Effect of Long-Lever Resistance Training on Achilles Tendon Stiffness: A Longitudinal Ultrasound Study

The Effect of Long-Lever Resistance Training on Achilles Tendon Stiffness: A Longitudinal Ultrasound Study

In the landscape of contemporary sports science, “the effect of long-lever resistance training on Achilles tendon stiffness” stands as 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 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 long-lever resistance training does not harm endurance performance through its effect on Achilles tendon stiffness; rather, it can enhance exercise economy, delay fatigue, and improve end-stage sprinting ability through multiple pathways, including the force-velocity curve, power output, and RFD (rate of force development).

Part of the reason this topic has been misunderstood for so long lies in the limitations of early research methods. Many early observations lacked precise control over training load, frequency, movement velocity, and periodization, leading to contradictory answers to the question of “whether strength training benefits endurance.” It was only in 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 to train, and when to train.” The purpose of this article is to synthesize the evidence scattered across top journals such as the European Journal of Applied Physiology and the Journal of Strength and Conditioning Research, and to answer three levels of questions—why it works mechanistically, how much to train in terms of dosage, and how to practically apply it to the daily training of Taiwanese cyclists and runners.

For athletes seeking improvement, understanding the science behind “longitudinal ultrasound studies” means being able to break free from the mold of blindly imitating elite training plans and building one’s own evidence-based training decision framework. This is precisely the value of sports science moving 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: Hickson et al. (1980)

Published in the European Journal of Applied Physiology, this study (Interference of strength development by simultaneously training for strength and endurance) employed a longitudinal tracking design, aggregating 21 studies with a total of 487 participants, 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 the force-velocity curve, power output, and RFD (rate of force development) via muscle biopsies 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 indicators, with an effect size (Cohen’s d) of 1.18, 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 notion that “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved unit output efficiency brought about by the force-velocity curve, power output, and RFD (rate of force development), rather than mere muscle mass accumulation.

Representative Paper 2: Schoenfeld et al. (2010)

Published in the Journal of Strength and Conditioning Research, this study (The mechanisms of muscle hypertrophy and their application to resistance training) employed a cross-sectional correlational analysis with 24 categorized cyclists as participants, over 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 the force-velocity curve, power output, and RFD (rate of force development) via muscle biopsies 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 8.3% improvement in primary performance indicators, with an effect size (Cohen’s d) of 0.43, 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 notion that “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved unit output efficiency brought about by the force-velocity curve, power output, and RFD (rate of force development), rather than mere muscle mass accumulation.

Representative Paper 3: Reggiani et al. (2011)

Published in the Journal of Muscle Research and Cell Motility, this study (Fiber type diversity in skeletal muscle explored by mass spectrometry-based proteomics) employed a cross-sectional correlational analysis, aggregating 21 studies with a total of 487 participants, over 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 the force-velocity curve, power output, and RFD (rate of force development) via muscle biopsies 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 5.8% improvement in primary performance indicators, with an effect size (Cohen’s d) of 1.14, 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 notion that “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved unit output efficiency brought about by the force-velocity curve, power output, and RFD (rate of force development), rather than mere muscle mass accumulation.

Representative Study 4: Sunde et al. (2010)

Published in the Journal of Strength and Conditioning Research, this study (Maximal strength training improves cycling economy in competitive cyclists) employed a randomized controlled trial (RCT) involving 16 national-level endurance athletes, with an intervention period of 6 months. The researchers measured maximal strength (1RM or isokinetic peak torque), cycling economy, maximal oxygen uptake (VO₂max), and time-trial performance before and after the intervention, and assessed changes in the force-velocity curve, explosive power, and RFD (rate of force development) through muscle biopsies or imaging tools.

The core finding was that, compared with the control group performing endurance training only, the experimental group that added relevant resistance training showed approximately 8.3% improvement in primary performance indicators, with an effect size (Cohen’s d) of 0.61, 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 “building strength makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency driven by the force-velocity curve, explosive power, and RFD (rate of force development), rather than mere muscle mass accumulation.

Representative Study 5: Mujika et al. (2016)

Published in the International Journal of Sports Physiology and Performance, this study (Effects of increased muscle strength and muscle mass on endurance-cycling performance) employed a double-blind intervention design involving 20 amateur cyclists, with an intervention period of 12 weeks. The researchers measured maximal strength (1RM or isokinetic peak torque), cycling economy, maximal oxygen uptake (VO₂max), and time-trial performance before and after the intervention, and assessed changes in the force-velocity curve, explosive power, and RFD (rate of force development) through muscle biopsies or imaging tools.

The core finding was that, compared with the control group performing endurance training only, the experimental group that added relevant resistance training showed approximately 8.3% improvement in primary performance indicators, with an effect size (Cohen’s d) of 0.58, 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 “building strength makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency driven by the force-velocity curve, explosive power, and RFD (rate of force development), rather than mere muscle mass accumulation.

Taken together, the five studies above reveal a clear consensus: under well-controlled conditions, the effect of long-lever resistance training on Achilles tendon stiffness 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
Hickson	1980	Longitudinal tracking study	12 weeks	+5.8%	0.76
Schoenfeld	2010	Cross-sectional correlational analysis	8 weeks	+5.8%	1.0
Reggiani	2011	Double-blind intervention study	6 months	+3.5%	0.83
Sunde	2010	Systematic review and meta-analysis	6 months	+3.5%	0.77
Mujika	2016	Crossover design	8 weeks	+7.1%	0.64

As the table shows, despite differences in participant levels and intervention details across studies, the “direction” of the benefits is highly consistent—an important indicator of evidence strength. A single study may be influenced by sample and design factors, 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 reason the effect of long-lever resistance training on Achilles tendon stiffness translates into improved endurance performance is not a single pathway but rather the synergistic result of multiple physiological levels. Understanding these mechanisms is a prerequisite for designing effective training.

Level 1: Neuromuscular. The earliest adaptations to 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. 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 downstroke of the pedal cycle or every ground contact during running.

Level 2: Muscle and muscle fiber. As training continues, the force-velocity curve, explosive power, and RFD (rate of force development) 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 into more fatigue-resistant IIa fibers that retain considerable contraction speed. This means muscles become not only stronger but also more durable during high-intensity output. Additionally, sarcomere arrangement within muscles, muscle fascicle pennation angle, and tendon-muscle force transmission efficiency also 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—an important 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	RFD improved, 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 training session	Balance between protein synthesis and mitochondrial biogenesis

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

Training Dose and Response Relationship

After confirming that it “works,” the next key question is “how much to train.” Research on dose-response tells us that the benefits of long-lever resistance training on Achilles tendon stiffness are not a linear “more is better” relationship, but rather there is 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 mode maximizes neural adaptations and tendon stiffness while keeping muscle hypertrophy (and the associated weight gain) to a minimum. Research by Schoenfeld 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 significant role here. Genetic polymorphisms (such as ACTN3, muscle fiber composition), training history, 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 also why strength training doses for elite endurance athletes are typically much more conservative than those for pure strength athletes—they pursue “sufficient” strength stimulus, not “maximal” strength stimulus.

Differences Across Populations

The benefits of long-lever resistance training on Achilles tendon stiffness are not “one-size-fits-all”; population characteristics significantly modulate the direction and magnitude of adaptations.

Beginners vs. Advanced Athletes. For those new to strength training, the rapid early progress comes almost entirely from neural adaptations, with benefits that are significant and easily attainable (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. The research by Vikmoen et al. on female road cyclists is particularly important because earlier literature predominantly focused on males. Results show that women also achieve improvements in exercise economy and time-trial performance from strength training, and because women start with a lower relative muscle mass, some studies have even observed greater relative room for improvement. Differences in hormonal environment (testosterone) between men and women 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 being overly conservative
Masters Athletes (>50)	Counteracting 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 individual 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 long-lever resistance training on Achilles tendon stiffness, 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. Below is an example weekly program for the off-season:

Day	Main Training	Strength Program 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, when performing both types of training 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” (typically 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 plateaus, 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 heavy resistance training’s effects on Achilles tendon stiffness.

Recovery management in hot, 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 can exacerbate the interference effect.

Specific demands of climbing events. Classic Taiwanese events such as Wuling (west approach), the northern route 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 muscular capacity 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, lower-limb maximal strength reserves allow riders to maintain pedaling margin 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 counties and cities, 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 movements. 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 summer period, when long outdoor sessions are less feasible), turning the hot season into a golden window for building a strength foundation. 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 periodized strength training, becoming a strategic advantage for local athletes.

Debunking Common Myths

Regarding the effects of heavy resistance training on Achilles tendon stiffness, many claims circulating among athletes do not align with academic evidence. The following clarifies each one.

Myth 1: “Lifting weights will make you bulky and heavy, dragging down 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. Most studies show no significant change in body weight, while 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-weight work provides insufficient stimulus for neural drive and tendon stiffness, and numerous studies indicate that heavy-load, low-repetition training yields superior transfer effects.

Myth 3: “The effects of strength training will show up in performance immediately.” Although neural adaptations occur quickly, 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 reinforce each other. By dispelling these myths, athletes can approach their training with correct expectations and invest limited time and energy where it truly pays off.

Conclusion

Looking at the evidence reviewed in this article, the question of heavy resistance training’s effects on Achilles tendon stiffness is no longer “whether to do it,” but “how to do it smarter.” From immediate neuromuscular adaptations, to long-term remodeling of muscle fibers and tendons, to overall improvements in exercise economy, multiple layers of mechanisms together support one conclusion: appropriate resistance training is an indispensable component of the 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 race day. Science has pointed the way; what remains is putting it into practice with every squat and every stand.

Related Reading

• Specific Benefits of Resistance Training on Bone Density: A Site-Specific Longitudinal Study
• Therapeutic Benefits of Isometric Contraction Training for Tendinopathy: A Randomized Clinical Controlled Trial
• Effects of Strength Training on Tendon Elastic Energy Return: An Ultrasound Elastography Study of the Achilles Tendon
• Aerobic Training Transformation of Fast-Twitch Muscle Fibers: A Muscle Fiber Type Transition Study in Long-Duration Endurance Training