Energy systems are the metabolic pathways through which the body converts fuel into usable energy for muscle contraction during running. Understanding these systems explains why different training intensities produce different adaptations and why marathon success depends primarily on aerobic fitness.
This article explains the aerobic and anaerobic energy systems, key performance markers including lactate threshold and VO₂max, the role of fat oxidation, and how specific workouts target each system.
Definition
Energy systems are biochemical pathways that break down fuel sources (carbohydrates, fats, and to a lesser extent proteins) to produce adenosine triphosphate (ATP), the molecular currency of energy used by muscle cells.
Three primary systems exist:
- Phosphagen system (immediate, very short duration)
- Anaerobic glycolysis (rapid, short to moderate duration)
- Aerobic system (slower but sustainable, long duration)
For marathon running, the aerobic system provides the vast majority of energy, with anaerobic systems contributing during surges, hills, and the final stretch.
The three energy systems
Phosphagen system (ATP-PC)
Duration: 0-10 seconds Fuel source: Stored ATP and creatine phosphate Oxygen required: No
Relevance to marathon running:
- Used during the first few strides of any run
- Powers brief maximal efforts (starting a race, short sprints)
- Minimal role during sustained marathon running
Key point: This system depletes rapidly and requires rest to regenerate. It plays almost no role in marathon pacing.
Anaerobic glycolysis (lactic acid system)
Duration: 10 seconds to ~2 minutes Fuel source: Glucose (from blood sugar or muscle glycogen) Oxygen required: No Byproduct: Lactate and hydrogen ions
Relevance to marathon running:
- Engaged during hard efforts above lactate threshold
- Contributes during uphill surges or late-race pushes
- Produces lactate, which accumulates faster than it can be cleared when intensity is high
Key point: While this system can produce energy quickly without oxygen, it cannot be sustained for long periods. Excessive reliance leads to rapid fatigue.
Aerobic system (oxidative phosphorylation)
Duration: Beyond ~2 minutes to many hours Fuel sources: Carbohydrates and fats (primarily) Oxygen required: Yes Byproducts: Carbon dioxide and water
Relevance to marathon running:
- Provides 95-99% of energy during a marathon
- Sustainable for extended periods when intensity is managed
- Improved through consistent aerobic training
Key point: Marathon performance is fundamentally an aerobic event. Training that develops the aerobic system forms the foundation of marathon success.
Key performance markers
VO₂max (maximal aerobic capacity)
Definition: The maximum rate at which the body can consume oxygen during exercise.
Why it matters:
- Represents the upper ceiling of aerobic power
- Higher VO₂max allows higher sustainable speeds
- Can be improved through high-intensity interval training
Typical values:
- Untrained: 35-45 ml/kg/min
- Trained recreational runner: 50-60 ml/kg/min
- Elite marathoner: 70-85 ml/kg/min
Training focus: Interval workouts at 3K-5K pace improve VO₂max.
Lactate threshold (anaerobic threshold)
Definition: The exercise intensity at which lactate begins to accumulate faster than the body can clear it.
Why it matters:
- Determines the highest sustainable pace for extended periods
- Well-trained marathoners run close to lactate threshold pace
- Strong predictor of marathon performance
Expression:
- Can be expressed as pace, heart rate, or percentage of VO₂max
- Elite marathoners may race at 85-90% of VO₂max
- Recreational runners often race at 75-85% of VO₂max
Training focus: Tempo runs and steady-state runs improve lactate threshold.
Running economy
Definition: The energy cost of running at a given pace. Better economy means less oxygen consumed per kilometer.
Why it matters:
- Two runners with identical VO₂max can perform very differently based on economy
- Improved economy allows faster paces at the same physiological cost
Factors influencing economy:
- Running mechanics and form
- Muscle fiber type and efficiency
- Training volume and consistency
- Body composition
Training focus: Easy mileage, strength training, and drills improve running economy.
Fuel utilization: carbohydrates vs fats
Carbohydrate metabolism
Characteristics:
- Provides energy more rapidly than fat
- Limited storage (~1,800-2,000 calories as glycogen)
- Becomes primary fuel at higher intensities
Marathon relevance:
- Glycogen depletion contributes to "hitting the wall"
- Training increases glycogen storage capacity
- Race fueling strategies aim to spare glycogen
Fat metabolism
Characteristics:
- Virtually unlimited storage in the body
- Provides energy more slowly than carbohydrates
- Predominant fuel source at easy aerobic paces
Marathon relevance:
- Enhanced fat oxidation spares limited glycogen stores
- Aerobic training increases fat-burning capacity
- Running at easy paces trains fat metabolism
Key concept: One goal of base building is improving the body's ability to burn fat at progressively faster paces, preserving glycogen for when it is most needed.
How different workouts target different systems
Easy runs (60-75% of max heart rate)
Primary system: Aerobic (fat oxidation dominant)
Adaptations:
- Increased mitochondrial density
- Improved capillary networks
- Enhanced fat metabolism
- Greater aerobic base
Tempo runs / threshold runs (80-88% of max heart rate)
Primary system: Aerobic near lactate threshold
Adaptations:
- Raised lactate threshold
- Improved lactate clearance and buffering
- Greater ability to sustain faster paces
- Enhanced carbohydrate metabolism efficiency
Intervals (90-100% of max heart rate)
Primary system: Aerobic at VO₂max with anaerobic contribution
Adaptations:
- Increased VO₂max
- Improved running economy
- Greater anaerobic capacity
- Enhanced cardiovascular power
Long runs (65-80% of max heart rate)
Primary system: Aerobic (gradual shift from fat to carbohydrate as duration extends)
Adaptations:
- Muscular endurance
- Glycogen storage capacity
- Mental resilience
- Fat oxidation at sustained efforts
Common misconceptions
"More anaerobic training makes you faster"
Reality: Marathon performance depends overwhelmingly on aerobic fitness. Excessive anaerobic work detracts from aerobic development and increases injury risk.
"Running easy doesn't make you faster"
Reality: Easy aerobic running drives fundamental adaptations—mitochondrial growth, capillary development, and fat metabolism—that underpin marathon success.
"Lactate causes muscle soreness"
Reality: Lactate itself is not harmful and is actually a useful fuel source. Muscle soreness (DOMS) results from structural damage, not lactate accumulation.
"You need to deplete glycogen on every long run"
Reality: While some long runs can strategically deplete glycogen to stimulate adaptation, most long runs should be fueled appropriately to support quality training.
Practical application in marathon training
Base building phase
Focus: Develop aerobic system and fat oxidation Primary efforts: Easy runs, long runs Energy system emphasis: Aerobic, fat metabolism
Build phase
Focus: Raise lactate threshold while maintaining aerobic base Primary efforts: Tempo runs, steady-state runs, long runs Energy system emphasis: Aerobic near threshold, lactate clearance
Peak phase
Focus: Fine-tune VO₂max and race-specific pacing Primary efforts: Threshold work, marathon-pace runs, some intervals Energy system emphasis: Aerobic capacity, sustained threshold work
Summary
Energy systems are the metabolic pathways that produce ATP for muscle contraction during running. The three systems—phosphagen, anaerobic glycolysis, and aerobic oxidation—contribute differently depending on exercise duration and intensity. Marathon running relies almost entirely on the aerobic system, making aerobic development the primary training focus. Key performance markers include VO₂max (maximal aerobic capacity), lactate threshold (highest sustainable pace), and running economy (energy cost per pace). Effective marathon training systematically targets these systems through varied intensities: easy runs build aerobic base and fat metabolism, tempo runs raise lactate threshold, and intervals improve VO₂max. Understanding energy systems allows runners to select appropriate workouts, manage pacing, and fuel correctly for optimal marathon performance.