Substrate utilization and application
Carbohydrates
- Primary fuel source for moderate- to high-intensity exercise.
- Stored as glycogen in the 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 necessary to sustain ATP production under anaerobic conditions.
Lactate as an energy source
- Lactate isn’t 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 pH within the muscle, which impairs enzyme function and contributes to fatigue. Lactate acts as a buffer by consuming H⁺ when it’s converted back to pyruvate.
Lactate threshold and OBLA
The lactate threshold (LT) is the point during exercise when blood lactate levels begin to rise exponentially. It’s a key marker of aerobic fitness and endurance capacity.
- Training effects: Regular endurance and high-intensity training can delay the onset of LT, allowing athletes to sustain higher intensities before lactate accumulation limits performance.
OBLA refers to the point where blood lactate reaches 4 mmol/L. It’s often used as a marker of anaerobic capacity and correlates with fatigue during high-intensity efforts.
Excess postexercise oxygen consumption (EPOC)
EPOC refers to elevated oxygen consumption after exercise. It reflects 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 produces greater EPOC because metabolic demands are higher.
- Exercise duration: Longer exercise increases EPOC because recovery processes take longer.
Training implications: High-intensity interval training (HIIT) maximizes EPOC, leading to greater post-exercise calorie expenditure and improvements in both aerobic and anaerobic fitness.
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 may require up to 48 hours for full recovery.
Factors affecting bioenergetics
- Exercise intensity and duration: Determines the predominant energy system.
- Training adaptations: Increase the 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.
Tables and diagrams
ATP yield by energy system
| System | Rate of ATP production | Total ATP yield |
|---|---|---|
| Phosphagen | Very high | 1 ATP |
| Glycolytic | High | 2-3 ATP |
| Oxidative | Low | ~38 ATP |
Effect of duration and intensity on energy system
| Duration | Intensity | Primary system |
|---|---|---|
| 0-6 seconds | Very high | Phosphagen |
| 6-30 seconds | High | Phosphagen and glycolysis |
| 30 sec - 2 min | Moderate | Glycolysis |
| 2+ minutes | Low | Oxidative |
Ranking of bioenergetic limiting factors
| Exercise | ATP and creatine phosphate | Muscle glycogen | Liver glycogen | Fat stores | Lower pH |
|---|---|---|---|---|---|
| Marathon | 5 | 5 | 1 | 2-3 | 5 |
| Triathlon | 1-2 | 5 | 4-5 | 1-2 | 1-2 |
| 5,000 m run | 2-3 | 5 | 3-4 | 1-2 | 3 |
| 1,500 m run | 2-3 | 3-4 | 4 | 1 | 3-4 |
| 400 m swim | 3 | 1-2 | 1 | 4-5 | 1 |
| Repeated snatch exercises at 60% of 1RM | 4-5 | 1 | 5 | 2-3 | 4-5 |
1=least probable limiting factor, 5=most probable limiting factor