Individual Variability in Endurance Training: A Genetic Analysis of the "Non-Responder" Phenomenon

The same training plan can send one athlete’s VO2max soaring while barely moving another’s—this is the “responder–non-responder” phenomenon. Large-scale studies such as HERITAGE have revealed that training response has a strong genetic basis, challenging the “one-size-fits-all” approach to training.

Based on research from leading international academic journals, this article systematically unpacks the scientific underpinnings of individual differences in training response. We will start from the methods and findings of key papers, delve into the underlying physiological mechanisms, quantify the relationship between training dose and effect, compare differences across populations, and ultimately translate these academic findings into actionable training recommendations for Taiwanese endurance athletes. This is not merely a compilation of knowledge, but a practical map leading from the laboratory to the training ground. In an era where it is difficult to distinguish truth from falsehood, returning to rigorous scientific evidence is the most worthwhile investment for any athlete who takes their training seriously.

Review of Academic Research

The most effective way to understand this topic is to directly examine representative studies from top international journals. Below is a compilation of several landmark or methodologically rigorous papers that, from different angles, collectively construct our current scientific understanding.

1. Bouchard et al. (1999, JAP)

This study used the HERITAGE Family Study. Approximately 47% of the variance in VO2max training response is heritable. The value of this study lies in its systematic approach to testing hypotheses, providing a quantifiable basis for subsequent training prescriptions and allowing us to move beyond the ambiguity of rule-of-thumb practices.

2. Timmons et al. (2010, JAP)

This study used gene markers of training response. It identified gene expression signatures that predict VO2max response. The value of this study lies in its systematic approach to testing hypotheses, providing a quantifiable basis for subsequent training prescriptions and allowing us to move beyond the ambiguity of rule-of-thumb practices.

3. Ross et al. (2015, Mayo Clin Proc)

This study used modified training to eliminate non-response. Increasing intensity or volume can elicit a response in non-responders. The value of this study lies in its systematic approach to testing hypotheses, providing a quantifiable basis for subsequent training prescriptions and allowing us to move beyond the ambiguity of rule-of-thumb practices.

4. Bonafiglia et al. (2016, J Physiol)

This study critiqued the concept of the non-responder. Individual responses must account for measurement error. The value of this study lies in its systematic approach to testing hypotheses, providing a quantifiable basis for subsequent training prescriptions and allowing us to move beyond the ambiguity of rule-of-thumb practices.

Looking across the literature above, a common trend emerges: contemporary exercise science increasingly emphasizes replacing intuition with objective data and rigorous experimental design. These studies echo and reinforce one another, converging on a consistent core conclusion that gives us greater confidence when formulating training strategies. The next section will delve deeper into the physiological mechanisms behind these phenomena.

Integration of Core Findings

Nearly half of the variance in VO2max response to standard training can be explained by genetics, indicating the existence of inherent “high responders” and “low responders.” However, a key finding is that so-called “non-responders” are often simply non-responsive to a “specific dose”—after increasing intensity or volume, or changing the training modality, most individuals still exhibit adaptation. True non-responders to all stimuli are extremely rare.

It is worth emphasizing that these findings are not isolated laboratory numbers, but robust conclusions repeatedly validated across different populations and study designs. Precisely for this reason, they can serve as the scientific cornerstone of training prescription. However, between “research findings” and “training application” lies a layer of mechanistic understanding—only by figuring out the “why” can we make correct adjustments in the face of individual differences and on-the-ground variables, rather than rigidly applying numbers. This is also the key dividing line between “executors who follow a plan mechanically” and “athletes who truly understand training”—the former merely replicate a schedule, while the latter can flexibly modify every training decision in response to their own condition, environmental changes, and race demands, maximizing the benefit of limited time and energy.

Core Physiological Mechanisms

Behind any training adaptation lies a cascade of physiological changes operating from the molecular and cellular levels up to the organ-system level. Understanding these mechanisms helps us judge which training methods truly target the limiting factors of performance and which merely add fatigue with limited benefit. The table below summarizes the key physiological mechanisms closely related to this topic and their functions:

Mechanism / Adaptation	Physiological Change	Impact on Performance
Genetic variation	Gene polymorphism	Determines response potential
Insufficient dose	Individual threshold not met	Apparent non-response
Mismatched modality	Stimulus type inappropriate	Response appears after change

These mechanisms do not operate independently but are interwoven and mutually influential within an integrated network. For example, without a simultaneous improvement in peripheral muscle metabolic capacity, the increased oxygen delivery from central cardiovascular adaptations cannot be effectively utilized; and vice versa. This “barrel effect” reminds us that a comprehensive and balanced training stimulus often yields more lasting progress than extreme focus on a single point.

More importantly, the “timing” of these adaptations varies. Some changes (such as plasma volume expansion, neural coordination) manifest within days to weeks, while others (such as cardiac structural remodeling, skeletal adaptations) require months or even years of accumulation. Understanding this temporal dimension helps us maintain reasonable expectations about training outcomes, avoiding the mistake of declaring a method ineffective before giving it sufficient time—a key reason why many people give up halfway.

Training Dose and Effect Relationship

“How much should I train?” is the question every athlete cares about most. Exercise science answers this using the concept of “dose-response”—a quantifiable relationship exists between training variables (intensity, frequency, duration, total volume) and the magnitude of adaptation, but this relationship is almost never simply linear. Understanding the shape of the dose-response curve helps us find the “sweet spot” with the highest return on investment, avoiding undertraining or overtraining.

The table below summarizes dose recommendations and expected effects under different scenarios as a practical planning reference:

Subject / Scenario	Recommended Dose	Expected Effect
Standard plan	Some show no response	Dose not individualized
Increased dose	Non-responders show response	Threshold surpassed
Changed modality	HIIT/volume adjustment	Adaptation induced

Several general principles can be drawn from the table. First, diminishing marginal returns: as fitness levels rise, the training stimulus required to achieve the same degree of improvement grows larger, which is why elite athletes’ progress is often measured in “fractions of a percent.” Second, ceiling effect: beyond a certain threshold, additional training volume not only yields sharply diminishing returns but may even backfire due to fatigue accumulation. Third, individual threshold: the minimum effective dose required to trigger adaptation differs for each person, explaining why the same plan produces vastly different results in different individuals.

Therefore, the smartest training strategy is not blindly pursuing “more,” but pursuing “just right”—providing a stimulus sufficient to trigger adaptation, paired with adequate recovery to allow the adaptation to actually occur. Periodization is designed precisely to achieve this goal: through planned fluctuations in load, it avoids linear fatigue accumulation and allows the body to peak at critical moments.

Differences Across Populations

When it comes to research on individual differences in training response, one recurring and unavoidable theme is “individual and population differences.” Applying the same conclusions indiscriminately to everyone is one of the most common errors in training prescription. Below, we analyze these differences across several key dimensions.

Beginners vs. Advanced Athletes: Beginners, being far from their physiological ceiling, respond significantly to almost any regular stimulus—this is the so-called “beginner’s bonus.” Advanced athletes, on the other hand, have limited adaptive capacity and require more precise, higher-intensity, or more varied stimuli to keep progressing. This means the optimal training strategies for the two groups are fundamentally different; advanced athletes especially need to prioritize training “quality” and “specificity” rather than simply stacking “volume.”

Men vs. Women: In absolute values (such as absolute VO₂max, muscle mass, and hemoglobin concentration), men generally exceed women, primarily due to differences in body size, hormones, and body composition. However, in “relative training response” (percentage improvement), the differences between sexes are often insignificant—women benefit just as fully from various types of training. Notably, women’s menstrual cycles, hormonal fluctuations, and energy availability (RED-S risk) need special consideration in training planning.

Age Differences: With advancing age, maximal heart rate, muscle mass, recovery speed, and hormonal environment all change. Yet extensive research confirms that even middle-aged and older populations retain the capacity to adapt to training—adaptation may simply be slower and require more adequate recovery. In other words, “it’s useless to train when you’re old” is a complete myth. Older adults, in fact, need regular training even more to combat sarcopenia, bone loss, and declining cardiorespiratory function.

Genetic Factors: Don’t forget the “responder–non-responder” phenomenon. Large family studies indicate that a considerable proportion of training response can be explained by genetics. This means that given the same training plan, some people improve rapidly while others progress slowly—often not due to insufficient effort, but to inherent differences in response potential. Recognizing this helps athletes maintain a healthier perspective on their own and others’ rates of progress, and makes them more willing to experiment with different training modes to find the stimulus that suits them.

Practical Training Application

The value of theory lies in guiding practice. Translating research findings on individual differences in training response into daily executable training requires grasping three core principles: “specificity,” “progression,” and “monitorability.”

The Specificity Principle: Training must target the energy systems and physiological adaptations required by the goal. If the goal is long-distance endurance, substantial aerobic base training is needed; if the aim is to break through VO₂max limits, targeted high-intensity interval stimuli are required. The most common problem with aimless training is falling into the “moderate-intensity black hole”—every session leaves you somewhat breathless but not intense enough, failing to effectively accumulate aerobic base while never reaching the critical high-intensity stimulus, ultimately leading to stagnation.

The Progression Principle: The body only adapts when faced with loads slightly above current capacity, but load increases must be gradual. A practical guideline is to “keep weekly training volume increases within about 10%,” and schedule a deload week every 3–4 weeks to allow accumulated fatigue to dissipate and adaptations to consolidate. Rushing progress is the number one cause of injury and overtraining in amateur athletes.

The Monitorability Principle: Replacing subjective feelings with objective data is the core of modern training. We recommend establishing the following monitoring habits:

• Morning resting heart rate and heart rate variability (HRV): These reflect recovery status and autonomic nervous system balance; an abnormally elevated resting heart rate or a sudden drop in HRV is a fatigue warning sign.
• Power or pace: Tracking output at the same intensity under standardized conditions is the most objective way to assess fitness progression.
• Subjective fatigue and sleep quality: Simple daily self-assessments capture overall status beyond the numbers.
• Periodic testing: Conduct a standardized test (such as threshold power or time trial) every 6–12 weeks to objectively evaluate training effectiveness and adjust accordingly.

Integrating these principles, a mature training plan should be “built on a large volume of low-intensity work to establish the base, elevated by a small amount of high-intensity work to raise the ceiling, consolidated by adequate recovery, and navigated by objective data.” Rather than blindly chasing mileage numbers every day, it is better to execute 1–2 high-quality sessions per week with discipline and truly relax the rest of the time—this is the essence of quality over quantity.

Local Application in Taiwan

For Taiwanese cyclists who have followed a generic training plan for months without improvement, there is no need to be discouraged or to conclude “I have no talent”—it is more likely that the dose or mode does not suit the individual. We suggest adjusting the intensity distribution, adding high-intensity stimuli, or changing the training format, while tracking responses with objective data.

Taiwan’s unique geography and climate mean that conclusions from international research must be localized before local application. The hot, humid summers, mountainous terrain, and dense, diverse racing culture are both challenges and advantages. By knowing how to leverage high-altitude resources such as Hehuan Mountain and Wuling for altitude stimulus, how to achieve heat adaptation and proper hydration and electrolyte replacement in hot, humid conditions, and how to adjust training focus according to the characteristics of Taiwanese races (such as a high proportion of climbing), Taiwanese endurance athletes can turn local conditions into a competitive advantage. Remember, any data from laboratories in temperate countries must be interpreted and applied against Taiwan’s real training environment—this is the final mile for scientific training to take root locally.

Debunking Common Myths

There is often a considerable gap between scientific findings and popular beliefs. Many “common sense” notions widely circulated in the sports community do not hold up to empirical scrutiny. Below, we debunk the common myths related to this topic:

Myth 1: No progress means no talent.

In reality, it is usually a mismatch of dose/mode, not true non-response. Blindly believing such myths wastes training time and energy at best, and leads to fatigue, stagnation, or even injury at worst.

Myth 2: One training plan works for everyone.

In reality, individual differences in training response are enormous. Blindly believing such myths wastes training time and energy at best, and leads to fatigue, stagnation, or even injury at worst.

Myth 3: Genetics determine everything and cannot be changed.

In reality, changing the stimulus can still induce adaptation. Blindly believing such myths wastes training time and energy at best, and leads to fatigue, stagnation, or even injury at worst.

The key to breaking myths lies in cultivating the habit of “demanding evidence.” Whenever you hear any training claim, ask yourself: “What research supports this? Which population does it apply to?” Only by grounding decisions in evidence can we avoid plausible-but-wrong traps in an age of information overload and make truly beneficial training decisions.

Conclusion: From Evidence to Action

Looking across the academic research on individual differences in training response, several clear conclusions emerge. First, endurance performance is the result of multiple physiological systems working in concert—no single metric or training method holds the exclusive key to success. Second, the essence of training is “precise stress plus adequate recovery,” not merely the accumulation of effort. Third, individual differences are everywhere; the best training plan is always the one “tailored to yourself and continuously adjusted based on data.”

Looking ahead, sports science is rapidly moving toward “precision individualization.” Advances in genomics, metabolomics, and wearable devices will eventually allow us to predict an individual’s response potential before training begins and fine-tune every session in real time based on physiological data. For Taiwanese athletes and coaches, building a local physiological database and developing training models adapted to the local climate and races are critical steps toward closing the gap with the world’s best.

For every reader, the most important action item remains the same: First, understand your physiological baseline through objective testing; then design your training with scientific principles; pair it with disciplined recovery and continuous monitoring; and be patient with your progress. There is no shortcut to building endurance, but there is a right direction. May this science-based analysis serve as a reliable guide on your training journey, accompanying you in pursuing your limits while also enjoying the purest joy of sport.

Related Reading

• Individual Differences in Training Adaptation: Genetic Factors Behind Performance Changes on the Same Training Plan
• Trainability of Endurance Talent: The HERITAGE Study, Responder Differences, and a Pragmatic Mindset
• Precision Sports Medicine: Personalized Training Plan Research Guided by Genomics
• Skeletal Muscle Microstructure in Endurance Athletes: Training Difference Research via Ultrastructural Analysis