Cardiac Output and Stroke Volume: The Engine of Endurance Sports — How Much Has Your Heart Actually Trained?

It Starts with a Student’s Heart Rate Numbers

I’ll never forget a student I coached a few years ago — let’s call him A-Kai. A-Kai was an engineer in Taipei, training on the riverside bike paths on weekends. He was in his early thirties, weighed about 72 kilograms. When he came to me, he threw out a very typical question: “Coach, my max heart rate is clearly 190 bpm, so why can’t I ride faster toward the end? Is my heart just not strong enough?”

That day I didn’t answer right away. Instead, I asked him: do you know how much blood your heart pumps with each beat? He was stunned. Most people only stare at the “heart rate” number, forgetting that heart rate is only half the story. What truly determines whether you can deliver oxygen nonstop to your legs is “cardiac output” — and the key engine behind cardiac output is “stroke volume.”

In this article, I want to take you through tearing this engine apart completely. We’ll talk about how the heart adapts to training, what the so-called “athlete’s heart” really is, where the limits of stroke volume lie, and finally give you a practical approach you can actually follow in training. Whether you’re a beginner just getting into cycling or a seasoned rider trying to break through an FTP plateau, understanding this will change how you view training.

The Conceptual Foundation: How Cardiac Output Is Calculated

Let’s lay out the core formula first. It’s so simple you’ll wonder why you never looked at it seriously before:

Cardiac Output = Stroke Volume × Heart Rate

In plain language: how many milliliters of blood your heart pumps with each contraction (stroke volume), multiplied by how many times it beats per minute (heart rate), gives you the total amount of blood delivered to your body in one minute. This number is usually expressed in “liters per minute.”

Blood is the delivery truck for oxygen. For your legs to produce power and your mitochondria to burn fuel, oxygen needs to keep arriving. So to a large extent, the size of your aerobic engine — your maximal oxygen uptake (VO₂max) — is capped by your maximal cardiac output. Research consistently shows that, in healthy people at sea level, the primary factor limiting VO₂max is cardiac output.

Let me also fill in the complete logic of VO₂max here, so you’ll see more clearly where the heart stands. VO₂max can be broken into two parts: one is “how much oxygen the heart sends out” (that’s cardiac output), and the other is “how much oxygen the muscles keep and use” (the arteriovenous oxygen difference). The former relies on the heart, the latter on the mitochondria and capillaries in the muscles. Elite athletes excel because both parts are highly trained, but in this central piece, cardiac output is almost the decisive gate. Since heart rate has a physiological ceiling (determined by age, sex, and genetics — very hard for you to change), the only variable with substantial room for improvement to push cardiac output higher is stroke volume. That’s why throughout this article I keep calling stroke volume “the engine” — it’s the core component you can genuinely reshape and enlarge through training.

Real Numbers from Three Groups

Here I’m citing actual measured data so you can get a concrete feel for “how big the gap is.” According to a study comparing elite distance runners, collegiate runners, and untrained students, the differences among the three groups at maximal exercise intensity are striking:

Group	Max Stroke Volume	Max Cardiac Output
Elite distance runners	~187 mL/beat	~33.8 L/min
Collegiate runners	~145 mL/beat	~26.3 L/min
Untrained students	~128 mL/beat	~21.3 L/min

See the key point? Elite athletes’ max heart rates are actually about the same as ordinary people’s — often even lower. But their stroke volume is a full 50% higher. That extra blood is what allows them to sustain oxygen delivery at high intensity and hold their pace. In other words, an endurance athlete’s heart is strong because it “pumps more per beat,” not because it “beats faster.”

Some textbooks give rougher ranges: untrained individuals’ max cardiac output falls around 15–20 L/min, generally trained individuals around 20–25 L/min, and highly trained endurance athletes can push it above 30 L/min. These are ranges, and individual variation is large, but the direction is very consistent.

Another Clue at Rest: Low Heart Rate

Many people, the first time they strap on a heart rate strap and measure their resting heart rate, get startled by the number — “How is it only 48 bpm? Is something wrong?” Actually, for regular endurance trainees, this is often a good sign.

The logic goes back to that formula. At rest, your body’s demand for blood flow is fixed (about 5 L/min is enough). If your stroke volume has increased through training, your heart only needs to beat slower to meet that demand. So a low resting heart rate is, to some extent, indirect evidence that your stroke volume has grown. Studies have compared this: endurance athletes have a resting left ventricular stroke volume of about 99 mL, while untrained individuals have about 74 mL — a clear gap.

This is also why I often ask students to measure their morning resting heart rate every day and treat it as a long-term tracking metric. As training accumulates, you’ll see this number gradually drift downward — that’s concrete evidence of the engine getting bigger. But a word of caution: a low heart rate is the result of “training adaptation,” not something you can achieve by deliberately suppressing your heart rate. And if it’s accompanied by dizziness, fatigue, or palpitations, that’s a different matter entirely — time to see a doctor.

Athlete’s Heart: What Actually Changes in the Heart

A-Kai heard this and asked me: “So what I need to train is to make my heart pump more blood per beat? Then how does the heart change?” Good question. This brings us to the concept of the “Athlete’s Heart.”

Long-term endurance training causes structural and functional adaptations in the heart. The most typical changes are an enlarged left ventricular volume, moderate thickening of the ventricular wall, and an overall more compliant heart. This change is called “eccentric hypertrophy” — the key point is that the chamber gets bigger and can hold more blood, rather than simply getting thicker and stiffer.

This is different from the cardiac adaptation direction of weightlifters and powerlifters. Pure high-intensity resistance training tends to produce “concentric hypertrophy,” where the ventricular wall thickens but the chamber doesn’t necessarily enlarge. Endurance athletes want a “big reservoir” — able to fill with more blood during diastole, so it can pump out more blood during systole.

Diastolic Function: The Real Dividing Line

Here’s a key point many people overlook. When studies compare elite endurance athletes with ordinary people, they find that the athletes’ biggest advantage actually lies in diastolic function — that is, the heart’s ability in the “relaxation and filling” phase.

One study of elite male distance runners specifically highlighted this: in generally untrained individuals, stroke volume reaches a plateau at a heart rate of about 120 bpm, and no matter how much more intensity you add, it won’t go higher. But in well-trained endurance athletes, stroke volume continues to rise along with heart rate, with almost no obvious plateau. The difference isn’t in how hard the heart contracts, but in the efficiency of ventricular “filling” — the heart can still fill quickly and completely within increasingly shorter diastolic times.

This has major implications for training. You’re not just training the force of “pumping out” — you’re also training the efficiency of “filling in.” The elasticity of the heart, the extensibility of the pericardium, the capacity for venous return, the expansion of plasma volume — all of these are part of this engine.

Why “Filling” Is So Hard at High Intensity

You might ask: since the heart’s job is to contract to pump blood and relax to fill, why is filling the bottleneck? Think about the numbers and it becomes clear. When your heart rate spikes to 180 bpm, one complete cardiac cycle lasts only about a third of a second. In that brief window, contraction and relaxation must each take their share, leaving an incredibly short relaxation time for the ventricles to “fill” with blood.

For the average person, that short filling time isn’t enough to fill the ventricles completely, so stroke volume can’t increase further. The only option is to rely on a higher heart rate to compensate—but heart rate also has a ceiling. This is why stroke volume plateaus at moderate intensity for most people.

The advantage endurance athletes develop is precisely this: even when time is compressed to the extreme, their ventricles can still fill rapidly thanks to better elastic recoil, stronger venous return, and greater blood volume. It’s an efficiency akin to “active suction,” not passively waiting for blood to flow in. Understanding this, you’ll see why endurance training emphasizes long-duration, regular aerobic stimulus—these adaptations are slow-crafted, and can’t be rushed.

Cycling vs. Running: Does the Heart Adapt Differently?

Riders often ask me: is a heart trained by cycling the same as one trained by running? Broadly speaking, any endurance sport that maintains a high cardiac output for extended periods will stimulate similar cardiac volume adaptations. Whether you cycle, run, swim, or row, as long as you regularly keep your heart working at high flow rates long enough, eccentric adaptation will occur.

There are some differences in the details. Cycling is seated, lower-limb dominant, with body weight supported by the frame, so the pattern of venous return differs slightly from running (where full body weight is borne and the up-and-down impact assists venous return); swimming involves a horizontal posture with water pressure aiding return, another scenario entirely. But for the vast majority of recreational athletes, these differences aren’t worth obsessing over. The key point is always: have you accumulated enough aerobic volume, and have you regularly stimulated a high cardiac output state? Cross-training across disciplines (e.g., cyclists running in winter, swimmers cycling on land) can actually maintain the conditioning of this cardiac engine while reducing the wear and tear of a single sport.

Plasma Volume: The Fastest and Most Overlooked Adaptation

Here I want to specifically highlight plasma volume. It’s one of the “fastest-acting” adaptations in endurance training. Within days to weeks of regular aerobic training, the body expands plasma volume, increasing total blood volume. With more blood, more returns to the heart via the veins, the ventricles can hold more during diastole, and stroke volume is naturally pushed higher.

This also explains a common phenomenon: why, after two to three weeks off, you come back and find your heart rate inexplicably spikes at the same intensity and you feel especially breathless? A large part of the reason is that plasma volume has dropped. In Taiwan’s hot, humid summers, sweat loss is high; if you don’t hydrate enough or manage electrolytes properly, plasma volume is even more susceptible, and the heart has to raise heart rate to barely maintain cardiac output—you feel tired and breathless, yet your pace isn’t necessarily faster.

The Limits of Stroke Volume: Can It Keep Increasing Forever?

This is a fascinating and still-debated topic in exercise physiology: at progressively higher exercise intensities, does stroke volume keep rising, plateau, or even decline?

Current evidence can be organized into several tiers:

• Average individuals / moderately trained: Most studies observe stroke volume plateauing at around 50% of VO₂max, or a heart rate near 120 bpm, after which cardiac output increases mainly by raising heart rate.
• Highly trained endurance athletes: Many studies show their stroke volume continues to rise during incremental intensity without a clear plateau, which is considered a major advantage.
• Controversy at maximal intensity: At extreme intensities near exhaustion, the data are inconsistent—some studies show continued increases, while others show a plateau or even a slight decline. This depends on measurement methods, subject characteristics, and posture (upright vs. supine).

One phenomenon worth special mention is the so-called “O₂ pulse plateau.” O₂ pulse can be roughly treated as an indirect indicator of stroke volume, and studies have observed that even among elite athletes with very high VO₂, a considerable proportion exhibit an O₂ pulse plateau at high intensity. In other words, stroke volume is highly trainable, but it is ultimately not unlimited—it’s constrained by a combination of cardiac structure, filling time, and blood volume.

I often tell my athletes: don’t make “infinitely increasing stroke volume” your goal—that’s unrealistic. What you should do is train “your own” stroke volume close to the ceiling allowed by your genetics and training history, and delay the point at which it plateaus as much as possible.

Practical Methods: How to Build This Engine

Theory aside, let’s talk about what you can actually do. To effectively stimulate cardiac volume adaptation and diastolic function, the core logic is: use “extended moderate-to-high intensity intervals” to keep the heart in a high cardiac output state long enough, and accumulate sufficient low-intensity aerobic volume as a foundation.

The zone chart below is a simplified version I use with my athletes. Heart rate zones are expressed as “percentage of maximum heart rate” because it’s the most intuitive; those with a power meter can map it to their FTP.

Zone	% Max HR	Perceived Exertion	Primary Training Goal
Zone 1 Recovery	50–60%	Very easy, can sing	Promote recovery, warm-up
Zone 2 Aerobic Base	60–70%	Can converse normally	Expand plasma volume, mitochondria, capillaries
Zone 3 Tempo	70–80%	Talking becomes effortful	Improve stroke volume, aerobic endurance
Zone 4 Threshold	80–90%	Can only speak short phrases	Raise maximal cardiac output, lactate threshold
Zone 5 Maximal	90–100%	Barely able to speak	Approach VO₂max, maximal cardiac work

Two Primary Workouts Targeting Stroke Volume

Many people think training the heart means hammering intervals. In fact, to stimulate cardiac volume adaptation, long-duration tempo riding in Zone 3 is often underrated. At this intensity, cardiac output is already high enough, yet it can be sustained for a relatively long time, repeatedly placing the heart in a “high-flow, prolonged filling” state.

Here are two workouts I commonly prescribe to my athletes:

Workout A: Aerobic Base Long Ride (suitable for accumulating volume during the week)

Segment	Duration	Intensity
Warm-up	15 min	Zone 1–2
Main set	90–150 min	Zone 2, steady without fading
Cool-down	10 min	Zone 1

This workout may look boring, but it’s the cornerstone for expanding plasma volume and increasing capillary density. Taiwan’s riverside bike paths (the Xindian River and Dahan River trails in Greater Taipei, or the Love River route in Kaohsiung) are ideal because there are few traffic lights, allowing you to maintain a steady heart rate without constant interruptions.

Workout B: Threshold Intervals (to push maximal cardiac output, suitable for weekends)

Segment	Duration	Intensity
Warm-up	20 min	Zone 1→2 progressive
Main set	4 × 8 min	Zone 4 (4 min Zone 1 rest between sets)
Cool-down	15 min	Zone 1

These 8-minute threshold intervals force the heart to work repeatedly near maximal cardiac output, making them one of the most efficient ways to stimulate cardiac functional adaptation. However, the intensity is high and recovery demands are significant—one to two sessions per week is sufficient.

How to Structure Your Week

A training plan alone isn’t enough—the order of sessions is where the art lies. Using a recreational rider with a solid base who can train 5 days a week as an example, here’s how I’d structure it:

Day	Content
Monday	Rest or stretching
Tuesday	Workout B: Threshold intervals
Wednesday	Zone 2 easy aerobic 60–90 min
Thursday	Rest or core/strength
Friday	Zone 3 tempo ride 60 min
Saturday	Workout A: Long ride
Sunday	Zone 1 recovery ride or rest

Key principle: There must be breathing room between high-intensity days and long-ride days, and low-intensity volume should far exceed high-intensity volume. This is the well-known “80/20 principle”—roughly 80% of training volume falls in easy Zone 1–2, and 20% falls in hard Zone 4–5. Most of the heart’s volume adaptation comes from that 80%, not from the all-out 20%.

Practical Considerations in the Taiwanese Context

Applying this framework to Taiwan’s real-world environment brings up several local nuances worth clarifying.

First is weather. Taiwan’s summers are hot and humid. At the same intensity, heart rate will run higher than in cooler conditions—this is a normal response as the body shunts blood to the skin for cooling, forcing the heart to pump harder. So when controlling intensity by “heart rate” in summer, be prepared: a higher heart rate doesn’t necessarily mean you’re working harder; it might just be the heat. In this case, I’d suggest those with a power meter switch to power-based intensity control, or simply move high-intensity sessions to cooler times like early morning or evening. Doing threshold intervals under the midday sun rarely yields quality work and increases heatstroke risk.

Second is terrain. Taiwan’s urban areas are full of traffic lights, making it hard to maintain a steady, continuous heart rate. This is particularly detrimental to tempo rides and long rides that require “sustained, stable high cardiac output.” So I strongly recommend scheduling these sessions on riverside bike paths or suburban mountain roads. Places like the Greater Taipei riverside paths, Fengguizui, and the Beiyi Highway; Dadu Mountain and Route 139 in central Taiwan; Meinong and Qishan in the south—all are excellent options for uninterrupted riding. A smart trainer is the option least affected by weather or road conditions, especially suited for precise interval workouts.

Third is recovery and lifestyle. Taiwanese office workers generally work long hours and don’t get enough sleep, which directly compresses the recovery needed for cardiac adaptation. Rather than forcing in more training volume, prioritize sleep first. Pushing high-intensity work while under-recovered not only fails to produce adaptation but also accumulates fatigue and raises injury risk. Your training plan should serve your life, not the other way around.

A Complete Case Study: A-Kai’s Six-Month Engine Project

Tables alone can feel abstract, so let me walk you through A-Kai’s six-month overhaul to give you a clearer picture.

A-Kai’s initial problem was classic: almost every ride sat in the same “grey zone”—he’d head out wanting to push the pace, with heart rate mostly in the 78–85% of max (upper Zone 3 to Zone 4). Three weekday rides plus one long weekend ride sounded like decent volume, but his performance had been stuck for nearly a year.

I first laid out his training distribution. The picture was clear: roughly 70% of his time was spent in moderate-to-high intensity, with almost no true Zone 2, and no properly executed threshold intervals. This is the classic “missing out on both ends.”

We split the adjustment into three phases:

Phase	Weeks	Focus	Main Content
Base	Weeks 1–8	Dig the big reservoir	Lots of Zone 2, one tempo ride per week, high intensity temporarily removed
Build	Weeks 9–16	Push the ceiling higher	Maintain Zone 2 volume, add 1–2 threshold interval sessions per week
Sharpen	Weeks 17–24	Integrate performance	Add a small amount of VO₂max intervals, taper before race day

The hardest part of the base phase was psychological. For the first two weeks he kept messaging me: “Coach, is riding this slow really working? I don’t even feel like I’m sweating.” I asked him to hold back, keep his heart rate around 65%, and focus purely on accumulating time. By week six, he noticed it himself: on the same warm-up stretch of Fengguizui, the pace that previously required 145 bpm now only needed 135 bpm. That’s direct evidence of plasma volume expansion and increased stroke volume—the engine got bigger, so the same output requires a lower heart rate.

In the build phase, we brought back threshold intervals, this time with quality. Because the base was solid, he could hold a stable power output during 4×8 min Zone 4 efforts, rather than blowing up after the first two sets. His lactate threshold clearly shifted upward during this phase.

In the sharpen phase, we added a small amount of short, sharp VO₂max intervals (e.g., 5×3 min near maximal effort) to squeeze out the engine’s top-end rpm, then tapered one to two weeks before race day to let accumulated fatigue dissipate and allow the adaptations to “surface.”

After six months of testing, his heart rate at the same power was a full ~10 bpm lower, and his ability to hold pace on long rides had improved dramatically. There was no magic in this case—only one thing done right: respecting easy volume, patiently accumulating, then applying high-intensity stimulus with quality.

Common Mistakes and Fixes

In all my years coaching athletes, I’ve seen enough mistakes to fill a book. Here are the ones most damaging to “heart engine” development.

Mistake 1: All training in the “in-between” grey zone

This is the most common error among Taiwanese recreational riders. Many people ride every session in the upper Zone 3 to lower Zone 4 range—not easy enough to be easy, not hard enough to be hard. The result: low-intensity volume is insufficient to effectively expand plasma volume and build the aerobic base, while high intensity isn’t sharp enough to push cardiac output limits. You miss out on both ends.

Fix: Be ruthless about making easy days truly easy. Many people feel deeply uncomfortable the first time they’re asked to ride an entire session in Zone 2, thinking, “Am I even training?” Yes, you are—you’re quietly digging the reservoir bigger.

Mistake 2: Only watching heart rate, ignoring stroke volume signals

A-Kai started out this way. A high heart rate doesn’t mean you’re training effectively, nor does it mean you’re in good shape. When you’re dehydrated, sleep-deprived, or overtired, heart rate at the same pace runs abnormally high—often a sign that stroke volume has dropped and the body is relying on a racing heart to compensate.

Fix: Learn to read the signals together. If heart rate at the same power/pace is inexplicably 5–10 bpm higher than usual and you feel unusually breathless, check sleep, hydration, and fatigue first—don’t push through. Pay extra attention in Taiwan’s summer; dehydration in hot, humid conditions directly reduces plasma volume and stroke volume.

Mistake 3: Neglecting hydration and electrolytes, letting plasma volume leak away

No matter how big the heart engine is, it can’t pump volume without enough blood. In Taiwan’s hot, humid climate, losing one to two kilograms of sweat on a two-hour ride is very common.

Fix: On long rides, replenish roughly 500–800 mL of fluid per hour; when sweating heavily or riding long, pair it with sodium-containing electrolyte supplementation. This isn’t about chugging—it’s about taking small, steady sips. Most people who eat out regularly get plenty of sodium day-to-day, but targeted electrolyte replacement is still needed after prolonged sweating.

Mistake 4: Treating “rest” as laziness

The heart’s structural adaptations happen during recovery, not while you’re riding. Continuous high intensity without recovery only accumulates fatigue, raises resting heart rate, and degrades stroke volume performance.

Fix: Schedule recovery into your plan and treat it as a formal part of training. Monitoring morning resting heart rate is a cheap, effective tool: if it runs 5–10 bpm above your baseline for several consecutive days, it usually means you’re under-recovered and should cut back.

Actionable Advice for Riders of Different Levels

If You’re a Beginner (Riding Less Than a Year)

Don’t rush into intervals or chase FTP numbers. The highest-return activity for you right now is steadily accumulating Zone 2 aerobic volume. Two to three easy rides per week, 45–90 minutes each, sustained for 8–12 weeks, will produce clear improvements in plasma volume, capillary density, and stroke volume. You’ll notice your pace increasing at the same heart rate—that’s the engine growing. Find a safe riverside bike path, wear a heart rate strap, keep your heart rate in Zone 2, and that’s it.

If You’re an Advanced Rider (1–2 Years of Foundation, Looking to Break Through a Plateau)

Take a close look at your training distribution—you’re likely stuck in the “grey zone” for too long. Try strictly adhering to the 80/20 rule: make your long rides longer and raise the quality of your high-intensity days (do proper threshold intervals instead of moderate intensity every day). At the same time, start paying attention to recovery metrics, so that every high-intensity session is done when your body is ready.

If You Want More Precision (Have a Power Meter, Want to Quantify)

Consider getting a professional VO₂max and lactate threshold test. Some sports medicine centers can even assess how your heart responds to exercise. In Taiwan, the sports medicine departments of certain hospitals, as well as professional sports science laboratories, offer these services. With objective data, your training zones will be more precise, and abnormalities can be detected earlier.

A Self-Checklist for Everyone

Check Item	Ideal State
Weekly low-intensity (Z1–2) share	Approximately 75–80%
Weekly high-intensity (Z4–5) share	Approximately 15–20%
Morning resting heart rate	Stable near your personal baseline
Hydration on long rides	500–800 mL per hour
Recovery after high-intensity days	At least one day of low intensity or rest in between

Frequently Asked Questions (FAQ)

These are the questions I’m most often asked about the heart and cardiac output when coaching athletes—compiled here for you.

Q1: Does a lower resting heart rate mean a healthier heart and better training?

Not exactly. For regular endurance athletes, a lower resting heart rate usually reflects an increased stroke volume, which is a positive adaptation. But resting heart rate is influenced by many factors—sleep, stress, caffeine, medication, and individual genetics all play a role. Instead of obsessing over the absolute number, track “your own trend”: a gradual long-term decline is a good sign; a sudden abnormal spike over a few days could mean inadequate recovery or an impending cold. If your resting heart rate is extremely low and is accompanied by dizziness, fatigue, or palpitations, be sure to seek medical evaluation.

Q2: Can I quickly enlarge my heart by doing only high-intensity intervals?

High-intensity intervals can indeed effectively stimulate maximal cardiac output, but doing “only” high-intensity work is the wrong path. Heart chamber adaptation and plasma volume expansion rely heavily on large volumes of low-intensity aerobic stimulation. Relying solely on high intensity not only gives you an incomplete adaptation profile, but also makes it very easy to accumulate fatigue and increase the risk of injury and overtraining. Think of it this way: low-intensity volume is responsible for digging the pond bigger, while high intensity is responsible for training the pump’s maximum RPM—both are indispensable.

Q3: I’m older (50+). Can I still develop these adaptations?

Yes. The heart’s plasticity in response to training exists across all age groups. Through regular endurance training, middle-aged and older adults can still expand blood volume and improve cardiac function and aerobic capacity. The difference is: you need to start more gradually, allow more recovery, and progress may be slower. Moreover, before middle-aged and older adults begin high-intensity training, they should first undergo cardiovascular evaluation. Under Taiwan’s National Health Insurance, arranging an exercise ECG or echocardiogram is not difficult—confirm safety first, then train with confidence.

Q4: Are the “cardiac output” or “stroke volume” readings from my watch accurate?

Currently, most consumer wearable devices cannot directly and accurately measure stroke volume or cardiac output; those numbers are mostly estimates derived from data like heart rate. They have some reference value for looking at “long-term trends,” but don’t treat them as laboratory-grade absolute values. For truly accurate measurement, you need professional testing (such as echocardiography or gas exchange analysis).

Q5: Which matters more for performance—training the heart or training the legs?

This is a false dichotomy. Aerobic performance is the result of both central (heart delivering oxygen) and peripheral (muscles utilizing oxygen) factors working together. The heart delivers the oxygen, while the mitochondria and capillary density in the muscles are responsible for using it—both need to be trained. The good news is: large volumes of regular aerobic training stimulate both central and peripheral adaptations simultaneously. You don’t need to choose between the two.

An Important Reminder: Don’t Ignore the Heart’s Signals

An “athlete’s heart” is a normal, healthy training adaptation. But the heart is still the heart—some signals must never be dismissed as “training hard enough”: chest tightness or pain during exercise, unusual breathlessness, dizziness, a feeling of near-fainting, or irregular, erratic palpitations—none of these are normal training responses.

If you experience these symptoms, or if you have high blood pressure, diabetes, or a family history of heart disease, be sure to consult a physician and undergo appropriate cardiovascular evaluation before starting high-intensity training. With Taiwan’s National Health Insurance, seeing a cardiology clinic, getting an exercise ECG, or an echocardiogram is relatively easy to arrange—don’t delay because of inconvenience. Distinguishing between “benign athlete’s heart adaptations” and “heart problems that need attention” is the job of a professional physician, not something to self-diagnose based on numbers from your watch.

Conclusion: Enlarging the Engine Is a Project of Patience

Back to A-Kai’s story. He later spent a full season honestly executing the 80/20 rule—easy days were truly easy, weekend threshold intervals were done properly, and he hydrated and slept well. Six months later, he came back for testing: at the same power output, his heart rate was noticeably lower, and his ability to hold pace on long distances had improved dramatically. He said to me with a smile: “Turns out what I needed to train wasn’t beating faster, but pumping more with each beat.”

That’s the core lesson cardiac output and stroke volume offer us. A large part of your endurance ceiling is written in whether your heart can pump more blood per beat and keep filling and delivering oxygen under high intensity. This engine can be trained, but it doesn’t require brute force—it requires patience, consistency, and respect for “easy volume.”

Next time you stare at the heart rate number on your bike computer, think of the heart behind it, silently pumping blood out again and again. Train it to be bigger, and every ride you take will feel easier, go farther, and be faster.

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This article is educational content and cannot replace individual diagnosis and treatment advice from a physician, physical therapist, or nutritionist.

References

• Endurance athletes’ stroke volume does not plateau: major advantage is diastolic function（PubMed）: https://pubmed.ncbi.nlm.nih.gov/7808245/
• Does Stroke Volume Increase During an Incremental Exercise? A Systematic Review（PMC）: https://pmc.ncbi.nlm.nih.gov/articles/PMC4896996/
• Athlete’s Heart Revisited: Historical, Clinical, and Molecular Perspectives（Circulation Research）: https://www.ahajournals.org/doi/10.1161/CIRCRESAHA.125.325638
• Cardiopulmonary Exercise Testing in Athletes: Pearls and Pitfalls（American College of Cardiology）: https://www.acc.org/Latest-in-Cardiology/Articles/2021/04/13/13/12/Cardiopulmonary-Exercise-Testing-in-Athletes

Related Reading

• Cardiac Output and Cycling Performance: Training Stroke Volume, Heart Rate, and Cardiac Output
• Training Adaptations in Cardiac Output and Stroke Volume: The Evolution of the Athlete’s Heart
• The Limitation of Cardiac Output on Endurance Performance: Research on Stroke Volume Trainability
• Cardiopulmonary Function and Cycling Power Output: The Relationship Between Heart Size and VO₂max