Key Takeaways
- ATP-PC system provides immediate energy (0-10 seconds) without oxygen; limited by phosphocreatine stores.
- Glycolysis breaks down glucose to produce ATP; fast glycolysis produces lactate, slow glycolysis feeds the oxidative system.
- The oxidative system produces the most ATP per molecule but is the slowest; dominant during low-intensity, long-duration activities.
- EPOC (excess post-exercise oxygen consumption) represents elevated metabolism after exercise for recovery processes.
- Substrate utilization shifts from primarily fat at low intensities to primarily carbohydrates at high intensities.
Bioenergetics and Metabolism
Quick Answer: Three energy systems produce ATP: the phosphagen system (immediate, 0-10 sec), glycolysis (short-term, 10 sec-2 min), and oxidative system (long-term, >2 min). All systems are always active, but their relative contribution depends on exercise intensity and duration. High-intensity activities rely more on anaerobic systems; low-intensity activities rely more on the oxidative system.
ATP: The Energy Currency
Adenosine triphosphate (ATP) is the only energy source directly used for muscle contraction:
- Stored ATP in muscle is very limited (~80-100g total body stores)
- Enough for only 1-2 seconds of maximal exercise
- Must be continuously regenerated from ADP (adenosine diphosphate)
ATP → ADP + Pi + Energy (for muscle contraction)
The three energy systems regenerate ATP at different rates and capacities.
The Three Energy Systems
| System | Primary Fuel | Rate of ATP Production | ATP Yield | Duration | Oxygen Required |
|---|---|---|---|---|---|
| Phosphagen (ATP-PC) | Creatine phosphate | Fastest | Limited | 0-10 seconds | No (anaerobic) |
| Glycolytic | Glucose/Glycogen | Fast | Moderate | 10 sec-2 min | No (anaerobic) |
| Oxidative | Carbs, Fats, Protein | Slowest | Highest | >2 minutes | Yes (aerobic) |
1. Phosphagen System (ATP-PC System)
The phosphagen system provides immediate energy for high-intensity, short-duration activities.
Key Characteristics
| Factor | Description |
|---|---|
| Substrates | ATP and creatine phosphate (CP) stored in muscle |
| Duration | 0-10 seconds of maximal effort |
| ATP production rate | Fastest of all systems |
| Limiting factor | Depletion of CP stores |
| Recovery | 50% in ~30 sec; full recovery in 3-5 minutes |
The Reaction
Creatine Phosphate + ADP → ATP + Creatine (via creatine kinase)
Applications
| Activity | Duration | Phosphagen Contribution |
|---|---|---|
| 40-yard dash | ~5 sec | Very high |
| Vertical jump | <1 sec | Primary |
| 1RM attempt | ~3 sec | Very high |
| Baseball swing | <1 sec | Primary |
Exam Tip: The phosphagen system is the dominant energy system for single maximal efforts lasting less than 10 seconds. Rest intervals of 2-5 minutes are needed for complete CP recovery between sets.
Creatine Supplementation
- Increases intramuscular creatine and phosphocreatine stores
- Improves performance in repeated high-intensity bouts
- Loading dose: 20g/day for 5-7 days, then 3-5g/day maintenance
- One of the most researched and effective legal supplements
2. Glycolytic System
The glycolytic system breaks down glucose (or glycogen) to produce ATP through a series of chemical reactions.
Two Forms of Glycolysis
| Type | End Product | Oxygen | ATP Yield | Speed |
|---|---|---|---|---|
| Fast (anaerobic) glycolysis | Lactate (lactic acid) | Not required | 2 ATP per glucose | Fast |
| Slow (aerobic) glycolysis | Pyruvate → enters oxidative system | Available | 2 ATP + more via oxidation | Slower |
Fast Glycolysis Pathway
Glucose → Pyruvate → Lactate + 2 ATP (net)
| Step | Details |
|---|---|
| Input | 1 glucose molecule (or 1 glycogen unit) |
| ATP cost | 2 ATP invested (1 if from glycogen) |
| ATP produced | 4 ATP |
| Net ATP | 2 ATP (3 if from glycogen) |
| End product | Lactate (when oxygen insufficient) |
Lactate: Not the Villain
Modern understanding of lactate:
| Old View | Current Understanding |
|---|---|
| "Waste product" | Valuable metabolic intermediate |
| Causes muscle soreness | DOMS is from muscle damage, not lactate |
| Limits performance | Important fuel source for heart, brain, and muscles |
| Should be avoided | Lactate clearance is a trainable adaptation |
Key Point: Lactate can be converted back to glucose in the liver (Cori cycle) or used directly as fuel by the heart and other tissues.
Lactate Threshold
- The intensity at which lactate production exceeds clearance
- Typically occurs at 50-60% VO2max in untrained individuals
- Can improve to 70-80% VO2max with training
- Important marker for endurance performance
3. Oxidative (Aerobic) System
The oxidative system produces ATP through aerobic metabolism in the mitochondria.
Components
- Aerobic glycolysis: Glucose → Pyruvate → Acetyl-CoA
- Krebs cycle (Citric Acid Cycle): Acetyl-CoA → CO2 + electron carriers
- Electron Transport Chain (ETC): Electron carriers → ATP + H2O
ATP Yield from Different Substrates
| Substrate | ATP Yield (approximately) |
|---|---|
| 1 glucose molecule | 30-32 ATP |
| 1 palmitate (fatty acid) | 106 ATP |
| 1 amino acid | Varies (typically 15-20 ATP) |
Fat Oxidation (Beta-Oxidation)
Fats provide the highest ATP yield per molecule but require:
- More oxygen per ATP produced
- More time to break down
- Lower intensity exercise for optimal use
| Factor | Carbohydrate | Fat |
|---|---|---|
| ATP per O2 | More efficient | Less efficient |
| Total ATP yield | 30-32 per glucose | 106+ per fatty acid |
| Speed of availability | Fast | Slow |
| Optimal intensity | High | Low to moderate |
The Crossover Concept
As exercise intensity increases, the body shifts from fat to carbohydrate oxidation:
| Intensity | Primary Fuel |
|---|---|
| Rest | ~60% fat, ~40% carbs |
| 25% VO2max | ~70% fat, ~30% carbs |
| 50% VO2max | ~50% fat, ~50% carbs |
| 65% VO2max | ~40% fat, ~60% carbs |
| 85% VO2max | ~15% fat, ~85% carbs |
| Maximal | ~100% carbs |
Exam Tip: Fat oxidation requires oxygen and is maximized at low to moderate intensities (around 60-65% VO2max). High-intensity exercise relies primarily on carbohydrate oxidation.
Energy System Contributions by Activity
| Activity | Phosphagen | Glycolytic | Oxidative |
|---|---|---|---|
| Shot put | 90% | 10% | 0% |
| 100m sprint | 50% | 45% | 5% |
| 400m sprint | 25% | 65% | 10% |
| 800m run | 10% | 50% | 40% |
| 1500m run | 5% | 25% | 70% |
| Marathon | 0% | 5% | 95% |
EPOC: Excess Post-Exercise Oxygen Consumption
EPOC (formerly called "oxygen debt") is the elevated oxygen consumption that occurs after exercise:
Causes of EPOC
| Fast Component (minutes) | Slow Component (hours) |
|---|---|
| Replenish muscle ATP | Elevated body temperature |
| Replenish creatine phosphate | Elevated heart rate and breathing |
| Reload muscle myoglobin with O2 | Tissue repair |
| Reload hemoglobin with O2 | Hormone effects (catecholamines) |
| Lactate conversion |
EPOC Magnitude Factors
| Factor | Effect on EPOC |
|---|---|
| Exercise intensity | Higher intensity = greater EPOC |
| Exercise duration | Longer duration = greater EPOC |
| Type of exercise | Resistance training > steady-state cardio |
| Training status | Trained individuals have faster recovery |
Exam Tip: EPOC is greater following high-intensity interval training (HIIT) and resistance training compared to steady-state aerobic exercise at the same total caloric expenditure.
Substrate Utilization Summary
Factors Favoring Fat Oxidation
- Low exercise intensity
- Longer exercise duration (after glycogen depletion begins)
- Fasted state
- Endurance training adaptations
- Higher aerobic capacity
Factors Favoring Carbohydrate Oxidation
- High exercise intensity
- Short duration, high-power activities
- Fed state (high carbohydrate availability)
- Untrained state
- Type II fiber recruitment
A football lineman performs a single maximal effort lasting approximately 5 seconds. Which energy system is the PRIMARY contributor?
Which energy system produces the MOST ATP per molecule of substrate?
At what exercise intensity is fat oxidation typically MAXIMIZED?
What is the approximate time needed for full phosphocreatine (PC) recovery after a maximal effort?