Energy Supply in Swimming Performance
The human organism is wiser than any scientist
The Principle of Intuition
Every athlete must rely in training not only on rules, methods, and scientific models, but also on intuition — because adaptive responses are always individual.
For this reason, the training process can and should be planned.
However, during its execution, the athlete must learn to listen to internal sensations and continuously compare them with both personal expectations and the coach’s intentions.
Regular monitoring of readiness across different systems and organs is essential.
This information, combined with scientific knowledge, forms the foundation of productive intuition — the ability to recognize subtle signals, make timely adjustments, and creatively refine the training process.
When working with a real athlete, theory alone is not enough.
There are moments when correction becomes necessary, and in those moments, intuition becomes the link between science and reality.
" Knowledge is blind without intuition "
For this reason, the training process can and should be planned.
However, during its execution, the athlete must learn to listen to internal sensations and continuously compare them with both personal expectations and the coach’s intentions.
Regular monitoring of readiness across different systems and organs is essential.
This information, combined with scientific knowledge, forms the foundation of productive intuition — the ability to recognize subtle signals, make timely adjustments, and creatively refine the training process.
When working with a real athlete, theory alone is not enough.
There are moments when correction becomes necessary, and in those moments, intuition becomes the link between science and reality.
" Knowledge is blind without intuition "
1
Biochemical Adaptations in the Training Process
Limits of Physiological Adaptation
Not all physiological systems adapt equally to training.
Each system has its own range, speed, and limit of adaptation.
Long-term, systematic training produces uneven but predictable changes across the body.
Some systems respond dramatically, others only marginally — regardless of effort.
Each system has its own range, speed, and limit of adaptation.
Long-term, systematic training produces uneven but predictable changes across the body.
Some systems respond dramatically, others only marginally — regardless of effort.
Aerobic System — The Greatest Adaptive Potential
The most pronounced adaptations occur in aerobic capacity.
With long-term endurance-oriented training, the following can increase significantly:
As a result, VO₂max can increase substantially ( ~15 - 40% )
However, genetic factors play a major role — individual responses vary widely, even under identical training loads.
With long-term endurance-oriented training, the following can increase significantly:
Aerobic metabolic enzymes ↑ (up to ~230%)
Mitochondrial content ↑ ( ~120% )
Myoglobin concentration ↑ ( ~80% )
Muscle capillarization ↑ ( ~40% )
As a result, VO₂max can increase substantially ( ~15 - 40% )
However, genetic factors play a major role — individual responses vary widely, even under identical training loads.
What Determines Aerobic Energy Potential?
Aerobic performance is not defined by a single factor, but by the interaction of many systems:
Efficiency of breathing and gas exchange
Cardiovascular capacity
Availability of energy substrates
Muscle fiber type distribution
Capillary density in muscle tissue
Number, size, and density of mitochondria
Activity of oxidative enzymes
Hormonal and regulatory mechanisms
Together, these determine how efficiently the body can sustain prolonged work.
Efficiency of breathing and gas exchange
Cardiovascular capacity
Availability of energy substrates
Muscle fiber type distribution
Capillary density in muscle tissue
Number, size, and density of mitochondria
Activity of oxidative enzymes
Hormonal and regulatory mechanisms
Together, these determine how efficiently the body can sustain prolonged work.
Aerobic VS Anaerobic Adaptations
Defines sustainable performance VS Defines peak power output
Large long-term adaptation range VS Limited adaptive range
Structural and metabolic changes VS Mainly biochemical changes
Sets balance between systems VS Drives short maximal efforts
Large long-term adaptation range VS Limited adaptive range
Structural and metabolic changes VS Mainly biochemical changes
Sets balance between systems VS Drives short maximal efforts
VO₂max — A Central Indicator
The full expression of aerobic capacity occurs at VO₂max.
All energy expenditure — including anaerobic contributions — ultimately depends on oxidative phosphorylation.
For this reason, VO₂max remains a highly sensitive indicator of aerobic potential.
VO₂max depends on:
Oxygen and substrate transport systems
Functional capacity of mitochondrial oxidative systems
All energy expenditure — including anaerobic contributions — ultimately depends on oxidative phosphorylation.
For this reason, VO₂max remains a highly sensitive indicator of aerobic potential.
VO₂max depends on:
Oxygen and substrate transport systems
Functional capacity of mitochondrial oxidative systems
Anaerobic Adaptations — Limited by Nature
Anaerobic systems adapt to a much smaller extent.
This applies especially to:
Anaerobic enzymes
Peak blood lactate levels (only modest increases, even after intense training)
A common misconception is that glycolysis occurs only when oxygen is lacking.
In reality, glycolysis plays a critical role during high-intensity work, regardless of oxygen availability.
Oxygen presence does not prevent lactate or pyruvate production.
This applies especially to:
Anaerobic enzymes
Peak blood lactate levels (only modest increases, even after intense training)
A common misconception is that glycolysis occurs only when oxygen is lacking.
In reality, glycolysis plays a critical role during high-intensity work, regardless of oxygen availability.
Oxygen presence does not prevent lactate or pyruvate production.
Phosphocreatine and Speed Potential
Phosphocreatine reserves — crucial for maximal speed and power — adapt differently depending on training type:
Endurance training: ~12% increase
Sprint-oriented training: up to ~42% increase
Endurance training: ~12% increase
Sprint-oriented training: up to ~42% increase
Cardiovascular Adaptations
The cardiovascular system strongly influences both aerobic and anaerobic performance.
With long-term training:
Maximal cardiac output increases by 50–75%
This increase is driven mainly by stroke volume
Maximal heart rate changes very little
With long-term training:
Maximal cardiac output increases by 50–75%
This increase is driven mainly by stroke volume
Maximal heart rate changes very little
Structural Adaptations
Long-term training also affects structure:
Muscle mass can increase by 10–40%
These changes directly influence force production and movement efficiency.
Muscle mass can increase by 10–40%
These changes directly influence force production and movement efficiency.
2
Energy Systems and Their Contribution as a Function of Swimming Duration
The key insight from the data is simple but often misunderstood:
The combined energy output of the phosphagen and lactacid systems is limited to ~100 kJ, regardless of race duration
These anaerobic systems function like a battery. They are charged during training and discharged during competition. No matter whether a race lasts one, two, or five minutes, this “battery” does not increase in capacity — only the rate of discharge changes.
In contrast, the oxidative (aerobic) system increases its contribution as race distance grows. This fundamental difference means that anaerobic and aerobic systems require completely different training strategies.
Ignoring this limitation — and individual athlete characteristics — often leads to overtraining, endocrine exhaustion, and cardiovascular–respiratory dysfunction.
Energy Systems:
In contrast, the oxidative (aerobic) system increases its contribution as race distance grows. This fundamental difference means that anaerobic and aerobic systems require completely different training strategies.
Ignoring this limitation — and individual athlete characteristics — often leads to overtraining, endocrine exhaustion, and cardiovascular–respiratory dysfunction.
Energy Systems:
- Phosphagen (ATP–PCr) — maximal power, minimal duration Eai (F.S.)
- Lactacid (glycolytic) — high power, limited capacity Eai (E.S.)
- Oxidative (aerobic) — lower power, virtually unlimited capacity Eai (O.S.)
Distance-Specific Metabolic Characteristics
Based on metabolic characteristics, success in 50 m swimming largely depends on the phosphagen energy system.
Many have encountered the expression “sprinters are born, not made.”
This can largely be explained by the fact that this type of athlete has a predominance of fast-twitch muscle fibers across most muscle groups. These fibers demonstrate a rapid ability to increase myofibrillar content and undergo hypertrophy under training loads, becoming highly glycolytic and high-threshold.
The ability to recruit the greatest possible number of muscle fibers through the central nervous system is what enables these athletes to reach extremely high levels of power and speed.
Many have encountered the expression “sprinters are born, not made.”
This can largely be explained by the fact that this type of athlete has a predominance of fast-twitch muscle fibers across most muscle groups. These fibers demonstrate a rapid ability to increase myofibrillar content and undergo hypertrophy under training loads, becoming highly glycolytic and high-threshold.
The ability to recruit the greatest possible number of muscle fibers through the central nervous system is what enables these athletes to reach extremely high levels of power and speed.
On the 100 m distance, the contribution of energy systems can follow several pathways:
- Dominant phosphagen–lactacid preparation, which is clearly reflected in the results of finals in the 50 m and 100 m events.
- Preparation with a significant emphasis on the oxidative energy system. These athletes are rarely among the fastest on 50 m but consistently rank among the strongest on 200 m.
- A third pathway, where the swimmer is not among the strongest on either 50 m or 200 m, yet demonstrates outstanding results specifically on 100 m.
The 200 m represents a metabolic transition.
There may be sufficiently endurance-trained sprinters capable of sustaining the 200 m distance, as well as high-speed distance swimmers who can also perform successfully. These two athlete types complete the same distance in fundamentally different ways.
Developing endurance in a sprinter through prolonged continuous work with short rest intervals is a mistaken approach. While recovery ability may improve, maximal power output can decline to the extent that the swimmer can no longer perform short distances as effectively as before. As a result, 200 m performance may improve only marginally.
A coach might conclude that the athlete swam “better” due to a reduced split differential between race segments, but overall race time does not meaningfully improve.
There may be sufficiently endurance-trained sprinters capable of sustaining the 200 m distance, as well as high-speed distance swimmers who can also perform successfully. These two athlete types complete the same distance in fundamentally different ways.
- The first group covers the distance at high speed due to strong anaerobic capacity.
- The second group may deliberately pace the distance and strategically determine when the aerobic mechanism becomes dominant.
Developing endurance in a sprinter through prolonged continuous work with short rest intervals is a mistaken approach. While recovery ability may improve, maximal power output can decline to the extent that the swimmer can no longer perform short distances as effectively as before. As a result, 200 m performance may improve only marginally.
A coach might conclude that the athlete swam “better” due to a reduced split differential between race segments, but overall race time does not meaningfully improve.
Research has shown that the Krebs Cycle (aerobic mechanism) becomes fully engaged during maximal power output (VO₂max) on the 400 m distance. On shorter distances, this mechanism does not have sufficient time to fully activate.
Longer distances are swum at lower speeds, with VO₂max remaining at or below the required threshold speed. The concept of a threshold here refers to the speed at which physiological parameters remain stable, allowing a swimmer to maintain a given velocity over a defined period of time.
Based on this, during the 400 m distance, the Krebs Cycle plays a maximal role in energy supply. This does not imply the absence of other energy sources.
Due to the lower maximal power of the Krebs Cycle compared to phosphagen and glycolytic mechanisms, a certain portion of energy is still derived from anaerobic glycolysis (ranging approximately from 6% to 60%, depending on training level and physiological characteristics).
However, for elite swimmers, from 400 m onward, the Krebs Cycle becomes the dominant energy mechanism. For this reason, the 400 m distance is commonly used to assess VO₂max in swimmers.
Swimming at speeds higher than those used in the 400 m event (i.e., shorter distances) requires increased involvement of the glycolytic and ATP–PCr systems.
Longer distances are swum at lower speeds, with VO₂max remaining at or below the required threshold speed. The concept of a threshold here refers to the speed at which physiological parameters remain stable, allowing a swimmer to maintain a given velocity over a defined period of time.
Based on this, during the 400 m distance, the Krebs Cycle plays a maximal role in energy supply. This does not imply the absence of other energy sources.
Due to the lower maximal power of the Krebs Cycle compared to phosphagen and glycolytic mechanisms, a certain portion of energy is still derived from anaerobic glycolysis (ranging approximately from 6% to 60%, depending on training level and physiological characteristics).
However, for elite swimmers, from 400 m onward, the Krebs Cycle becomes the dominant energy mechanism. For this reason, the 400 m distance is commonly used to assess VO₂max in swimmers.
Swimming at speeds higher than those used in the 400 m event (i.e., shorter distances) requires increased involvement of the glycolytic and ATP–PCr systems.
3
What Really Happens at High Intensity
Aerobic and Anaerobic Thresholds
Most discussions of aerobic and anaerobic thresholds oversimplify the problem into a question of “oxygen availability.” In reality, the transition between these thresholds is governed by metabolic flux, mitochondrial capacity, and redox balance, not by the presence or absence of oxygen.
- The Role of NAD⁺: The Real Limiting FactorWhen exercise intensity exceeds the mitochondria’s capacity to oxidize pyruvate — for example, ~15 seconds after the start of a 100 m freestyle swim — glycolysis continues only if NAD⁺ is regenerated.
This occurs through the conversion of pyruvate to lactate via lactate dehydrogenase (LDH).
The purpose of this reaction is not to “remove oxygen debt,” but to restore NAD⁺, without which glycolysis — and ATP production — would stop entirely.
Glycolysis is limited by NAD⁺ availability, not oxygen availability. - Lactate Is Not the ProblemLactate formation allows carbohydrate metabolism to continue at high power outputs.
At physiological pH, lactic acid immediately dissociates into:- lactate (C₃H₅O₃⁻)
- hydrogen ions (H⁺)
- reduced muscle force production
- altered enzyme activity
- metabolic acidosis during intense exercise
- Aerobic and Anaerobic Metabolism Are Not OppositesThe aerobic stage of energy metabolism begins as soon as pyruvate enters the mitochondria.
Through the pyruvate dehydrogenase complex, pyruvate is converted into acetyl-CoA, which enters the tricarboxylic acid (TCA) cycle.
From this point:- carbon is fully oxidized
- high-energy electrons are captured as NADH and FADH₂
- ATP is synthesized via oxidative phosphorylation in the electron transport chain
- The Krebs Cycle and Sustainable Energy ProductionEach turn of the Krebs cycle fully oxidizes one acetyl-CoA molecule and produces:
- 3 NADH
- 1 FADH₂
- 1 GTP (ATP equivalent)
- 2 CO₂
- acetyl-CoA supply is maintained
- mitochondrial oxidative capacity is sufficient
What the “Threshold” Actually Represents
The aerobic and anaerobic thresholds reflect:
- the balance between glycolytic rate and mitochondrial oxidation
- redox state (NADH/NAD⁺ ratio)
- hydrogen ion accumulation
They do not represent a switch between oxygen and no oxygen.
Thresholds are metabolic balance points, not binary states.
- the balance between glycolytic rate and mitochondrial oxidation
- redox state (NADH/NAD⁺ ratio)
- hydrogen ion accumulation
They do not represent a switch between oxygen and no oxygen.
Thresholds are metabolic balance points, not binary states.
Lactate production is a survival mechanism for high-intensity metabolism.
Fatigue emerges not from lactate itself, but from the limits of redox balance and mitochondrial capacity.
Understanding this distinction changes how we train, test, and interpret performance.
Fatigue emerges not from lactate itself, but from the limits of redox balance and mitochondrial capacity.
Understanding this distinction changes how we train, test, and interpret performance.
Maxim
4
Why Lactate Is Energy, Not Waste
Origin and Transformation of Lactic Acid
Lactate is often misunderstood as a byproduct that causes fatigue. In reality, it is a central component of energy redistribution during and after exercise.
Lactate Utilization in the Body
The heart and slow-twitch muscle fibers possess a high capacity to remove lactate from the bloodstream and use it directly for energy production. This ability is largely explained by the presence of specific lactate dehydrogenase (LDH) isoenzymes.
LDH isoenzymes expressed in cardiac muscle and slow oxidative fibers preferentially catalyze the reverse reaction — the conversion of lactate back into pyruvate. This facilitates the net removal of lactate from the blood and allows it to be oxidized in the mitochondria, ultimately producing carbon dioxide and water.
In this context, lactate conversion is an oxidative mitochondrial process, not an anaerobic dead end.
LDH isoenzymes expressed in cardiac muscle and slow oxidative fibers preferentially catalyze the reverse reaction — the conversion of lactate back into pyruvate. This facilitates the net removal of lactate from the blood and allows it to be oxidized in the mitochondria, ultimately producing carbon dioxide and water.
In this context, lactate conversion is an oxidative mitochondrial process, not an anaerobic dead end.
Maxim
Lactate is not the enemy of performance.
It is a mobile energy currency, continuously produced, transported, reused, and recycled.
Understanding lactate metabolism shifts training focus from “avoiding lactate” to managing production, utilization, and recovery capacity.
It is a mobile energy currency, continuously produced, transported, reused, and recycled.
Understanding lactate metabolism shifts training focus from “avoiding lactate” to managing production, utilization, and recovery capacity.
5
Why Different Athletes Win Different Races
Muscle Recruitment and Race Distance
Race performance is not determined by a single energy system, but by how muscle fibers are recruited over time and how long they can sustain force.
The central nervous system recruits muscle fibers according to demand:
low-threshold (oxidative) fibers first, then progressively high-threshold (glycolytic) fibers as speed and force increase.
This recruitment pattern explains why different distances favor different athlete profiles.
The central nervous system recruits muscle fibers according to demand:
low-threshold (oxidative) fibers first, then progressively high-threshold (glycolytic) fibers as speed and force increase.
This recruitment pattern explains why different distances favor different athlete profiles.
Performance is dominated by:
Oxidative capacity plays almost no limiting role.
Key limiter: phosphagen capacity and neural recruitment speed.
- rapid recruitment of high-threshold glycolytic fibers
- ATP–CP availability
- neural drive and rate of force development
Oxidative capacity plays almost no limiting role.
Key limiter: phosphagen capacity and neural recruitment speed.
This distance is decided by:
However, those with insufficient oxidative support experience rapid power decay.
Key limiter: glycolytic fiber reserve and acid tolerance.
- fast initial recruitment of glycolytic fibers
- ability to sequentially recruit new fibers as earlier ones fatigue
- tolerance to acidosis
However, those with insufficient oxidative support experience rapid power decay.
Key limiter: glycolytic fiber reserve and acid tolerance.
The 200 m represents a recruitment transition point.
Two athlete types can succeed:
Glycolytic-dominant athletes
Key limiter: ability to stabilize power near the anaerobic threshold.
Two athlete types can succeed:
Glycolytic-dominant athletes
- high early power
- gradual loss of force
- slightly lower peak power
- earlier stabilization near anaerobic threshold
Key limiter: ability to stabilize power near the anaerobic threshold.
Here, performance is governed by:
Key limiter: oxidative capacity and sustainable recruitment.
- dominance of oxidative fibers
- early attainment of oxygen uptake plateau
- minimal reliance on glycolytic reserves
Key limiter: oxidative capacity and sustainable recruitment.
Success depends almost entirely on:
Key limiter: efficiency, not power.
- proportion and efficiency of oxidative fibers
- metabolic economy
- minimal unnecessary muscle mass
Key limiter: efficiency, not power.
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