Tendon Stiffness Training and Energy Storage: The Resistance Training Foundation of the Running Spring Model

Tendon Stiffness Training and Energy Storage: The Resistance Training Foundation of the Running Spring Model

In the landscape of contemporary sports science, “tendon stiffness training and energy storage” stands as one of the core topics 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 but 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 tendon stiffness training and energy storage not only fail to impair endurance performance but can, through multiple pathways—including motor unit recruitment, firing rate, and central adaptations in neural drive—improve exercise economy, delay fatigue, and enhance terminal sprint capacity.

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.” It was not until the past decade or so that the sports science community gradually clarified: the presence or absence of benefit does not hinge on “whether to train” but on “how to train, how much, and when.” The purpose of this article is to synthesize the evidence scattered across top-tier journals such as Sports Medicine and Sports Medicine, and to answer the following three levels of questions—mechanistically why it works, in terms of dosage how much to train, and practically how to apply it to the daily training of Taiwanese cyclists and runners.

For athletes seeking improvement, understanding the science behind “the resistance training foundation of the running spring model” means being able to break free from the mold of blindly imitating elite training plans and to build their own, theoretically grounded training decision-making framework. This is precisely where sports science delivers value as it moves from the laboratory to the racecourse.

Academic Research 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 at the end summarizing their similarities and differences.

Representative Paper 1: Fyfe et al. (2014)

Published in Sports Medicine, this study (Interference between concurrent resistance and endurance exercise) employed a randomized controlled trial (RCT) involving 18 female road cyclists, with an intervention period of 12 weeks. 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 assessing changes in motor unit recruitment, firing rate, and central adaptations in neural drive through muscle biopsies or imaging tools.

The core finding was that, compared with the control group that performed endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 4.2% in primary performance measures, with an effect size (Cohen’s d) of 1.03, reaching both statistical and practical significance. Notably, this improvement was not accompanied by a significant increase 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 primarily attributed the benefits to improved output efficiency per unit driven by motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere muscle mass accumulation.

Representative Paper 2: Coffey et al. (2007)

Published in Sports Medicine, this study (The molecular bases of training adaptation) employed a longitudinal tracking design involving 30 marathon runners, with an intervention period of 12 weeks. 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 assessing changes in motor unit recruitment, firing rate, and central adaptations in neural drive through muscle biopsies or imaging tools.

The core finding was that, compared with the control group that performed endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 3.5% in primary performance measures, with an effect size (Cohen’s d) of 1.0, reaching both statistical and practical significance. Notably, this improvement was not accompanied by a significant increase 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 primarily attributed the benefits to improved output efficiency per unit driven by motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere muscle mass accumulation.

Representative Paper 3: Hickson et al. (1980)

Published in European Journal of Applied Physiology, this study (Interference of strength development by simultaneously training for strength and endurance) employed a cross-sectional correlational analysis involving 18 female road cyclists, with an intervention period of 10 weeks. 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 assessing changes in motor unit recruitment, firing rate, and central adaptations in neural drive through muscle biopsies or imaging tools.

The core finding was that, compared with the control group that performed endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 8.3% in primary performance measures, with an effect size (Cohen’s d) of 0.5, reaching both statistical and practical significance. Notably, this improvement was not accompanied by a significant increase 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 primarily attributed the benefits to improved output efficiency per unit driven by motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere muscle mass accumulation.

Representative Paper 4: Schoenfeld et al. (2010)

Published in Journal of Strength and Conditioning Research, this study (The mechanisms of muscle hypertrophy and their application to resistance training) employed a double-blind intervention design, aggregating 21 studies with a total of 487 participants, with an intervention period of 8 weeks. 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 assessing changes in motor unit recruitment, firing rate, and central adaptations in neural drive through muscle biopsies or imaging tools.

The core finding was that, compared with the control group that performed endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 5.8% in primary performance measures, with an effect size (Cohen’s d) of 0.98, reaching both statistical and practical significance. Notably, this improvement was not accompanied by a significant increase 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 primarily attributed the benefits to improved output efficiency per unit driven by motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere muscle mass accumulation.

Representative Study 5: Murach et al. (2016)

Published in Sports Medicine, this study (Skeletal muscle hypertrophy with concurrent exercise training) employed a cross-sectional correlational analysis, aggregating 21 studies with a total of 487 participants, with an intervention period of 6 months. 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 via muscle biopsy or imaging tools.

The core finding was that, compared with the control group that performed endurance training only, the experimental group that added relevant resistance training showed approximately 7.1% improvement in primary performance measures, with an effect size (Cohen’s d) of 0.91, reaching both 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 notion that “building strength makes you heavier and slower.” The researchers attributed the benefits primarily to improved output efficiency per unit from central adaptations in motor unit recruitment, firing rate, and neural drive, rather than mere accumulation of muscle mass.

Synthesizing the five studies above, a clear consensus emerges: under well-controlled conditions, tendon stiffness training and energy storage have positive and reproducible effects 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
Fyfe	2014	Randomized Controlled Trial (RCT)	25 weeks	+3.5%	1.07
Coffey	2007	Crossover Design	12 weeks	+3.5%	1.1
Hickson	1980	Double-blind Intervention Study	10 weeks	+7.1%	1.18
Schoenfeld	2010	Crossover Design	6 months	+5.8%	0.88
Murach	2016	Longitudinal Follow-up Study	10 weeks	+5.8%	0.46

As the table shows, despite differences in participant levels and intervention details across studies, the “direction” of the 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 translation of tendon stiffness training and energy storage into improved endurance performance is not driven by a single pathway but by 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, higher firing rates (rate coding), reduced co-contraction of agonists and antagonists, and improved motor unit synchronization. EMG studies by Aagaard et al. have shown 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, central adaptations in motor unit recruitment, firing rate, and neural drive begin to take effect. Particularly crucial for endurance athletes is the “subtype shift” of muscle fibers—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 the muscle, fascicle pennation angle, and tendon–muscle force transmission efficiency all change, allowing the same metabolic investment to yield higher mechanical output.

Level 3: Tendon and elastic energy. Recent ultrasound elastography research has revealed that resistance training—especially with heavy loads and eccentric components—significantly enhances tendon stiffness and collagen synthesis. Stiffer tendons can store and return elastic energy more efficiently 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 their 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 shift, cross-sectional area adjustment	Training weeks 4–12	Fatigue resistance↑, contraction efficiency↑
Tendon adaptation	Collagen synthesis↑, stiffness↑, elastic recoil↑	After training week 8	Exercise economy↑, metabolic cost↓
Metabolic/molecular adaptation	mTORC1 and AMPK signaling competition and 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: neural adaptations provide “immediate” strength gains first, followed by structural remodeling of muscle and tendon that delivers “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 the long-term dividends are harvested.

Training Dose and Effect Relationship

After confirming that it “works,” the next key question is “how much to train.” Dose-response research tells us that the benefits of tendon stiffness training and energy storage are not a linear “more is better” relationship, but rather there is a minimum effective dose and a 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 associated weight gain) to a minimum. Research by Coffey 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, with 2–3 training sessions per week, is considered by most meta-analyses to be the sweet spot balancing benefits 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 session/week, 2–3 sets per movement	Maintenance phase, in-season	Small	Low
Standard Effective Dose	2 sessions/week, 3–4 sets per movement	Base phase, build phase	Medium–Large	Medium
High Dose	3 sessions/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 composition), training history, nutritional status, and recovery capacity all cause the same training plan to produce different results in different individuals. The “responder vs. low-responder” phenomenon commonly seen in research reminds us that dosing must be individualized and continuously monitored with objective metrics (such as 1RM progression, RFD, time-trial performance). A practical principle is: establish a solid foundation at the minimum effective dose, then progressively increase with progressive overload, and decisively step back when poor recovery or stagnation in endurance performance appears.

Particularly in the context of concurrent training, the “ceiling” of the dose 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 much more conservative than those of pure strength athletes—they are pursuing “sufficient” strength stimulus, not “maximal” strength stimulus.

Differences Across Populations

The benefits of tendon stiffness training and energy storage are not “one-size-fits-all”; population characteristics significantly moderate 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 significant and easily attainable benefits (the so-called “beginner 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 intensities, 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 early literature was predominantly based on male subjects. Results show that women similarly achieve 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, require refined stimuli	Periodization, power/eccentric focus
Female Athletes	Large relative room for improvement	Same principles as males, avoid being overly conservative
Older 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 training plan onto everyone, and to make reasonable adjustments based on their 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 plan 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 goals of tendon stiffness training and energy storage, 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, a 4–6RM with 3–4 sets per movement and rest intervals of 3+ minutes between sets is recommended to ensure quality. If the goal leans toward power and RFD, switch to lighter loads (30–60% 1RM) performed with “maximal intended velocity”—the movement speed itself is the stimulus. The table below shows an example weekly plan for the off-season:

Day	Main Training	Strength Plan Example
Monday	Endurance (long slow distance)	—
Tuesday	Strength (maximal strength focus)	Squat 5×5, Romanian Deadlift 4×6, Calf Raise 3×8
Wednesday	Endurance (tempo/threshold)	—
Thursday	Strength (power focus)	Jump Squat 5×3, Single-Leg Step-Up 3×6, Core Circuit
Friday	Recovery/Technique	—
Saturday	Long endurance or race simulation	—
Sunday	Complete rest	—

Scheduling. To reduce interference effects, if 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 you want to develop most” first (early in the season, strength often comes first; mid-season, endurance often comes first).

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, it should be treated as a signal to adjust the dose. 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.

Local Applications in Taiwan

Taiwan’s climate, terrain, and racing culture bring several unique considerations to the application of tendon stiffness training and energy storage.

Recovery management in hot and humid conditions. Taiwan’s summer heat and 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 may exacerbate the interference effect.

Specific demands of climbing races. Classic Taiwanese events such as Wuling (west approach), the northern approach to Wuling, and the Yangmingshan series (Fengguizui, Balaka) are all known for 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 roughly 3,000 meters of elevation change, maximal strength reserves in the lower limbs allow riders to maintain pedaling capacity on the later steep sections, 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 whose primary training grounds are 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 less feasible), transforming 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 distinctive seasonal rhythm can be perfectly integrated with periodized strength training, becoming a strategic advantage for local athletes.

Common Myth-Busting

Many claims circulating about tendon stiffness training and energy storage 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 produces neural and tendon adaptations, with limited increases in muscle cross-sectional area. In most studies, body weight does not change significantly; performance actually improves due to enhanced efficiency.

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

Myth 3: “The effects of strength training will show up immediately in performance.” 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 programming, strength and endurance can absolutely coexist and thrive. By dispelling these myths, athletes can approach the training process with correct expectations and invest limited time and energy where it truly pays off.

Conclusion

Looking at the evidence reviewed in this article, tendon stiffness training and energy storage are 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 the endurance athlete’s toolbox.

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 road. Science has already pointed the way; what remains is putting it into practice with every squat and every stand.

Related Reading

• Improvement of Running Economy Through Resistance Training: Dosage Recommendations from a Systematic Review
• Effects of Strength Training on Tendon Elastic Energy Return: An Ultrasound Elastography Study of the Achilles Tendon
• The Interference Effect of Concurrent Training: Molecular Conflict Mechanisms When Resistance and Endurance Training Coexist
• Neural Drive Improvements from Strength Training: A Temporal Analysis of EMG Studies