Does muscle hyperplasia occur in humans?

While often stated that hyperplasia (increase in muscle fiber number) doesn't occur in humans, it's more accurate to say it hasn't been directly observed experimentally due to ethical and practical limitations of human research. However, indirect evidence from animal models and cadaver studies suggests it likely does occur in humans to some extent, even with normal resistance training.

How do muscle fibers grow, referring to longitudinal and radial hypertrophy?

Individual muscle fibers can grow in two main ways: longitudinally (increasing in length) or radially (increasing in diameter/thickness). Longitudinal hypertrophy involves adding sarcomeres in series, making the fiber longer, often influenced by stretching and eccentric contractions. Radial hypertrophy involves adding sarcomeres in parallel, making the fiber thicker, often associated with concentric contractions.

How effective are stretching interventions for muscle growth in humans?

Stretching interventions in humans can induce hypertrophy, but the effects are generally small compared to resistance training. They typically require high intensity and longer durations (over 1.5 hours per week) to yield meaningful changes, particularly in fascicle length (longitudinal hypertrophy). Lower intensity stretching or shorter durations show negligible to no significant muscle size improvements.

Why is it argued that increased growth from lengthened strength training is not truly 'stretch-mediated'?

The term 'stretch-mediated' implies that the stretch itself is the primary causal factor. However, studies showing more growth from training at longer muscle lengths (e.g., partial squats to 90-100 degrees) involve very little actual stretch, far less than what's needed for independent hypertrophic effects from devoted stretching. The duration and intensity of the 'stretch' during these exercises are insufficient to mediate the observed growth differences, suggesting other mechanisms are at play.

Which type of muscle contraction is better for increasing fascicle length?

Eccentric-only training (muscle lengthening under tension) consistently leads to greater increases in fascicle length compared to concentric-only training (muscle shortening under tension). This is thought to contribute to longitudinal hypertrophy. Conversely, concentric training tends to increase pennation angle more, associated with radial hypertrophy.

Do changes in pennation angle and fascicle length always reflect radial and longitudinal hypertrophy accurately?

Changes in pennation angle and fascicle length are often used as proxies for radial and longitudinal hypertrophy, respectively. However, there are inherent assumptions and measurement errors. Correlations between these architectural changes and overall muscle size are only moderate, indicating that neither variable singularly explains hypertrophy, and direct measures (like biopsies for fiber cross-sectional area) don't always fully align with proxy measures.

How can I practically incorporate lengthened training into my routine?

You can use lengthened partials as your primary training method, aiming for a consistent joint angle (e.g., 90 degrees of knee or elbow flexion at the bottom of a rep). Alternatively, you can perform lengthened partials after full range of motion failure at the end of a set (lengthened supersets) to extend the stimulus. Consider the safety of the movement and use machines or spotter arms where free weights might be risky for failure in a stretched position.

Should I increase the weight for lengthened partials due to the shorter range of motion?

It depends on the movement. For exercises hardest in the shortened position (e.g., most back movements), you might be able to use more weight. However, for movements hardest in the lengthened to mid-range (e.g., squats, bench press), you might need to use less weight because you're cutting out the easier lockout portion and maintaining continuous tension. It's best to assess this on a movement-by-movement basis.

Does the benefit of lengthened training 'hit a wall' for advanced lifters or run out over time?

While fascicle length adaptations are theoretically finite, the idea that they max out after a few months and cease providing benefits is likely unsubstantiated. Evidence shows trained individuals can still experience increases in fascicle length. Moreover, lengthened training also promotes radial hypertrophy. Therefore, it's expected to continue being beneficial for muscle growth, though the rate of additional gains might slow over time as certain adaptations approach their physiological maximum.

Key Moments

Is Stretch-Mediated Hypertrophy Overhyped? (Episode 132)

Stronger By Science

Sports6 min read205 min video

May 1, 2024|7,308 views|195|59

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Key Moments

TL;DR

Stretch-mediated hypertrophy is a specific concept that likely doesn't explain muscle growth from lengthened training.

Key Insights

Stretch-mediated hypertrophy originated from animal studies in the 1970s, where chronic, intense stretching led to significant muscle growth and hyperplasia, even in denervated muscles.

True stretch-mediated hypertrophy, independent of active contraction, appears to occur in animals with very high loads (10-35% body weight) and long durations (hours daily for weeks).

In humans, stretching interventions yield modest hypertrophy, with significant benefits requiring high intensity and prolonged duration (over 1.5 hours/week).

Evidence suggests that hypertrophic adaptations to lengthened resistance training (e.g., partials in a stretched position) are unlikely to be 'stretch-mediated' in the scientific sense.

The range of motion, intensity, and duration of stretch in most lengthened resistance training studies are insufficient to independently cause the observed growth if it were purely stretch-mediated.

Different training types, particularly eccentric contractions, may preferentially increase fascial length (longitudinal hypertrophy), while concentric contractions may favor pennation angle (radial hypertrophy).

Training through longer muscle lengths generally promotes more overall muscle growth, including both fascial length and pennation angle increases, regardless of a subjective 'stretch' sensation.

THE SCIENTIFIC ORIGIN OF STRETCH-MEDIATED HYPERTROPHY

The concept of stretch-mediated hypertrophy (SMH) traces its roots to animal studies from the 1970s, notably research by Sola and colleagues. These pioneering experiments typically involved applying significant loads (10-35% of body weight) to animal muscles, such as the anterior latissimus dorsi in birds, for extended durations, often 1 to 24 hours daily over several weeks. Such chronic and intense stretching consistently led to substantial muscle growth, sometimes exceeding 140-170% increases in muscle mass, alongside muscle hyperplasia (the creation of new muscle fibers). Crucially, these effects were observed even in denervated muscles, indicating that SMH can occur independently of active muscle contraction.

HYPERPLASIA: AN UNRESOLVED HUMAN QUESTION

While animal studies consistently report both hypertrophy and hyperplasia, the occurrence of hyperplasia in humans remains a subject of debate. Direct experimental observation of hyperplasia in humans is practically unfeasible due to ethical and methodological challenges, as it would require invasive procedures like removing entire muscles for fiber counting. However, indirect evidence, such as the consistent observation of hyperplasia across various animal species and in cadaver studies comparing non-dominant versus dominant limbs, strongly suggests its potential presence in humans. Despite this, the lack of direct evidence means that the precise extent and mechanisms of hyperplasia in human training remain largely unknown.

UNDERSTANDING MUSCLE FIBER GROWTH: PARALLEL VERSUS SERIES ADDITION

Muscle fiber growth can primarily occur in two ways: in parallel (radial hypertrophy) or in series (longitudinal hypertrophy). Radial hypertrophy involves an increase in the diameter of muscle fibers, akin to a cylinder becoming thicker. Longitudinal hypertrophy, on the other hand, refers to an increase in muscle fiber length, like a cylinder becoming longer. Both occur through the addition of sarcomeres, either side-by-side or end-to-end, respectively. In human research, fascial length (bundles of muscle fibers) is often used as a proxy for fiber length and longitudinal hypertrophy, while pennation angle (the angle of muscle fibers relative to the tendon) is frequently used as a proxy for radial hypertrophy. These concepts are crucial for understanding how different training stimuli might differentially impact muscle architecture.

HUMAN RESPONSES TO STRETCHING INTERVENTIONS

Translating animal SMH findings to humans reveals significant differences. Human stretching protocols are typically much shorter in duration and lower in intensity due to ethical constraints. Meta-analyses on human stretching interventions show only modest muscle growth. A narrative review by Warne and colleagues, for instance, found that only 6 of 10 studies reported significant hypertrophy. The magnitude of hypertrophy from stretching in humans (average small effect size of 0.27 with high-intensity stretching) is considerably less than that seen in animal models (effect size of 8.5) and lower than traditional resistance training (average effect size of 0.34). Effective fascial length increases in humans typically require over 1.5 hours of high-intensity stretching per week.

THE MISNOMER OF 'STRETCH-MEDIATED' RESISTANCE TRAINING

The application of the term 'stretch-mediated hypertrophy' to lengthened resistance training is likely inaccurate in its scientific sense. For an effect to be truly stretch-mediated, the stretch component would need to be the primary causal factor explaining the additional growth. However, most studies comparing lengthened versus shortened resistance training, even those reporting superior growth with the former (e.g., Pedrosa study on quads), involve ranges of motion (e.g., 90-100 degrees of knee flexion) that induce minimal subjective 'stretch.' Furthermore, the total time spent in these 'stretched' positions during typical resistance training (often only a few minutes per week) is far below the duration and intensity required for independent hypertrophic effects from passive stretching, as shown in human stretching studies. Thus, the observed hypertrophy from lengthened training is unlikely to be mediated solely by a significant stretch.

ECCENTRIC VS CONCENTRIC TRAINING: ARCHITECTURAL ADAPTATIONS

Different modes of muscle contraction tend to elicit distinct architectural adaptations. Research suggests that eccentric-only training (muscle lengthening under tension) consistently leads to greater increases in fascial length (longitudinal hypertrophy) compared to concentric-only training (muscle shortening under tension). Conversely, concentric-only training may preferentially increase pennation angle (radial hypertrophy). This differential response highlights that integrating both eccentric and concentric phases in training is likely beneficial for comprehensive muscle development. Factors like higher loads and faster eccentric velocities can further amplify increases in fascial length, as seen in exercises like Nordic curls.

LENGTHENED TRAINING: BEYOND THE 'STRETCH'

Despite the misapplication of the term SMH, training at longer muscle lengths consistently appears to be beneficial for hypertrophy. Studies comparing lengthened versus shortened resistance training, even when equating for contraction type, generally show greater increases in fascial length and pennation angle, and ultimately, overall muscle size, in the lengthened groups. This suggests that lengthened training promotes both longitudinal and radial hypertrophy. Therefore, while a subjective sensation of deep stretch might not be present or even necessary, the mechanical advantage or other stimuli associated with working muscles at longer positions contribute to superior growth outcomes across various measures of muscle size and architecture.

PRACTICAL IMPLICATIONS FOR TRAINING

For practitioners, the takeaway is to prioritize training through the longest range of motion that is safe and comfortable. The perceived 'stretch' sensation is not a prerequisite for gaining benefits from lengthened training. Even partials performed in a lengthened position can be highly effective. For example, incorporating lengthened partials after full-range-of-motion failure (lengthened supersets) has shown promising results for muscle growth, particularly in movements where the muscle is challenged in its lengthened state. However, it's crucial to select exercises and setups that allow for safe execution of lengthened partials, considering potential risks with free-weight compound movements near failure without proper spotting or safety equipment.

THE 'STRETCH POTENTIAL' FALLACY

The argument that the benefits of lengthened training 'hit a wall' after a few months, due to muscles reaching their 'stretch potential,' largely lacks strong empirical support. This argument often assumes that fascial length increases are the sole benefit of lengthened training and that these adaptations are very short-lived. However, evidence suggests that lengthened training also promotes radial hypertrophy (pennation angle increases). Moreover, several studies, including those in highly trained populations, have observed sustained increases in fascial length beyond a few weeks or months of training. While fascial length adaptations are theoretically finite (as extreme, unchecked lengthening would impair muscle function), practically it is unlikely that individuals in a typical training context would quickly exhaust this potential. The benefits of lengthened training likely persist over time, even if the rate of specific architectural changes might slow down.

Mentioned in This Episode

●Studies Cited

Lengthened Training for Hypertrophy: Practical Considerations

Practical takeaways from this episode

Do This

Train through the longest safe and comfortable range of motion.

Use lengthen partials specifically for movements that are easiest in the shortened position (e.g., back training, dumbbell pullovers).

Consider doing lengthen partials after full range of motion failure as an intensity technique.

Use specific landmarks like 90-degree joint angles for consistency in lengthened partials on compound movements.

Incorporate weighted stretching or long muscle length partials into your warm-up to improve range of motion.

Be consistent with your chosen range of motion and load, adjusting as you learn the movement.

Avoid This

Don't assume more muscle growth from longer muscle lengths is solely due to 'stretch-mediated' effects, especially if no intense stretch is felt.

Don't force yourself into painful or unsafe positions seeking a 'deep stretch' if the underlying mechanism isn't purely stretch-mediated.

Don't rely on lengthening partials for movements that are unsafe to fail in a stretched position without proper safety equipment (e.g., free-weight barbell bench, squats without spotter arms).

Don't expect linear increases in weight for all lengthen partial movements; some (like squats/bench press) might require less weight due to continuous tension.

Don't assume the benefits of lengthened training 'run out' quickly for advanced lifters, as there's evidence for continued adaptation.

Common Questions

Stretch-mediated hypertrophy refers to muscle growth stimulated by chronic stretching, distinct from active contraction. It was first conceptualized in the 1970s through animal studies (e.g., Sola and colleagues, 1973) where muscles were stretched with significant loads for long durations, even in denervated (inactive) muscles, leading to substantial increases in muscle mass and even hyperplasia (new muscle fiber creation).

Topics

Stretch-mediated Hypertrophy Lengthened Training Muscle Growth Mechanisms Longitudinal Hypertrophy Radial Hypertrophy Fascicle Length Pennation Angle Resistance Training Intensity Concentric Training Exercise Programming

Mentioned in this video

Studies & Research

Anaki and colleagues study

A study in highly trained male throwers (average 405 lb squat) that still observed increases in fascicle length during a phase of their 25-week training macrocycle.

Pedrosa study

One of the first studies comparing long-length partials to full range of motion training for quad hypertrophy, which is used to illustrate issues with the 'stretch-mediated' hypothesis.

Baroni and colleagues study

One of several studies finding substantial increases in fascicle length beyond the initial few weeks of training, countering the idea that fascicle adaptations are always short-lived.

McMahon study

A study on quad training using various exercises, finding more growth with a 90-degree knee flexion range of motion compared to 50 degrees, despite the researchers labeling it as 'stretch-mediated.'

Hartman (2013) paper

A review paper that shifted the paradigm regarding deep squats and knee safety, showing that forces on the knees increase with knee joint angle but remain below tissue loading capacities.

Corey paper

An older study often cited to claim that drug-free individuals cannot achieve a fat-free mass index (FFMI) above 25, which the hosts criticize for drawing overly strong inferences from an insufficient sample.

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