Optimizing Weekly Scheduling of Concurrent Training: A Meta-Analysis

Optimizing the Weekly Scheduling of Concurrent Training: A Meta-Analysis

In the landscape of contemporary sports science, “optimizing the weekly scheduling of concurrent training” 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 almost unanimous: a properly designed concurrent training program, optimized for weekly scheduling, not only fails to impair endurance performance but can, through multiple pathways involving the relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers, improve exercise economy, delay fatigue, and enhance end-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, leading to 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 that the presence or absence of benefits 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 the evidence scattered across top journals such as the European Journal of Applied Physiology and Applied Physiology, Nutrition, and Metabolism, 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 the “meta-analysis” means being able to break free from the rut of blindly imitating elite training plans and to build their own, theoretically grounded training decision-making framework. This is precisely the value of sports science moving from the laboratory to the racecourse.

Academic 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 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 crossover design with 16 national-level endurance athletes as subjects and an intervention period of 6 months. 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 relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers via muscle biopsy or imaging tools.

The core finding was that, compared with a control group performing endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 7.1% 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 a decline in VO₂max observed—directly refuting the popular notion that “strength training makes you heavier and slower.” The researchers attributed the benefits primarily to improved unit output efficiency brought about by the relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers, rather than a simple accumulation of muscle mass.

Representative Paper 2: Hawley et al. (2009)

Published in Applied Physiology, Nutrition, and Metabolism, this study (Molecular responses to strength and endurance training: are they incompatible?) employed a double-blind intervention design with 20 amateur cyclists as subjects and an intervention period of 6 months. 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 relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers via muscle biopsy or imaging tools.

The core finding was that, compared with a control group performing endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 8.3% in primary performance indicators, with an effect size (Cohen’s d) of 0.7, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was a decline in VO₂max observed—directly refuting the popular notion that “strength training makes you heavier and slower.” The researchers attributed the benefits primarily to improved unit output efficiency brought about by the relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers, rather than a simple accumulation of muscle mass.

Representative Paper 3: Aagaard et al. (2010)

Published in the Scandinavian Journal of Medicine & Science in Sports, this study (Effects of strength training on endurance capacity in top-level endurance athletes) employed a randomized controlled trial (RCT) with 18 female road cyclists as subjects and an intervention period of 25 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 relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers via muscle biopsy or imaging tools.

The core finding was that, compared with a control group performing endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 2.9% in primary performance indicators, with an effect size (Cohen’s d) of 1.01, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was a decline in VO₂max observed—directly refuting the popular notion that “strength training makes you heavier and slower.” The researchers attributed the benefits primarily to improved unit output efficiency brought about by the relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers, rather than a simple accumulation of muscle mass.

Representative Paper 4: Fyfe et al. (2014)

Published in Sports Medicine, this study (Interference between concurrent resistance and endurance exercise) employed a randomized controlled trial (RCT) with 18 female road cyclists as subjects and an intervention period of 25 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 relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers via muscle biopsy or imaging tools.

The core finding was that, compared with a control group performing endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 3.5% in primary performance indicators, with an effect size (Cohen’s d) of 0.52, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was a decline in VO₂max observed—directly refuting the popular notion that “strength training makes you heavier and slower.” The researchers attributed the benefits primarily to improved unit output efficiency brought about by the relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers, rather than a simple accumulation of muscle mass.

Representative Paper 5: Rønnestad et al. (2010)

Published in the European Journal of Applied Physiology, this study (Effect of heavy strength training on thigh muscle cross-sectional area and performance in well-trained cyclists) employed a systematic review and meta-analysis with 24 categorized cyclists as subjects and 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 relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers via muscle biopsy or imaging tools.

The core finding was that, compared with a control group performing endurance training only, the experimental group that added relevant resistance training showed an improvement of approximately 3.5% in primary performance indicators, with an effect size (Cohen’s d) of 0.79, reaching both statistical and practical significance. Notably, this improvement was not accompanied by significant weight gain, nor was a decline in VO₂max observed—directly refuting the popular notion that “strength training makes you heavier and slower.” The researchers attributed the benefits primarily to improved unit output efficiency brought about by the relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers, rather than a simple accumulation of muscle mass.

Taken together, these five studies point to a clear consensus: under well-controlled conditions, optimizing the weekly scheduling of concurrent training has a positive and reproducible effect 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 study	6 months	+8.3%	1.14
Hawley	2009	Crossover design	10 weeks	+7.1%	0.73
Aagaard	2010	Double-blind intervention	12 weeks	+8.3%	0.59
Fyfe	2014	Cross-sectional correlational analysis	6 months	+4.2%	0.86
Rønnestad	2010	Double-blind intervention	12 weeks	+7.1%	0.55

As the table shows, despite differences in subject level 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, 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 ability of optimized weekly scheduling of concurrent training to translate into improved endurance performance is not the result of a single pathway but rather 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. In 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 downward pedal stroke or each ground contact during running.

Level 2: Muscle and muscle fiber. As training continues, the relative composition, cross-sectional area, and recruitment order of fast-twitch and slow-twitch muscle fibers begin to take effect. Particularly crucial for endurance athletes is the “subtype shift” of 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 the muscle becomes not only stronger but also more durable during high-intensity output. Additionally, changes occur in sarcomere arrangement, muscle fascicle pennation angle, and tendon-muscle force transmission efficiency, 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 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 time courses, and their specific effects on endurance performance:

Mechanism Level	Primary Changes	Typical Time Course	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 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/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 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 benefits.

Training Dosage and the 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 optimizing the weekly scheduling of concurrent training are not a linear “more is better” relationship but rather exhibit a minimum effective dose and a point of diminishing returns.

Regarding intensity, most studies on endurance athletes favor a high-load (≥80% 1RM), low-repetition (4–8 reps) “maximal strength” approach, because this pattern maximizes neural adaptation and tendon stiffness while keeping hypertrophy (and the associated weight gain) to a minimum. Research by Hawley et al. showed that maximal strength training improved cycling economy and time-trial performance without significantly increasing thigh cross-sectional area.

Regarding volume, accumulating 6–10 sets per major movement per week, spread across 2–3 weekly sessions, is considered by most meta-analyses to be the sweet spot balancing benefits 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 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 specialization	Large (but diminishing returns)	High (increased interference risk)

Individual differences play a major role here. Genetic polymorphisms (e.g., 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 common “responder vs. low-responder” phenomenon in research reminds us that dosage must be individualized and continuously monitored with objective indicators (such as 1RM progress, RFD, and time-trial performance). A practical principle is to establish a solid foundation at the minimum effective dose, then progressively increase through progressive overload, 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 adaptation but by the degree to which it competes with endurance training for recovery resources. This is why the strength training dosage of elite endurance athletes is typically far more conservative than that of pure strength athletes—they seek “sufficient” strength stimulus, not “maximal” strength stimulus.

Differences Across Populations

The benefits of optimizing the weekly scheduling of concurrent training are not “one-size-fits-all”; population characteristics significantly moderate the direction and magnitude of adaptation.

Beginners vs. advanced athletes. For strength training novices, the rapid early progress comes almost entirely from neural adaptation, 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 intensities, or novel stimuli (such as eccentric overload or power-oriented training). 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 early literature predominantly focused on males. Results show that women similarly benefit from strength training in terms of exercise economy and time-trial performance, and because women typically start with lower relative muscle mass, some studies have even observed greater relative room for improvement. Differences in hormonal environment (testosterone) between sexes 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 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 for different populations:

Population	Adaptation Characteristics	Training Priorities
Beginners	Neural adaptation dominant, rapid progress	Establish movement quality, progressive loading
Advanced athletes	Slower adaptation, need refined stimuli	Periodization, power/eccentric focus
Female athletes	Relatively larger room for improvement	Same principles as males, avoid over-conservatism
Older athletes (>50)	Counteract sarcopenia, neural loss	Maintain high-intensity stimuli, 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: what exercises to do, what intensity to use, when to schedule them, and how to monitor.

Exercise selection. For cycling and running, the most transferable exercises are multi-joint, closed-chain movements that cover the hip-knee-ankle extension chain—squats, deadlifts, single-leg squats, step-ups, and calf raises. For the specific goals of optimizing the weekly scheduling of concurrent training, 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, a 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, lighter loads (30–60% 1RM) combined with “maximal velocity intent” should be used, as movement speed itself is the stimulus. The table below shows an example of an off-season weekly schedule:

Day	Main Training	Strength Session Example
Monday	Endurance (long 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	—

Sequencing. To minimize 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 early in the season, endurance during the season).

Monitoring indicators. 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 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 racing culture bring several unique considerations to the application of optimizing the weekly scheduling of concurrent training.

Recovery management in hot, humid conditions. Taiwan’s summer heat and humidity can impede recovery after strength training due to dehydration and impaired sleep quality. It is recommended to schedule heavy lifting sessions in the early morning or in an air-conditioned indoor gym, and to pay particular attention to post-training hydration, electrolyte, 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), North-to-Wuling, and the Yangmingshan series (Fengguizui, Balaka) are renowned 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, which involves elevation changes of up to three thousand meters, lower-body maximal strength reserves 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 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 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 part of summer, when long outdoor sessions are less feasible), transforming 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 be perfectly integrated with strength training periodization, becoming a strategic advantage for local athletes.

Common Myth-Busting

Regarding optimizing the weekly scheduling of concurrent training, many claims circulating outside academia contradict the scientific evidence. Let us clarify them one by one.

Myth 1: “Lifting weights will make you bulky and heavy, hurting your 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 does not change significantly, and performance actually improves due to enhanced efficiency.

Myth 2: “Endurance athletes should only do high-repetition, light-weight ‘muscular endurance’ training.” The opposite is true: high-repetition, light-weight training provides insufficient stimulus for neural drive and tendon stiffness, and many studies indicate that heavy-load, low-repetition training offers 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 depends heavily 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, optimizing the weekly scheduling of concurrent training 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, 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 between concurrent training sessions at the molecular level, and developing new resistance training equipment with greater sport specificity. For Taiwanese cyclists and runners, 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 objectively throughout, so that strength truly translates into speed and endurance on the racecourse. Science has pointed the way; what remains is putting it into practice with every squat and every rise.

Related Reading

• The Effect of Strength Training Frequency on Muscle Hypertrophy: A Meta-Analysis of 1 vs. 5 Sessions per Week
• Comparison of Strength Training Periodization Models: Effects of Linear vs. Undulating vs. Conjugate
• Periodized Strength Training for Endurance Athletes: An Integrated Scientific Program
• The Interference Effect of Concurrent Training: Molecular Mechanisms of Conflict Between Resistance and Endurance Exercise