Cardiac Output and Stroke Volume Training Adaptations: The Evolutionary Path of the Athlete’s Heart

Introduction

Maximal oxygen uptake (VO₂max) is the gold standard indicator of endurance performance, and the most important factor determining VO₂max is not the lungs’ gas exchange capacity, nor merely the muscles’ oxygen utilization efficiency—but rather the heart’s ability to pump oxygenated blood to the body, namely cardiac output (Q̇). According to the Fick equation:

VO₂ = Q̇ × (CaO₂ - CvO₂)

where cardiac output (Q̇) = heart rate (HR) × stroke volume (SV). This article explores in depth how long-term endurance training remodels the heart, enhancing stroke volume and maximal cardiac output.

Basic Physiology of Cardiac Output

At Rest

The resting cardiac output of an average adult is approximately 5 L/min:

• Heart rate: ~70 bpm
• Stroke volume: ~70 mL

During Maximal Exercise

Parameter	Average Person	Trained Athlete	Elite Endurance Athlete
Maximal heart rate	~195 bpm	~190 bpm	~185 bpm
Maximal stroke volume	~100 mL	~140 mL	~180-200 mL
Maximal cardiac output	~20 L/min	~27 L/min	~35-40 L/min

Note a key fact: the maximal heart rate of elite athletes is typically not higher than that of the average person (and may even be lower), yet their cardiac output is nearly double. The difference comes entirely from a substantial increase in stroke volume.

Athlete’s Heart

Long-term endurance training induces a series of structural and functional adaptations in the heart, collectively referred to as the “athlete’s heart.”

Ventricular Structural Remodeling

Eccentric Hypertrophy

The characteristic adaptation produced by endurance training (such as long-distance cycling) is left ventricular eccentric hypertrophy:

• Increased chamber volume: Left ventricular end-diastolic volume (LVEDV) can increase from ~120 mL in the average person to 170-220 mL
• Moderate thickening of the ventricular wall: Wall thickness increases to maintain normal wall stress (Laplace’s law)
• Normal or low wall thickness-to-chamber diameter ratio: Distinguishes it from pathological hypertrophy

This remodeling is the heart’s adaptation to repeated volume overload—during exercise, venous return increases, and the ventricle must handle a greater preload.

Comparison with Resistance Training

Pure high-resistance training (such as weightlifting) tends to produce concentric hypertrophy:

• The ventricular wall thickens but chamber size changes little
• Primarily a response to pressure overload
• The increase in stroke volume is smaller

Cycling combines endurance and resistance elements (especially climbing), so cyclists’ cardiac adaptations often present a mixed pattern.

Enhanced Frank-Starling Mechanism

Increased ventricular volume directly extends the operating range of the Frank-Starling mechanism:

1. Greater end-diastolic volume → myocardial fibers are stretched to a longer initial length before contraction
2. Longer initial length → actin-myosin overlap approaches optimal → stronger contractile force
3. Stronger contractile force → greater ejection fraction → higher stroke volume

This is like a longer rubber band with greater elastic potential energy to release.

Improved Ventricular Compliance

A well-trained heart exhibits better diastolic function:

• Faster active relaxation: Improved sarcoplasmic reticulum calcium reuptake efficiency
• Increased passive compliance: Better ventricular wall elasticity, allowing more rapid filling
• More efficient use of diastolic filling time: Maintains adequate filling volume even at high heart rates

This is especially important during high-intensity exercise, because a high heart rate means a greatly shortened diastole.

Dynamic Changes in Stroke Volume During Exercise

Classic View vs. Modern Understanding

Traditionally, stroke volume was thought to reach a plateau at 40-50% VO₂max, after which increases in cardiac output rely entirely on heart rate elevation. However, studies of well-trained athletes reveal a different picture:

• Elite athletes’ SV may continue to increase up to near-maximal exercise intensity
• This is related to a stronger Frank-Starling mechanism and better diastolic filling
• It may also involve a more effective muscle pump action, increasing venous return

Factors Affecting Stroke Volume

Preload

• Plasma volume: Training increases plasma volume → increased venous return
• Muscle pump: Leg muscle contractions squeeze blood back to the heart
• Body position: Venous return in an upright posture (such as cycling) is lower than in a supine position

Afterload

• Systemic vascular resistance: During exercise, vasodilation in active muscles lowers total peripheral resistance
• Arterial compliance: Training improves arterial elasticity, reducing afterload

Myocardial Contractility

• Sympathetic activation: During exercise, adrenaline increases myocardial contractility
• Training adaptation: Improved calcium handling efficiency in cardiac muscle cells

Time Course of Training-Induced Cardiovascular Adaptations

Cardiovascular adaptations are not achieved overnight:

Adaptation	Onset	Significant Improvement	Near Maximum
Plasma volume expansion	Days	1-2 weeks	2-4 weeks
Resting heart rate reduction	1-2 weeks	4-8 weeks	Months
Stroke volume increase	2-4 weeks	8-12 weeks	1-2 years
Left ventricular remodeling	Weeks	Months	2-5 years
Maximal cardiac output increase	4-8 weeks	Months	Years

It is worth noting that plasma volume expansion is one of the earliest adaptations to appear—it can increase preload and thereby elevate SV in the early stages of training, even before significant structural remodeling of the heart has occurred.

Resting Heart Rate and Heart Rate Reserve

Well-trained cyclists often have resting heart rates as low as 35-50 bpm (some elite athletes even below 30 bpm), resulting from a combination of factors:

• Increased parasympathetic tone: Enhanced vagal nerve activity
• Reduced intrinsic firing rate of the sinoatrial node: Adaptation of cardiac automaticity
• Increased stroke volume: With unchanged resting demands, a larger SV means fewer heartbeats are needed

A lower resting heart rate provides a greater heart rate reserve (maximal heart rate − resting heart rate), theoretically allowing a wider range of cardiac output regulation.

Detraining Effects

After cessation of training, the regression of cardiovascular adaptations also follows its own time course:

• Plasma volume: Declines significantly within 1-2 weeks of detraining (the fastest adaptation to regress)
• Stroke volume: Decreases noticeably within 2-4 weeks
• Maximal cardiac output: Begins to decline within 2-4 weeks
• Ventricular structure: Takes months to years to fully reverse

This explains why even a brief training interruption can cause a noticeable drop in performance—the rapid loss of plasma volume directly affects stroke volume.

Practical Implications

Training Design

1. Importance of the aerobic base: Large volumes of low-intensity training are the fundamental stimulus for ventricular remodeling
2. Patience and consistency: Structural adaptations of the heart require years of accumulation
3. Avoid prolonged detraining: Even during tapering periods, maintain a minimum training volume to preserve cardiovascular adaptations
4. A multi-year perspective: An elite athlete’s heart is the product of years of training, not something that can be built in a single season

Monitoring Indicators

• Morning resting heart rate: Long-term trends reflect cardiovascular adaptation status
• Submaximal exercise heart rate: A lower heart rate at the same power output indicates improved cardiovascular efficiency
• Heart rate recovery: The rate of heart rate decline after exercise reflects parasympathetic function

Conclusion

The increase in cardiac output—especially through an increase in stroke volume—is the most central cardiovascular adaptation to endurance training. From the expansion of ventricular volume to the improvement of diastolic function, from plasma volume expansion to the enhancement of the Frank-Starling mechanism, every component contributes to supplying more oxygen for your next all-out sprint. This is a cardiac remodeling project that spans years, and every training session is an indispensable step along the way.

Related Reading

• The Limitation of Cardiac Output on Endurance Performance: A Study on the Trainability of Stroke Volume
• The Ceiling of VO2max: Does Cardiac Output or Muscle Oxygen Utilization Determine the Upper Limit
• Cardiopulmonary Function and Cycling Power Output: The Relationship Between Heart Size and Maximal Oxygen Uptake
• Cardiac Output and Stroke Volume: The Engine of Endurance Sports—How Much Has Your Heart Actually Been Trained?