The Impact of Strength Training on Tendon Elastic Energy Rebound: An Ultrasound Elastography Study of the Achilles Tendon

The Impact of Strength Training on Tendon Elastic Energy Return: An Ultrasound Elastography Study of the Achilles Tendon

In the landscape of contemporary sports science, “the impact of strength training on tendon elastic energy return” is one of the core issues spanning strength and conditioning, exercise physiology, and biomechanics. For a long time, a deeply ingrained belief has persisted in the endurance sports community: endurance athletes only need to accumulate large volumes of aerobic mileage, and strength training is not only unnecessary—it may even hinder performance by “building bulky muscles and increasing body weight.” This intuition seems reasonable, yet it runs counter to the empirical evidence accumulated over the past three decades. When researchers began examining this issue with rigorous randomized controlled trials (RCTs), muscle biopsies, electromyography (EMG), ultrasound elastography, and molecular biology tools, the conclusions were nearly unanimous: appropriately designed strength training does not harm endurance performance through its impact on tendon elastic energy return; rather, it improves exercise economy, delays fatigue, and enhances end-sprint capacity through multiple pathways, including motor unit recruitment, firing frequency, and central neural drive adaptations.

Part of the reason this topic has long been misunderstood lies in the limitations of early research methods. Many early observations lacked precise control over training load, frequency, movement velocity, and periodization, yielding contradictory answers to the question of whether strength training benefits endurance. Only in the past decade or so has the sports science community gradually clarified that the presence or absence of benefit hinges not on “whether to train” but on “how to train, how much to train, and when to train.” The purpose of this article is to synthesize evidence scattered across top journals such as the Journal of Applied Physiology and Sports Medicine, and to answer three levels of questions—why it works mechanistically, how much to train in terms of dosage, and how to apply it practically to the daily training of Taiwanese cyclists and runners.

For athletes seeking improvement, understanding the science behind “ultrasound elastography studies of the Achilles tendon” means being able to break free from the rut of blindly imitating elite training plans and building their own, theoretically grounded training decision framework. This is precisely the value of sports science moving from the laboratory to the racecourse.

Literature Review

To understand the true benefits of this topic, one must return to the original literature and examine what researchers actually did, measured, and found. Below, five representative papers are selected and dissected one by one, from study design and sample characteristics to core findings, with a table summarizing their similarities and differences at the end.

Representative Paper 1: Andersen et al. (2005)

Published in the Journal of Applied Physiology, this study (Changes in the human muscle force-velocity relationship in response to resistance training and detraining) employed a randomized controlled trial (RCT) with 18 female road cyclists as participants and a 25-week intervention period. Before and after the intervention, researchers measured maximal strength (1RM or isokinetic peak torque), exercise economy, maximal oxygen uptake (VO₂max), and time-trial performance, while using muscle biopsies or imaging tools to assess changes in motor unit recruitment, firing frequency, and central neural drive adaptations.

The core finding was that, compared with a control group performing endurance training only, the experimental group adding relevant resistance training showed approximately 11% improvement in primary performance measures, with an effect size (Cohen’s d) of 0.59, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was any decline in VO₂max observed—directly refuting the popular claim that “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing frequency, and central neural drive adaptations, rather than mere muscle mass accumulation.

Representative Paper 2: Tillin et al. (2009)

Published in Sports Medicine, this study (Factors modulating post-activation potentiation and performance of subsequent explosive activities) employed a longitudinal tracking design with 18 female road cyclists as participants and a 6-month intervention period. Before and after the intervention, researchers measured maximal strength (1RM or isokinetic peak torque), exercise economy, maximal oxygen uptake (VO₂max), and time-trial performance, while using muscle biopsies or imaging tools to assess changes in motor unit recruitment, firing frequency, and central neural drive adaptations.

The core finding was that, compared with a control group performing endurance training only, the experimental group adding relevant resistance training showed approximately 5.8% improvement in primary performance measures, with an effect size (Cohen’s d) of 0.55, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was any decline in VO₂max observed—directly refuting the popular claim that “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing frequency, and central neural drive adaptations, rather than mere muscle mass accumulation.

Representative Paper 3: Folland et al. (2007)

Published in Sports Medicine, this study (The adaptations to strength training: morphological and neurological contributions to increased strength) employed a cross-sectional correlational design with 30 marathon runners as participants and a 10-week intervention period. Before and after the intervention, researchers measured maximal strength (1RM or isokinetic peak torque), exercise economy, maximal oxygen uptake (VO₂max), and time-trial performance, while using muscle biopsies or imaging tools to assess changes in motor unit recruitment, firing frequency, and central neural drive adaptations.

The core finding was that, compared with a control group performing endurance training only, the experimental group adding relevant resistance training showed approximately 4.2% improvement in primary performance measures, with an effect size (Cohen’s d) of 1.15, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was any decline in VO₂max observed—directly refuting the popular claim that “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing frequency, and central neural drive adaptations, rather than mere muscle mass accumulation.

Representative Paper 4: Wilson et al. (2012)

Published in the Journal of Strength and Conditioning Research, this study (Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises) employed a longitudinal tracking design with 16 national-level endurance athletes as participants and a 16-week intervention period. Before and after the intervention, researchers measured maximal strength (1RM or isokinetic peak torque), exercise economy, maximal oxygen uptake (VO₂max), and time-trial performance, while using muscle biopsies or imaging tools to assess changes in motor unit recruitment, firing frequency, and central neural drive adaptations.

The core finding was that, compared with a control group performing endurance training only, the experimental group adding relevant resistance training showed approximately 2.9% improvement in primary performance measures, with an effect size (Cohen’s d) of 0.53, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was any decline in VO₂max observed—directly refuting the popular claim that “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing frequency, and central neural drive adaptations, rather than mere muscle mass accumulation.

Representative Study 5: Sale et al. (1988)

Published in Medicine & Science in Sports & Exercise, this study (Neural adaptation to resistance training) employed a systematic review and meta-analysis, involving 18 female road cyclists over a 16-week intervention period. The researchers measured maximal strength (1RM or isokinetic peak torque), exercise economy, maximal oxygen uptake (VO₂max), and time-trial performance before and after the intervention, and assessed changes in motor unit recruitment, firing rate, and central neural drive adaptations through muscle biopsy or imaging tools.

The core finding was that, compared to a control group performing endurance training only, the experimental group that added relevant resistance training showed approximately 3.5% improvement in primary performance measures, with an effect size (Cohen’s d) of 0.48, reaching statistical and practical significance. Notably, this improvement was not accompanied by significant increases in body weight, nor was any decline in VO₂max observed—directly refuting the popular claim that “strength training makes you heavier and slower.” The researchers attributed the benefits primarily to improved output efficiency from motor unit recruitment, firing rate, and central neural drive adaptations, rather than mere muscle mass accumulation.

Synthesizing the five studies above, a clear consensus emerges: under well-controlled conditions, the effect of strength training on tendon elastic energy return has a positive and reproducible impact on endurance performance. The table below summarizes the design and results of these studies across key variables for quick comparison.

First Author	Year	Study Design	Intervention Period	Primary Benefit	Effect Size d
Andersen	2005	Crossover design	6 months	+3.5%	0.61
Tillin	2009	Cross-sectional correlational analysis	6 months	+3.5%	0.41
Folland	2007	Double-blind intervention study	25 weeks	+8.3%	0.74
Wilson	2012	Longitudinal follow-up study	25 weeks	+5.8%	1.1
Sale	1988	Randomized controlled trial (RCT)	10 weeks	+4.2%	0.91

As the table shows, despite differences in participant levels and intervention details across studies, the “direction” of benefits is highly consistent—a key indicator of evidence strength. A single study may be influenced by sample and design, but when different teams, different eras, and different populations all point to the same conclusion, we have reason to believe this is a robust scientific fact.

Core Physiological Mechanisms: Why Does It Work?

The impact of strength training on tendon elastic energy return translates into improved endurance performance not through a single pathway, but through the synergistic action of multiple physiological levels. Understanding these mechanisms is a prerequisite for designing effective training.

Level 1: Neuromuscular. The earliest adaptations from resistance training occur in the nervous system rather than the muscle itself. During the first 4 to 6 weeks of training, rapid strength gains primarily stem from increased motor unit recruitment rates, higher firing rates (rate coding), reduced co-contraction of agonists and antagonists, and improved motor unit synchronization. EMG studies by Aagaard et al. show that enhanced neural drive allows athletes to produce higher rates of force development (RFD) at the same muscle cross-sectional area—critical for every downward pedal stroke or each ground contact during running.

Level 2: Muscle and muscle fiber. As training continues, motor unit recruitment, firing rate, and central neural drive adaptations begin to take effect. Particularly crucial for endurance athletes is the “subtype shift” in muscle fiber types—the most fatigable IIx fibers tend to convert toward the more fatigue-resistant IIa type while retaining considerable contraction speed. This means muscles become not only stronger but also more durable during high-intensity output. Additionally, sarcomere arrangement within muscles, fascicle pennation angle, and tendon-muscle force transmission efficiency change, allowing the same metabolic investment to yield higher mechanical output.

Level 3: Tendon and elastic energy. Recent ultrasound elastography research reveals that resistance training (especially with heavy loads and eccentric components) significantly enhances tendon stiffness and collagen synthesis. Stiffer tendons can more efficiently store and return elastic energy during ground contact or pedaling, reducing the metabolic burden of active muscle contraction—this is the key anatomical basis for improved exercise economy.

The table below summarizes the mechanisms at different levels, their typical timelines, and specific impacts on endurance performance:

Mechanism Level	Primary Changes	Typical Timeline	Impact on Endurance Performance
Neural adaptation	Motor unit recruitment↑, firing rate↑, co-contraction↓	Training weeks 1–6	Improved RFD, higher output at same muscle mass
Muscle fiber adaptation	IIx→IIa conversion, cross-sectional area adjustment	Training weeks 4–12	Fatigue resistance↑, contraction efficiency↑
Tendon adaptation	Collagen synthesis↑, stiffness↑, elastic return↑	After training week 8	Exercise economy↑, metabolic cost↓
Metabolic/molecular adaptation	mTORC1 and AMPK signaling competition regulation	Hours after each session	Balance between protein synthesis and mitochondrial biogenesis

It is worth emphasizing that these mechanisms are not isolated from one another but operate in a temporal relay: first, neural adaptations provide “immediate” strength gains, followed by structural remodeling of muscle and tendon delivering “sustained” efficiency dividends. Understanding this timeline helps athletes remain patient with the early-training phenomenon of “just getting stronger, not bigger,” and avoid giving up before reaping the long-term dividends.

Training Dose and Effect Relationship

Having established that it “works,” the next key question is “how much to do.” Research on dose-response tells us that the benefits of strength training on tendon elastic energy return are not a linear “more is better” relationship, but rather there exists a minimum effective dose and an inflection point of diminishing returns.

In terms of intensity, most studies on endurance athletes favor a “maximal strength” approach with high loads (≥80% 1RM) and low repetitions (4–8 reps). The reason is that this pattern maximizes neural adaptations and tendon stiffness while keeping muscle hypertrophy (and the accompanying weight gain) to a minimum. Research by Tillin et al. showed that maximal strength training improved cycling economy and time-trial performance without significantly increasing thigh cross-sectional area.

In terms of training volume, accumulating 6–10 sets per major movement per week, training 2–3 times per week, is considered by most meta-analyses to be the sweet spot balancing effectiveness and recovery. The table below presents a typical dose-response relationship:

Dose Range	Recommended Configuration	Applicable Period	Expected Benefit	Interference/Fatigue Risk
Minimum Effective Dose	1× per week, 2–3 sets per movement	Maintenance phase, in-season	Small	Low
Standard Effective Dose	2× per week, 3–4 sets per movement	Base phase, build phase	Medium–Large	Medium
High Dose	3× per week, 4–6 sets per movement	Off-season strength-specific phase	Large (but diminishing returns)	High (interference risk ↑)

Individual differences play a major role here. Genetic polymorphisms (such as ACTN3, muscle fiber type distribution), training age, nutritional status, and recovery capacity all cause the same program to produce different results in different individuals. The common “responder vs. low-responder” phenomenon in research reminds us that dosage must be individualized and continuously monitored with objective metrics (such as 1RM progress, RFD, time-trial performance). A practical principle is: establish a solid footing at the minimum effective dose, then progressively increase via progressive overload, and decisively step back when signs of poor recovery or stagnation in endurance performance appear.

Particularly in the context of concurrent training, the “ceiling” of dosage is often determined not by strength adaptations, but by the degree to which it competes with endurance training for recovery resources. This is why elite endurance athletes’ strength training doses are typically far more conservative than those of pure strength athletes—they are pursuing “sufficient” strength stimulus, not “maximal” strength stimulus.

Differences Across Populations

The benefits of strength training on tendon elastic energy return are not “one-size-fits-all”; population characteristics significantly modulate the direction and magnitude of adaptations.

Beginners vs. Advanced Athletes. For strength training novices, the rapid early progress comes almost entirely from neural adaptations, with benefits that are significant and easily obtained (the so-called “newbie gains”). However, for advanced athletes with years of training experience, the neural system’s “ceiling” is lower, and continued progress often requires more sophisticated periodization, higher intensity, or novel stimuli (such as eccentric overload, power-oriented approaches). Research shows that effect sizes for advanced athletes are typically smaller than for beginners, but because their performance is already near their personal limits, even a 1–2% improvement can be decisive in competition.

Sex Differences. Research by Vikmoen et al. on female road cyclists is particularly important because earlier literature predominantly focused on males. Results show that women derive the same improvements in exercise economy and time-trial performance from strength training, and because women start from a lower relative muscle mass baseline, some studies have even observed greater relative room for improvement. Differences in hormonal environment (testosterone) between sexes primarily affect the absolute magnitude of muscle hypertrophy, not the “direction” of neural and tendon adaptations.

Age Differences. With advancing age, the loss of fast-twitch muscle fibers and motor units (sarcopenia) transforms strength training from “icing on the cake” to “indispensable.” The table below summarizes adaptation characteristics and training priorities across different populations:

Population	Adaptation Characteristics	Training Priorities
Beginners	Neural adaptations dominate, rapid progress	Establish movement quality, progressive loading
Advanced Athletes	Adaptations slow, need refined stimuli	Periodization, power/eccentric focus
Female Athletes	Greater relative room for improvement	Same principles as males, avoid over-conservatism
Masters Athletes (>50)	Counteract sarcopenia, neural loss	Maintain high-intensity stimulus, emphasize RFD
Adolescents	Prioritize movement technique and safety	Start with bodyweight, avoid early heavy loading

Understanding these differences allows athletes and coaches to avoid forcing a single program onto everyone, and to make reasonable adjustments based on one’s own stage and conditions. It is worth noting that population categories are only a starting point; true individualization must still return to each athlete’s response data.

Practical Training Application

Translating research into a training program requires answering four questions: which exercises to do, what intensity to use, when to schedule them, and how to monitor.

Exercise Selection. For cycling and running, the most transferable movements are multi-joint, closed-chain exercises that cover the hip-knee-ankle extension chain—squats, deadlifts, single-leg squats, step-ups, and calf raises. For the specific goal of strength training’s effect on tendon elastic energy return, corresponding accessory exercises (such as eccentric components, plyometric jumps, or core stability work) can be added.

Intensity and Sets. When maximal strength is the primary goal, 4–6RM with 3–4 sets per movement and rest intervals of 3 minutes or more to ensure quality is recommended. If the goal leans toward power and RFD, switch to lighter loads (30–60% 1RM) executed with “maximal velocity intent”—movement speed itself is the stimulus. The table below shows a sample off-season weekly schedule:

Day	Main Training	Strength Session Example
Monday	Endurance (long slow distance)	—
Tuesday	Strength (maximal strength focus)	Back squat 5×5, Romanian deadlift 4×6, calf raises 3×8
Wednesday	Endurance (tempo/threshold)	—
Thursday	Strength (power focus)	Jump squats 5×3, single-leg step-ups 3×6, core circuit
Friday	Recovery/Technique	—
Saturday	Long endurance or race simulation	—
Sunday	Complete rest	—

Scheduling. To reduce interference effects, when both types of training are performed on the same day, it is recommended to separate strength and high-intensity endurance sessions by at least 6 hours, or place them on different days; when they must be done on the same day, prioritize the “capacity to be developed first” (often strength first in the pre-season, endurance first in-season).

Monitoring Metrics. Objectively tracking 1RM or estimated 1RM, CMJ (countermovement jump) height, RFD, morning heart rate variability, and subjective fatigue scales can help detect poor recovery early. When CMJ declines consecutively or time-trial performance stagnates, this should be treated as a signal to adjust dosage. Remember: strength training is the “auxiliary engine” for endurance performance—its purpose is to make you more efficient on the course, not to lift heavier in the gym. Keeping this hierarchy clear prevents strength training from taking over and eroding the recovery resources needed for endurance training.

/no_think

Local Applications in Taiwan

Taiwan’s climate, terrain, and racing culture bring several unique considerations to the application of strength training’s effects on tendon elastic energy return.

Recovery management in hot and humid conditions. Taiwan’s summer heat and high humidity can hinder recovery after strength training due to dehydration and reduced sleep quality. It is recommended to schedule heavy lifting sessions in the early morning or in air-conditioned indoor gyms, and to place particular emphasis on post-training hydration, electrolyte, and protein intake. Avoid stacking high-intensity endurance and strength stimuli during hot afternoons, as this can exacerbate the interference effect.

Specific demands of climbing events. Classic Taiwanese events such as Wuling (West Approach), the North Route to Wuling, and the Yangmingshan series (Fengguizui, Balaka) are renowned for their long distances and massive elevation gain. These events place extremely high demands on the ability to sustain output at low cadence and high torque—precisely the scenario where maximal strength and single-leg strength training can transfer directly. For challenges like Wuling, which involves elevation changes of up to three thousand meters, maximal strength reserves in the lower limbs allow riders to maintain pedaling ease on the final steep gradients, avoiding the predicament of “legs giving out first.”

Local training resources and seasonal rhythm. Gyms are widely available in most Taiwanese cities and counties, allowing cyclists to use the free-weight area for squats and deadlifts. Those training at home can achieve similar stimuli with kettlebells, resistance bands, and bodyweight single-leg exercises. For riders who primarily train on Yangmingshan, Beiyi, and the Central Cross-Island Highway, it is recommended to concentrate a dedicated strength block during the off-season (typically the hottest part of summer, when long outdoor sessions are impractical), turning the hot season into a golden window for building a strength base. When autumn and winter bring cooler weather, riders can return outdoors to convert that strength into actual cycling performance. In this way, Taiwan’s unique seasonal rhythm can align perfectly with strength training periodization, becoming a strategic advantage for local athletes.

Debunking Common Myths

Many claims circulating about the effects of strength training on tendon elastic energy return do not align with the academic evidence. The following clarifies each one.

Myth 1: “Lifting weights will make you bulky and heavier, dragging down your endurance.” Evidence shows that maximal-strength-oriented training (high intensity, low volume) primarily drives neural and tendon adaptations, with limited increases in muscle cross-sectional area. In most studies, body weight shows no significant change, while performance improves due to enhanced efficiency.

Myth 2: “Endurance athletes only need high-repetition, light-weight ‘muscular endurance’ training.” The opposite is actually true. High-repetition, light-weight training provides insufficient stimulus for neural drive and tendon stiffness, and many studies indicate that heavy loads with low repetitions yield better transfer effects.

Myth 3: “The effects of strength training will show up in performance immediately.” Although neural adaptations are fast, tendon remodeling and muscle fiber transformation take weeks to months. Giving up too early is a common mistake.

Myth 4: “The interference effect of concurrent training will cancel out the benefits of strength training.” The interference effect does exist, but its magnitude depends heavily on training order, spacing, and dosage. With proper planning, strength and endurance can absolutely coexist and thrive. By dispelling these myths, athletes can approach their training with the right expectations and invest limited time and energy where it truly pays off.

Conclusion

Looking at the evidence reviewed in this article, the impact of strength training on tendon elastic energy return is no longer a question of “whether to do it,” but rather “how to do it smarter.” From immediate neuromuscular adaptations, to long-term remodeling of muscle fibers and tendons, to comprehensive improvements in exercise economy, multiple layers of mechanisms jointly support one conclusion: appropriate resistance training is an indispensable component of an endurance athlete’s toolkit.

Future research directions include predicting individual responses using genetic and molecular markers, clarifying the optimal molecular-level interval for concurrent training, and developing new resistance training equipment with greater sport specificity. For cyclists and runners in Taiwan, the most practical course of action is: build a solid maximal strength foundation in the off-season, maintain it with the minimum effective dose during the season, and monitor progress with objective metrics throughout, so that strength truly translates into speed and endurance on the course. Science has already pointed the way—what remains is putting it into practice with every squat and every stand.

Related Reading

• Tendon Stiffness Training and Energy Storage: The Resistance Training Foundation of the Running Spring Model
• The Effects of Strength Training on Ground Reaction Forces in Running: A Biomechanical Adaptation Study
• The Effects of Strength Training Frequency on Muscle Hypertrophy: A Meta-Analysis of 1 vs. 5 Sessions per Week
• Power Training and Its Transfer Effects on Slow-Endurance Sports: A Research Review