Effects of Hip Flexor Strengthening on Cycling Climbing Power: A Randomized Controlled Trial

The Effect of Hip Flexor Strengthening on Cycling Climbing Power: A Randomized Controlled Trial

In the landscape of contemporary sports science, “the effect of hip flexor strengthening on cycling climbing power” 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 but could even hinder performance by “building bulky muscles and increasing body weight.” This intuition seems reasonable, yet it contradicts 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 almost unanimous: appropriately designed hip flexor strengthening not only fails to harm endurance performance but can, through multiple pathways such as 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 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 “is strength training beneficial for 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 evidence scattered across top journals such as the Journal of Strength and Conditioning Research and Sports Medicine, and to answer three levels of questions—why it works mechanistically, how much to do 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 “randomized controlled trials” means being able to break free from the mold of blindly imitating elite training plans and building their own, theoretically grounded training decision-making framework. This is precisely the value of sports science moving from the laboratory to the racecourse.

Review of Academic Research

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: 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 systematic review and meta-analysis, involving 18 female road cyclists over a 12-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 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 to a control group performing endurance training only, the experimental group that added relevant resistance training showed approximately 8.3% improvement in primary performance measures, with an effect size (Cohen’s d) of 0.4, 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 “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere accumulation of muscle mass.

Representative Paper 2: Coffey et al. (2007)

Published in Sports Medicine, this study (The molecular bases of training adaptation) employed a crossover design, involving 30 marathon runners over an 8-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 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 to a control group performing 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 0.49, 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 “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere accumulation of muscle mass.

Representative Paper 3: Aagaard et al. (2002)

Published in the Journal of Applied Physiology, this study (Increased rate of force development and neural drive following resistance training) employed a randomized controlled trial (RCT), aggregating 21 studies with a total of 487 participants over 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 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 to a control group performing endurance training only, the experimental group that added relevant resistance training showed approximately 8.3% improvement in primary performance measures, with an effect size (Cohen’s d) of 1.02, 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 “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere accumulation of muscle mass.

Representative Paper 4: Damas et al. (2015)

Published in Sports Medicine, this study (A review of resistance training-induced changes in muscle protein synthesis and hypertrophy) employed a longitudinal tracking design, involving 20 amateur cyclists over 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 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 to a control group performing endurance training only, the experimental group that added relevant resistance training showed approximately 5.8% improvement 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 significant increases in body weight, 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 output efficiency per unit from motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere accumulation of muscle mass.

Representative Paper 5: 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 cross-sectional correlational design, involving 16 national-level endurance athletes over a 12-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 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 to a control group performing endurance training only, the experimental group that added relevant resistance training showed approximately 2.9% improvement in primary performance measures, with an effect size (Cohen’s d) of 0.81, 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 “strength training makes you heavier and slower.” The researchers primarily attributed the benefits to improved output efficiency per unit from motor unit recruitment, firing rate, and central adaptations in neural drive, rather than mere accumulation of muscle mass.

Taken together, these five studies converge on a clear consensus: under well-controlled conditions, the effect of hip flexor strengthening on cycling climbing power is positive and reproducible. The table below organizes 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
Schoenfeld	2010	Systematic review and meta-analysis	6 months	+7.1%	0.59
Coffey	2007	Longitudinal tracking study	6 months	+5.8%	0.64
Aagaard	2002	Systematic review and meta-analysis	6 months	+8.3%	0.66
Damas	2015	Crossover design	12 weeks	+3.5%	0.84
Sunde	2010	Crossover design	6 months	+8.3%	0.61

As the table shows, despite differences in participant levels and intervention details across studies, the “direction” of benefit is highly consistent—an important 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 reason the effect of hip flexor strengthening on cycling climbing power translates into improved endurance performance is not a single pathway but the synergistic result 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. In the first 4 to 6 weeks of training, rapid strength gains primarily come from improved motor unit recruitment, increased firing rate (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 push-off in 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” in muscle fiber types—the most fatigable IIx fibers tend to convert to IIa fibers, which are more fatigue-resistant while retaining considerable contraction speed. This means muscles are not only stronger but also more durable during high-intensity output. Additionally, sarcomere arrangement within muscles, pennation angle of fascicles, and tendon-muscle force transmission efficiency also change, allowing the same metabolic investment to yield higher mechanical output.

Level 3: Tendons 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 push-off 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 timelines, and their specific effects on endurance performance:

Mechanism Level	Primary Changes	Typical Timeline	Effect 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 adjustments	Training weeks 4–12	Improved fatigue resistance↑, contraction efficiency↑
Tendon adaptation	Collagen synthesis↑, stiffness↑, elastic recoil↑	After training week 8	Improved 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: first, neural adaptations provide “immediate” strength gains, then structural remodeling of muscle and tendon delivers “sustained” efficiency dividends. Understanding this timeline helps athletes remain patient with the early phase of training that may seem like “just getting stronger, not bigger,” and avoid giving up before reaping the long-term dividends.

Training Dosage and Dose-Response Relationship

Having confirmed that it “works,” the next key question is “how much to do.” Dose-response research tells us that the benefits of hip flexor strengthening on cycling climbing power are not a linear “more is better” relationship; rather, there is a minimum effective dose and a point of diminishing returns.

Regarding intensity, most studies on endurance athletes favor a “maximal strength” approach with heavy loads (≥80% 1RM) and low repetitions (4–8 reps), because this pattern maximizes neural adaptations and tendon stiffness while keeping 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.

Regarding training volume, accumulating 6–10 sets per major exercise per week, with 2–3 sessions per week, is considered by most meta-analyses to be the sweet spot balancing benefit and recovery. The table below presents a typical dose-response relationship:

Dosage Range	Recommended Configuration	Applicable Period	Expected Benefit	Interference/Fatigue Risk
Minimum effective dose	1 session/week, 2–3 sets per exercise	Maintenance phase, in-season	Small	Low
Standard effective dose	2 sessions/week, 3–4 sets per exercise	Base phase, build phase	Medium–large	Medium
High dose	3 sessions/week, 4–6 sets per exercise	Off-season strength specialization	Large (but diminishing returns)	High (increased 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 program to produce different results in different people. 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 foothold at the minimum effective dose, then progressively overload incrementally, and decisively step back when signs of poor recovery or stagnation in endurance performance appear.

Especially 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 typically use much more conservative strength training doses than pure strength athletes—they seek “sufficient” strength stimulus, not “maximal” strength stimulus.

Differences Across Populations

The benefits of hip flexor strengthening on cycling climbing power 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 benefits that are significant and easily obtained (the so-called “beginner gains”). However, for advanced athletes with years of training experience, the nervous system’s “ceiling” is lower, and further progress often requires more sophisticated periodization, higher intensities, or novel stimuli (such as eccentric overload or 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 study by Vikmoen et al. on female road cyclists is particularly important because early literature predominantly focused on males. Results show that women equally benefit from strength training in terms of improved exercise economy and time-trial performance, and because women’s relative muscle mass starting point is lower, some studies even observed greater relative room for improvement. Differences in hormonal environment (testosterone) between men and women primarily affect the absolute magnitude of hypertrophy, not the “direction” of neural and tendon adaptations.

Age differences. With advancing age, the loss of fast-twitch fibers and motor units (sarcopenia) makes strength training shift 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
Older athletes (>50)	Counteracting sarcopenia, neural loss	Maintain high-intensity stimuli, emphasize RFD
Adolescents	Prioritize movement technique and safety	Start with bodyweight, avoid early heavy loads

Understanding these differences allows athletes and coaches to avoid rigidly applying a single program to 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 return to each athlete’s response data.

Practical Training Application

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

Exercise selection. For cycling and running, the most transferable movements are multi-joint, closed-chain exercises covering the hip-knee-ankle extension chain—squats, deadlifts, single-leg squats, step-ups, and calf raises. For the specific goal of hip flexor strengthening’s effect on cycling climbing power, supplementary exercises (such as eccentric components, plyometric jumps, or core stability work) can be added accordingly.

Intensity and sets. When maximal strength is the primary goal, 4–6RM with 3–4 sets per exercise 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) performed with “maximal velocity intent”—the movement speed itself is the stimulus. The table below shows a sample weekly program for the off-season:

Day	Main Training	Strength Program Example
Monday	Endurance (long-distance aerobic)	—
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	—

Timing and sequencing. 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 on the same day, prioritize the “capacity to be developed first” (often strength early in the season, endurance 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, it 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 racecourse, 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 Application in Taiwan

Taiwan’s climate, terrain, and race culture bring several unique considerations to the application of hip flexor strengthening’s effect on cycling climbing power.

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 pay special attention to post-training hydration, electrolytes, and protein intake, avoiding stacking high-intensity endurance and strength stimuli on hot afternoons to prevent exacerbating interference effects.

Specific demands of climbing races. Classic Taiwanese events such as Wuling (west approach), the northern route to Wuling, and the Yangmingshan series (Fengguizui, Balaka) are known for long distances and massive elevation gain. These events place extremely high demands on “sustained output at low cadence and high torque,” a scenario where maximal strength and single-leg strength training can transfer directly. For challenges like Wuling with its 3,000-meter elevation change, lower-body maximal strength reserves allow riders to maintain pedaling margin on the latter steep sections, avoiding the dreaded “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 weights area for squats and deadlifts; those training at home can achieve similar stimuli with kettlebells, resistance bands, and bodyweight single-leg movements. For cyclists whose primary training grounds are Yangmingshan, Beiyi, and the Central Cross-Island Highway, it is recommended to concentrate a strength specialization block during the off-season (typically the hottest summer period, unsuitable for long outdoor sessions), turning the hot season into a golden window for building a strength foundation, then returning outdoors in the cooler autumn and winter to convert that strength into actual riding performance. In this way, Taiwan’s unique seasonal rhythm can perfectly align with strength training periodization, becoming a strategic advantage for local athletes.

Common Myth-Busting

Regarding the effect of hip flexor strengthening on cycling climbing power, many claims circulating publicly contradict academic evidence. Let us clarify them one by one.

Myth 1: “Lifting weights will make you bulky, heavier, and hurt endurance.” Evidence shows that maximal strength-oriented training (high intensity, low volume) primarily induces neural and tendon adaptations, with limited increases in muscle cross-sectional area; in most studies, body weight did not change significantly, and performance actually improved due to enhanced efficiency.

Myth 2: “Endurance athletes should only do high-rep, light-weight ‘muscular endurance’ training.” The opposite is true: high-rep, light-weight training provides insufficient stimulus for neural drive and tendon stiffness, and many studies indicate that heavy-load, low-rep training has better transfer benefits.

Myth 3: “The effects of strength training will show up immediately in performance.” Although neural adaptations are fast, tendon remodeling and muscle fiber conversion 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 is highly dependent on training order, spacing, and dosage; with proper arrangement, strength and endurance can absolutely coexist and thrive. Dispelling these myths allows athletes to 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, the effect of hip flexor strengthening on cycling climbing power is no longer a question of “whether to do it,” but “how to do it smarter.” From immediate neuromuscular adaptations, to long-term remodeling of muscle fibers and tendons, to comprehensive improvements in exercise economy, multi-layered 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 interval for concurrent training at the molecular level, 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 throughout with objective metrics, allowing strength to truly translate into speed and endurance on the racecourse. Science has provided the direction; what remains is putting it into practice with every squat and every stand.

Related Reading

• Correlation Between Squat Maximal Strength and Cycling Power Output: A Meta-Analysis Study
• The Effect of Strength Asymmetry on Cycling Pedaling Efficiency: A 3D Motion Analysis Study
• EMG Analysis of Standing Pedaling in Cycling: Which Muscles Are Actually Working
• Application of Isokinetic Strength Testing in Power Prediction for Cyclists: A Biomechanical Study