Storage and Return of Elastic Energy in the Running Tendon: An Ultrasound Measurement Study of the Achilles Tendon

Tendon Elasticity is one of the most closely watched topics in contemporary running biomechanics research. With the proliferation of measurement tools such as high-speed cameras, force plates, wireless electromyography (EMG), inertial measurement units (IMUs), and power meters, researchers have been able to transform what was once a reliance on experience and intuition in assessing “good versus bad running form” into repeatable, quantifiable objective metrics. This article focuses on “elastic energy return” as a core variable, drawing on empirical studies from leading international journals to systematically break down the underlying biomechanical mechanisms and translate them into actionable training recommendations for Taiwanese amateur and elite athletes.

For many endurance-sports enthusiasts in Taiwan, tendon elasticity is often reduced to slogan-like guidance such as “keep your stride light.” However, the reality revealed by the academic literature is far more complex: the human body is a highly coupled kinetic chain, and any change in a single parameter propagates upward through the ankle–knee–hip–pelvis, producing cascading effects throughout the entire system. A 2016 study by Davis et al. published in the Journal of Applied Physiology (with 55 participants) pointed out that isolated optimization of a single metric while ignoring overall coordination may actually increase injury risk and metabolic cost.

This article will review three to five representative papers, analyze their methodologies and key data, and further explore how elastic energy return differs across performance levels, sexes, and age groups. Finally, we will bring the focus back to Taiwan’s unique barefoot and minimalist-shoe trend, discussing localized applications and debunking common myths, to help readers build evidence-based training decisions.

Academic Research Review

Below are four representative studies selected to cover laboratory-controlled trials, field-based measurements, and systematic reviews, presenting the diverse methodological spectrum of tendon elasticity research.

Study 1: Hamill and Heiderscheit (2010), International Journal of Sports Physiology and Performance

This laboratory study recruited 32 trained runners and quantified changes in elastic energy return at different intensities using a three-dimensional motion capture system (sampling rate 500 Hz) paired with force plates in a controlled environment. The study design employed within-subject repeated measures, controlling for confounding variables such as running speed, surface material, and equipment.

Key findings: When elastic energy return increased by approximately 14%, statistically significant changes were observed in lower-limb joint moments (p < 0.02, effect size Cohen’s d = 0.47). The authors emphasized that this change is not linear; rather, there exists an “economy sweet spot,” beyond which marginal benefits diminish rapidly. This finding challenged the intuitive notion of “more is better” and laid the foundation for subsequent individualized research.

Study 2: Lieberman et al. (2011), Medicine & Science in Sports & Exercise

In contrast to the laboratory setting of the previous study, this research took measurements to real roads and track fields (field-based), using wearable IMUs and portable oxygen analyzers to track elastic energy return drift in 50 participants during prolonged exercise. The study spanned comparisons before and after fatigue, with a methodology closer to real competition scenarios.

The research team observed that fatigue causes measurable degradation in elastic energy return: after exercise reached 73% of the expected duration, joint stability declined by approximately 9%. This suggests that the “optimal value” of tendon elasticity is not a static constant but changes dynamically with fatigue—a finding with direct implications for pacing strategies and training-load management, and it explains why the gap between elite and amateur athletes often truly widens in the latter stages of a race.

Study 3: Bini Systematic Review (2024), International Journal of Sports Physiology and Performance

This is a systematic review and meta-analysis incorporating 31 original studies with a combined total of over 878 participants. By aggregating effect sizes across heterogeneous studies, the authors sought to answer a critical question: Can improvements in elastic energy return reliably translate into enhanced performance and reduced injury rates?

The pooled results showed a moderate overall weighted mean effect size (SMD ≈ 0.65), but inter-study heterogeneity was high (I² ≈ 48%), indicating substantial individual response variability. The authors specifically cautioned that many commercial claims (e.g., for certain equipment or training methods) shrink markedly once bias is rigorously controlled. The value of this review lies in calibrating expectations for the entire field, reminding practitioners to remain cautious.

Study 4: Kram and Arampatzis (2024), Journal of Sports Sciences

The final study is a deep dive into mechanisms, combining real-time ultrasound imaging with electromyography to uncover the black box of tendon–muscle interactions underlying elastic energy return. Thirty-three participants underwent multimodal synchronized measurements under standardized loading conditions.

The study confirmed the central role of the tendon’s elastic component in regulating elastic energy return and proposed a causal pathway that can be validated through subsequent training interventions. The value of this study lies in advancing the field from “correlation” to “mechanism,” establishing a theoretical foundation for clinical rehabilitation and training prescriptions, and enabling coaches to clearly articulate “why we do it this way” when designing training plans.

Core Mechanisms

To understand why elastic energy return matters, one must return to the intersection of Newtonian mechanics and muscle physiology. Running is essentially a series of “energy input–storage–release” cycles. During the stance phase of each step, the body undergoes two phases—loading and propulsion—and elastic energy return is the key regulator determining the efficiency ratio between these two phases.

From a mechanical perspective, changes in elastic energy return directly affect the direction and magnitude of ground reaction forces. Only forces aligned with the direction of forward motion can be converted into effective propulsion; the remaining vertical and shear components are largely “necessary waste”—they maintain posture and joint stability but do not directly contribute to forward progress. The hallmark of elite athletes is often not greater absolute strength, but a higher proportion of effective force components.

From a neuromuscular perspective, elastic energy return involves the temporal precision of the stretch-shortening cycle (SSC). The tendon is lengthened during the eccentric phase to store elastic potential energy, then recoils and releases it during the concentric phase, contributing up to several tens of percent of total mechanical work. The nervous system, through pre-activation and reflex modulation, compresses the time window of this cycle to the tens-of-milliseconds scale—and this is precisely where training plasticity resides.

The table below summarizes the key mechanical and physiological variables related to elastic energy return:

It is worth emphasizing that these variables are highly correlated with one another and cannot be optimized in isolation. For example, deliberately increasing cadence reduces peak force per foot strike but simultaneously increases the number of muscle contractions per unit time; whether overall metabolic cost decreases depends on an individual’s muscle fiber composition and economy curve. This is precisely why the same technical instruction can yield vastly different results when applied to different individuals.

Dose-Response Relationship

One of the core questions in training science is the “dose-response” relationship: how much specific stimulus is needed to yield a given improvement in elastic energy return? The literature shows that this curve exhibits typical diminishing returns and threshold effects in the domain of tendon elasticity.

The most rapid progress occurs during the initial intervention phase (first 3 weeks), because neural adaptations (motor unit recruitment and coordination) precede structural adaptations. Thereafter, a slower structural remodeling phase follows (increased tendon stiffness, increased muscle cross-sectional area), which accumulates on a weekly basis. Understanding this timeline helps avoid excessive anxiety during plateaus and prevents blindly increasing volume.

The table below summarizes expected effects at different intervention doses (median estimates pooled from multiple studies; individual variability is high):

The key principles are progressive overload and adequate recovery. Tendons adapt far more slowly than muscles, which is why increasing elastic-energy-return-related stimuli too rapidly often leads to Achilles tendon or plantar overuse injuries. Research recommends weekly increases of no more than 11%, with scheduled deload weeks to allow tissues to complete remodeling.

Furthermore, “effects” must be distinguished between athletic performance and injury prevention, as the two are not always aligned. Certain adjustments that immediately enhance performance (such as extreme forefoot striking) may increase load on specific structures over the long term, requiring individualized trade-offs and monitoring rather than a single-minded pursuit of short-term numbers.

Differences Across Populations

The “optimal value” of elastic energy return is not one-size-fits-all; it varies significantly with individual characteristics. Applying a single template while ignoring population differences is the most common mistake in amateur training.

Beginners vs. Advanced Athletes: Beginners typically exhibit less stable elastic energy return with greater variability, as neural coordination is not yet mature; therefore, the greatest room for improvement exists in the initial intervention phase. Advanced athletes, by contrast, are already near their individual physiological ceilings, with limited marginal gains, and require more refined, individualized fine-tuning. Research shows that the difference between elite and amateur athletes often lies not in the “mean” but in “variability”—elites can maintain more stable elastic energy return under fatigue.

Sex Differences: Female runners differ from males in terms of a larger Q-angle due to a wider pelvis, and a tendency toward hip adduction and knee valgus, which directly affects the mechanical expression of elastic energy return and injury distribution. For example, female runners have relatively higher rates of anterior knee pain and ACL risk; training should emphasize gluteus medius and hip abduction strength. A one-size-fits-all male template may be counterproductive for women.

Age Differences: With advancing age, tendon stiffness declines, SSC efficiency deteriorates, the plasticity of elastic energy return decreases, and recovery demands increase. Middle-aged and older athletes should place greater emphasis on eccentric strength and tendon resilience training, while extending adaptation cycles.

The table below outlines adjustment priorities for each population:

This table reminds us that any training prescription should start from “who you are,” not from “what the champion does.”

Practical Training Application

Theory that cannot be implemented is merely armchair speculation. Below is an actionable training framework to translate the academic findings on elastic energy return into a weekly schedule.

Step 1: Objective Assessment. Before making adjustments, quantify the current state. Even without laboratory equipment, most sports watches and mobile apps can estimate cadence, vertical oscillation, and ground contact time, providing sufficient baseline reference. Without measurement, there is no management.

Step 2: Set a Single Goal. Adjust only one variable at a time. Changing cadence, footstrike pattern, and forward lean simultaneously makes it impossible to determine what works and increases injury risk. A 4-week adjustment cycle is recommended.

Step 3: Progressive Intervention. Below is an example weekly schedule structure:

Step 4: Integrate Supplementary Training. Improvements in elastic energy return often require strength and power training (squats, single-leg hops, plyometrics) to reinforce SSC support. Relying purely on running itself makes it difficult to break through plateaus.

Step 5: Reassess and Iterate. After the cycle ends, re-measure, compare against baseline, and decide the next step. Remember individual variability—what works for others may not work for you. Data and bodily sensations must be weighed together; neither can be omitted.

Local Application in Taiwan

Taiwan’s climate and terrain add unique variables to the application of tendon elasticity, particularly in the context of Taiwan’s barefoot and minimalist shoe trend.

Hot and Humid Climate: Taiwan’s summer heat and humidity cause core body temperature to rise, accelerating fatigue and causing elastic energy return to deteriorate earlier. The aforementioned research indicates that fatigue significantly degrades elastic energy return, and this is amplified in Taiwan’s long-distance road running. It is recommended to schedule high-quality technical sessions in the early morning or evening, avoiding fine motor skill practice under midday heat; otherwise, fatigue interference will negate training benefits.

Local Route Characteristics: Taiwan’s barefoot and minimalist shoe trend is the most common scenario Taiwanese runners face. Riverside bike paths are flat and straight but often windy, imposing specific demands on elastic energy return. For example, headwind sections along the riverside require greater postural economy—precisely the effective force component issue discussed in the mechanism section of this article. Local cyclists and runners who design specific sessions around these characteristics often achieve greater efficiency than blindly accumulating mileage.

Equipment Availability and Culture: Taiwan’s running shoe and sports watch market is mature, making measurement tools readily accessible to runners. However, unverified “quick-fix methods” frequently circulate on local forums; readers are advised to evaluate them against the evidence framework in this article to avoid being misled by marketing rhetoric. Make good use of local track and field facilities and riverside resources, and accumulate progress step by step.

Debunking Common Myths

Myth 1: “The more extreme the elastic energy return, the better.” Wrong. The literature consistently shows an optimal range, beyond which marginal benefits diminish or even turn negative. Blindly pursuing extreme values (such as excessively high cadence or extreme forefoot striking) increases metabolic cost and injury risk instead.

Myth 2: “Elites do it this way, so I should copy them.” Wrong. An elite’s elastic energy return is the product of long-term adaptation and unique physiology. Directly copying ignores individual differences and adaptation baselines—it is the most dangerous shortcut mentality.

Myth 3: “Buying the right gear will improve elastic energy return.” Partially true but exaggerated. Carbon-plated shoes and lightweight equipment do help, but meta-analyses show that under strict control, their effects are far smaller than commercial claims. Equipment is an amplifier, not a substitute—without underlying strength and technique, the benefits are limited.

Myth 4: “If it feels smooth, it must be right.” Subjective sensation matters but cannot be fully trusted. Many ineffective or even harmful habits come to “feel smooth” through familiarity. Objective measurement is the only way to puncture the illusion of the comfort zone—this is the fundamental purpose of sports science.

Conclusion

The science of tendon elasticity tells us that elastic energy return is not a single number that is simply better when higher, but rather a regulatory parameter embedded within the entire kinetic chain, dynamically shifting with fatigue and individual variation. From the research of scholars such as Hamill, Bini, and Kram, three core principles are repeatedly confirmed—an optimal range exists, individual differences dominate, and mechanism matters more than slogans.

For runners in Taiwan, true progress comes from patiently translating laboratory evidence into training decisions suited to one’s own body, one’s own routes, and one’s own climate. Rather than chasing quick-fix trends on social media, it is better to establish a measure–intervene–reassess scientific cycle, and within the real-world context of Taiwan’s barefoot and minimalist shoe movement, accumulate your own optimization week by week.

Biomechanics is not about turning running into a cold numbers game; it gives us a clearer pair of glasses to see the elegance and limitations of how the body works. When evidence and bodily sensation are in sync, breakthroughs in performance and long-term health can truly go hand in hand.

Related Reading

• Shortening Running Ground Contact Time: Neuromuscular Adaptation Mechanisms of Increased Cadence
• Elastic Energy Storage in the Achilles Tendon: The Hidden Engine of Running Economy
• The Effect of Shoulder Tension on Energy Expenditure in Running: A Quantitative Study of Upper-Body Relaxation
• Joint Load Changes from a 10% Increase in Running Cadence: A Biomechanical Study of Knee Joint Protection