The heart is also muscle
Strong Heart Alone Does Not Guarantee Performance
Central and Peripheral Components
When we talk about athletic performance, we inevitably come to the question of what actually limits it. In physiological terms, performance can be constrained by central factors—primarily the cardiovascular system—or by peripheral factors, namely the working muscles themselves.
In swimming, this distinction becomes especially important. Each swimming stroke relies on a specific technical pattern and involves slightly different muscle groups. Smaller stabilizing muscles may vary from stroke to stroke, but the major contributors to propulsion remain largely the same. Muscles such as the quadriceps, hamstrings, and deltoids appear consistently across all strokes and play a dominant role in generating forward movement.
This leads to a fundamental question:
-How much oxygen do these muscles actually require to perform at a high level?
-How much oxygen do these muscles actually require to perform at a high level?
Oxygen Consumption of Skeletal Muscle
From a theoretical standpoint, this question is not difficult to answer. If we assume that a muscle is fully prepared—that is, trained to its physiological limit—then 1 kilogram of active muscle mass is capable of consuming approximately 0.2–0.3 liters of oxygen per minute, provided that all oxidative muscle fibers are involved in the work.
The next step is straightforward: this value is multiplied by the amount of active muscle mass participating in the movement, under the assumption that this muscle mass is maximally prepared.
But this raises another important questions:
The next step is straightforward: this value is multiplied by the amount of active muscle mass participating in the movement, under the assumption that this muscle mass is maximally prepared.
But this raises another important questions:
- What Does “Maximally Prepared” Actually Mean?Maximal preparation is not about effort or motivation. It is a structural state of the muscle.
Within a fully prepared muscle, oxidative muscle fibers, myofibrils, and mitochondria are arranged in an optimal ratio. The density of mitochondria surrounding the myofibrils is so high that no further structural adaptation is possible. In this state, the muscle resembles myocardial tissue, where every contractile element is tightly surrounded by mitochondria to ensure uninterrupted aerobic energy production.
Under these conditions:- To consume 3 liters of oxygen per minute, approximately 10 kg of active muscle mass is required.
- To consume 6 liters of oxygen per minute, about 20 kg of active muscle mass is sufficient.
- This gives us a clear framework for estimating muscular oxygen demand.
- How Much Oxygen Can the Heart Deliver?Now we turn to the other side of the equation: oxygen delivery.
If we assume that 1 liter of blood carries approximately 160 ml of oxygen (with normal hemoglobin levels), then the heart’s ability to deliver oxygen depends entirely on cardiac output, which is the product of stroke volume and heart rate.
In an average adult male:- Stroke volume is approximately 120–130 ml per beat
- At a heart rate of 190 beats per minute
The calculation is simple and direct.
In elite endurance athletes, however, the situation is very different. Stroke volume can reach 240 ml per beat, allowing oxygen delivery in the range of 7–8 liters per minute.
Matching Muscle Demand and Cardiac Supply
We previously established that 20 kg of active muscle mass can consume approximately 6 liters of oxygen per minute. If we consider a swimmer whose legs alone contain 20–25 kg of muscle, and then add the muscles of the trunk, back, and arms, total active muscle mass can easily exceed 30 kg.
Even allowing for the fact that not all muscle groups will operate at maximal oxygen consumption simultaneously, it is reasonable to estimate that 40 kg of active muscle mass could require close to 8 liters of oxygen per minute under optimal conditions.
This means that, in theory, the heart must be capable of delivering up to 8 liters of oxygen per minute in order to fully support the muscles when they are maximally prepared.
Even allowing for the fact that not all muscle groups will operate at maximal oxygen consumption simultaneously, it is reasonable to estimate that 40 kg of active muscle mass could require close to 8 liters of oxygen per minute under optimal conditions.
This means that, in theory, the heart must be capable of delivering up to 8 liters of oxygen per minute in order to fully support the muscles when they are maximally prepared.
Two Absolute Limits
At this point, we encounter two important physiological boundaries:
- FirstFrom the scientific literature, it is well established that delivering 8 liters of oxygen per minute through the cardiovascular system represents a practical upper limit for the human heart.01
- SecondFctual measurements of muscular oxygen consumption rarely exceed 6–6.5 liters per minute. Values approaching 7 liters per minute are extremely uncommon and have not been consistently documented.02
This means that performance may be limited either:
- by the heart’s ability to deliver oxygen, or
- by the muscles’ ability to utilize it
Case Example: Strong Heart, Weak Muscles
Consider a swimmer undergoing physiological testing. The results appear disappointing at first glance. Measurements show that the muscles of the legs consume only 3.5 liters of oxygen per minute at the anaerobic threshold. This is clearly insufficient for high-level performance.
The immediate question becomes:
Is the heart the problem?
When we examine the step-test data, we observe that during the initial stages—when only oxidative muscle fibers are recruited—there is a near-linear relationship between heart rate and power output. As intensity increases, this relationship begins to change, and heart rate rises more steeply.
If we extrapolate the initial linear segment to a heart rate of 190 beats per minute, we can estimate the heart’s potential oxygen delivery capacity under purely oxidative conditions.
In this case, the heart is capable of delivering approximately 7 liters of oxygen per minute.
This indicates a large, well-developed heart that does not require additional aerobic conditioning. The limiting factor is clearly peripheral: the muscles, particularly in the legs, are poorly prepared and must be trained.
Consider a swimmer undergoing physiological testing. The results appear disappointing at first glance. Measurements show that the muscles of the legs consume only 3.5 liters of oxygen per minute at the anaerobic threshold. This is clearly insufficient for high-level performance.
The immediate question becomes:
Is the heart the problem?
When we examine the step-test data, we observe that during the initial stages—when only oxidative muscle fibers are recruited—there is a near-linear relationship between heart rate and power output. As intensity increases, this relationship begins to change, and heart rate rises more steeply.
If we extrapolate the initial linear segment to a heart rate of 190 beats per minute, we can estimate the heart’s potential oxygen delivery capacity under purely oxidative conditions.
In this case, the heart is capable of delivering approximately 7 liters of oxygen per minute.
This indicates a large, well-developed heart that does not require additional aerobic conditioning. The limiting factor is clearly peripheral: the muscles, particularly in the legs, are poorly prepared and must be trained.
What Would Adequate Preparation Look Like?
For this swimmer to achieve competitive results, leg oxygen consumption would need to increase to approximately 4.5 liters per minute. At this level, the athlete would be capable of stable inclusion in a national team environment.
Crucially, this level of oxygen consumption should occur at a heart rate of around 150 beats per minute, not 190. This heart rate reserve is essential, as it allows additional oxygen demand from the arms during swimming.
If testing confirms:
When arm testing is added, it may reveal an additional 1.5 liters per minute of oxygen consumption. Combined, this produces a total oxygen uptake of 6 liters per minute.
For a swimmer weighing 70 kg, this corresponds to approximately 85 ml/kg/min, which represents a very high level of aerobic performance.
Crucially, this level of oxygen consumption should occur at a heart rate of around 150 beats per minute, not 190. This heart rate reserve is essential, as it allows additional oxygen demand from the arms during swimming.
If testing confirms:
- 4.5 L/min in the legs at 150 bpm
- followed by the onset of acidosis
When arm testing is added, it may reveal an additional 1.5 liters per minute of oxygen consumption. Combined, this produces a total oxygen uptake of 6 liters per minute.
For a swimmer weighing 70 kg, this corresponds to approximately 85 ml/kg/min, which represents a very high level of aerobic performance.
Opposite Scenario: Weak Heart, Strong Muscles
Now consider a different swimmer.
Testing reveals:
This swimmer does not tolerate training well, struggles to maintain workloads, and gradually loses performance. This pattern is especially common among older athletes.
Importantly, the heart itself is not defective—it is overstressed.
Now consider a different swimmer.
Testing reveals:
- 4.5 L/min oxygen consumption in the legs
- but at a heart rate of 190 beats per minute
This swimmer does not tolerate training well, struggles to maintain workloads, and gradually loses performance. This pattern is especially common among older athletes.
Importantly, the heart itself is not defective—it is overstressed.
Recovery and Reversibility
When volume-based training is removed and the program is reduced to short, high-intensity, sprint-oriented work, the heart begins to recover. Over a period of 4–5 months, cardiac function normalizes, and oxygen delivery capacity increases from 4.5 to nearly 8 liters per minute.
Arm oxygen consumption may nearly double, while leg performance remains stable. The same 4.5 L/min in the legs is now achieved at a heart rate of 160 bpm, leaving sufficient reserve to incorporate the arms and reach maximal effort at 190 bpm.
At this point, the athlete can sustain extremely high workloads.
The limiting factor was never the quality of the heart—it was the absence of adequate recovery.
Arm oxygen consumption may nearly double, while leg performance remains stable. The same 4.5 L/min in the legs is now achieved at a heart rate of 160 bpm, leaving sufficient reserve to incorporate the arms and reach maximal effort at 190 bpm.
At this point, the athlete can sustain extremely high workloads.
The limiting factor was never the quality of the heart—it was the absence of adequate recovery.
