogen is replenished within 24 hours with carbohydrate intake of 5-7 g/kg of body weight.
Prolonged, high-intensity exercise requires up to 48 hours for full recovery.
Factors affecting bioenergetics
Exercise intensity and duration: Determines the predominant energy system.
Training adaptations: Increase efficiency of energy systems and enhance substrate utilization.
Nutrition: Adequate carbohydrate intake supports glycogen replenishment and performance.
Practical applications
High-intensity interval training (HIIT):
Alternates between intense exercise and recovery intervals.
Improves both aerobic and anaerobic capacity.
Work-to-rest ratios:
Tailored to the energy system:
Phosphagen: 1:12 to 1:20
Glycolytic: 1:3 to 1:5
Oxidative: 1:1 to 1:3
Combination training:
Incorporates aerobic and anaerobic modalities to enhance overall fitness and performance.
