Stretch-mediated hypertrophy: the terminology confusion also involves researchers

In recent years, a new term in the muscle hypertrophy field has begun to emerge, namely stretch-mediated hypertrophy. However, its precise meaning is much more confusing and controversial than it may seem.

Witnessing the evolution of the various meanings and debates on the topic, the article attempts to reconstruct the facts. The purpose is to focus on the various proposed meanings of the terms by numerous experts and highlight the existing controversies in the field rather than delve deeper into the debated mechanisms.

Beardsley’s original definition

It seems that the first author to discuss stretch-mediated hypertrophy (SMH) in the exercise and hypertrophy field by providing a precise definition was Chris Beardsley in an article on his blog in 2018 (1) and then in his e-book in 2019 (2), well before it was addressed in research. This gives Beardsley particular significance in the overall narrative, though he will later need to grapple with scientific rigor. 

To be precise, the term “stretch-mediated hypertrophy” had already been utilized in some research on animal cardiomyocytes starting less than a decade earlier (3-5), but since it pertains to a completely different field, we will accept that these are phenomena and research that are not related to the domain of exercise at all.

Anyway, Beardsley initially asserted that SMH occurs when the muscle reaches lengths at which passive muscle structures dominate in force production. In technical terms, the loaded muscle should reach the descending portion of its length-tension curve; alternatively, it should experience passive mechanical tension (1,2). 

Only in 2020 did he also add that SMH was characterized by an increase in the number of fascicles or sarcomeres in series and that it would rarely be observed after the early months of training (6).

Here is a little problem: the author did not cite any references to confirm these definitions, it seems he invented them. However, thanks to his contribution and influence, the term began taking on a certain meaning and diffusion for the first time in the international exercise/resistance training field.

Henselmans’ and Nuckols definitions

In 2021, researcher and educator Menno Henselmans dedicated an article to the topic on his blog; however, he gave it a slightly different definition, namely, the most literal one: SMH is the surplus hypertrophy obtained by achieving high muscle lengthening during dynamic resistance training (7). Regardless of the mechanisms by which it occurs, if more hypertrophy is gained with high muscle lengthening, then that is SMH. 

The following year, another influential researcher and educator, Greg Nuckols, also dedicated an article to the topic. He shared Henselmans’ definition, describing SMH as a factor that enhances hypertrophy during resistance training due to the inherent effect of muscle lengthening in dynamic resistance training (8).

On the other hand, the article focused on reviewing a study on hypertrophy caused by static stretching (9). Nuckols highlighted stretching as an exercise stimulus to achieve SMH outside of regular resistance training. This may be the first time stretching has been described as an important cause of SMH, evidently even before research, since Beardsley and Henselmans had only mentioned it tangentially.

SMH term entry in exercise research

In 2022, the term seems to have been mentioned for the first time in research related to resistance training in a study conducted by Kassiano et al. concerning the effect of different range of motion (ROM) angles and muscle lengths on hypertrophy. However, they associate it with passive tension and the descending portion of the muscle’s length-tension curve (10). 

In other words, whoever first introduced the term SMH into exercise research adopted a definition very similar to the one Beardsley proposed in his blog (1), a detail that likely went unnoticed among experts. The researchers state that SMH is obtained by an “additional passive tension from elastic elements (e.g., titin) [which may elicit] superior muscle growth because of the potential additive effects of stretch and contraction.”

To confirm this definition, they cite McMahon et al., who, throughout the paper, speak generically of “stretch-mediated responses, effects, or adaptations” referring to greater adaptations obtained with isolated lengthening compared to isolated shortening dynamic resistance training (11). The problem is that they do not mention the term SMH, much less describe it as a well-defined phenomenon. 

It should be noted that both Kassiano and McMahon discussed adaptations induced by resistance training in lengthening compared to other ROM angles, confirming that originally, SMH always referred to the inherent effect of lengthening in regular dynamic resistance training rather than stretching. This is consistent with the meaning originally conveyed by Beardsley.

Beardsley’s revisited definition

In 2023, the fitness social media landscape has seen the emergence of SMH as a “pop” term. Meanwhile, Beardsley rides the wave and strengthens his (arbitrary) revisited definition: for SMH to be such, there must also be an increase in serial sarcomeres (or sarcomerogenesis) (12,13). The title of one of his articles makes this position very explicit: “Stretch-mediated hypertrophy = sarcomerogenesis” (12).

Therefore, it is not enough for hypertrophy to develop with high lengthening; this must occur through passive tension and sarcomerogenesis; otherwise, it cannot be called SMH, even if hypertrophy is observed through other mechanisms (13).

However, since sarcomerogenesis would occur only in novices, he reiterates that SMH is not achieved after the early couple of months of training (12,13); in other words, SMH would not be observed in non-novices or trained subjects.

Alternative new SMH meaning in research

In 2023, the first review entirely dedicated to SMH was finally published by Warneke et al., which, however, fuels the ambiguities. According to them, SMH basically includes very prolonged and often high-intensity static stretching stimuli (which, in many cases, could be considered as very prolonged lengthening isometrics) (14). 

They admit that the mechanisms are unclear, but the studies examined have nothing to do with normal dynamic resistance training at different ROMs and lengths; i.e., those considered, for example, by Kassiano and McMahon (10,12). That is, Warneke et al. referred to SMH as hypertrophy caused by stretching and not by the muscle lengthening achieved with resistance training as originally intended also in research (in fact, they do not cite the works of Kassiano, McMahon, and others in this area).

Note that Warneke et al. initially did not use the term SMH in their studies (like the one published the year before and reviewed by Nuckols) (8,9), suggesting that the exclusive reference to stretching was an arbitrary choice proposed only a posteriori. Perhaps the researchers felt justified in referring only to stretching because, in this way, they could completely isolate the “muscle lengthening variable”, excluding dynamic resistance training as an alleged “confounder“. This, however, would still be an arbitrary and confusing perspective.

Ironically, even Beardsley changed his position again, asserting that SMH “is a term that was originally coined to describe the muscle growth caused by static stretching”, but again, without citing any confirming references (13). His change is likely explained by the publication of Warneke’s review, which was apparently the first in research to explicitly associate the term SMH solely with stretching. 

At this point in history, at least three different definitions of SMH have been attributed to or used by several figures actively engaged in the research field (for those who don’t know, even Beardsley, Henselmans, and Nuckols have published research), and one of these definitions, the most recent one, strays farthest from the original conception.

My critics

In January 2024, I wrote a long post on Facebook explaining that Beardsley (and therefore his “parrot” colleague Paul Carter) had made up the meaning of SMH. The post received some positive feedback, as it was shared, for example, by Lyle McDonald and Carlo Buzzichelli in their Facebook groups and appreciated by a few other experts and scientists (see the comments in the post). 

Only about ten days later, Henselmans raised curiously similar criticisms about Carter (which actually echoed Beardsley’s arguments), and a month later, Hanson did the same. Finally, the field began to better recognize the confusion surrounding SMH and that what Beardsley (and his “arm”, Carter) claimed was a personal view, although later partially accepted in research by some (10).

The Wolf’s intervention

At this point, the researcher Milo Wolf offers his own perspective, but unfortunately, he doesn’t clarify the confusion either (15). In short, Wolf asserts that SMH is being mistaken for emphasized lengthening resistance training, such as full ROM and lengthened partials. According to him, the correct definition of SMH is essentially that of Warneke et al., which refers to what is achieved with prolonged and sometimes extreme static stretching (or stretching isometrics) protocols. 

Wolf inadvertently questions the original definition first reported in research by Kassiano et al. (10), who essentially recognized SMH similarly to Beardsley in the early years (1, 2).

Wolf also indirectly criticizes Henselmans and Nuckols’ positions (7,8), while ironically, both he and Nuckols are part of the same team, Stronger By Science. In other words, Wolf criticizes the views that his close colleague expressed on the same website, perhaps without even realizing it.

Finally, Wolf also took a dig at Beardsley (and his “arm” Carter), criticizing the notion that SMH is synonymous with sarcomerogenesis and that it only occurs in novices. This is the same criticism that I and others expressed a few months earlier.

To recap

In research, the term stretch-mediated hypertrophy (SMH) originally emerged in areas completely different from exercise and hypertrophy training, specifically in the study of certain phenomena in cardiomyocytes.

The first to link the term SMH with the exercise/hypertrophy domain was likely Chris Beardsley in his blog in 2018, but his definition was not substantiated; he seemingly invented it, since it was not utilized in research at that time. 

Over the years, the term gained traction through fitness social media, prompting other science writers and scientists to adopt it, albeit with a slightly altered meaning. 

Its popularity likely also encouraged some scientists to introduce it into exercise research, proposing a definition similar to the one Beardsley originally assigned. The concept was consistently associated with muscle lengthening in regular dynamic resistance training.

Beardsley then redefined the term, asserting that for SMH to be such, it had to involve sarcomerogenesis; if the further hypertrophy favored by lengthening did not occur through this process, according to him, it could not be defined as SMH. 

Warneke et al. then apparently published the first review dedicated to the topic, but the authors attributed a different meaning to SMH compared to the original one coined by Beardsley (later taken up in research by Kassiano) and from that redefined by Henselmans (later shared by Nuckols). Warneke et al. arbitrarily related SMH only to stretching and not to resistance training. 

Finally, Wolfe criticized both Beardsley’s and Henselmans’ definitions, asserting that the most correct definition of SMH is the one used by Warneke’s team.

Implications

SMH represents one of the typical cases in which a term without a well-accepted universal definition, is redefined at will by various authors (including scientists), creating a lot of confusion and misunderstanding. Which definition is scientifically the most correct or “definitive” remains a matter of debate. 

• According to some, it would be appropriate to accept Beardsley’s definition, who first coined it in the exercise context, even though he did so outside of research (but those who first discussed it in research adopted his original definition, which is relevant). 
• According to others, the most sensible meaning would be the literal one originally recognized by Henselmans: if achieving greater lengthening allows for more hypertrophy compared to not achieving it, then that surplus is SMH. 
• According to others, the most accurate meaning would be the one coined by those who first published a review, although it is the last definition chronologically, as well as the one furthest from the original conception.  
• Others might instead conclude that SMH refers to any additional hypertrophy inherently caused by muscle lengthening, regardless of whether this is achieved through resistance training or stretching.

Personally, I would agree with the second option originally proposed by Henselmans, because it is the only one that would be useful for athletes and field workers. Knowing that achieving greater muscle lengthening or emphasizing stretch under load may optimize hypertrophy (at least in many cases) is relevant to exercisers and trainers. Referring mainly to dynamic resistance training is consistent with its original use, while static stretching rarely generates significant hypertrophy as it is normally applied.

Understanding SMH in strictly mechanistic terms would be useful for exercise scientists, who, however, have yet to clarify among themselves which of the two definitions (Kassiano vs. Warneke) is more accurate. In the meantime, I believe it is acceptable for those of us in the industry to use the more literal expression.

References:

1. Beardsley C. How can exercise strength curves affect hypertrophy? sandcresearch.medium.com. Sep 5, 2018.
2. Beardsley C. Hypertrophy: Muscle Fiber Growth Caused by Mechanical Tension. Strength and Conditioning Research Limited; 2019.
3. Blaauw E et al. Stretch-induced hypertrophy of isolated adult rabbit cardiomyocytes. Am J Physiol Heart Circ Physiol. 2010 Sep;299(3):H780-7.
4. Scimia MC et al. APJ acts as a dual receptor in cardiac hypertrophy. Nature. 2012;488,394–398.
5. Matter ML. Induction of VEGF secretion in cardiomyocytes by mechanical stretch. Methods Mol Biol. 2015:1332:67-74.
6. Beardsley C. Range of motion. patreon.com. Dec 17, 2020. 
7. Henselmans M. Beyond full ROM: 3 Lessons about stretch-mediated hypertrophy. mennohenselmans.com. 2021, May 27.  
8. Nuckols G. Can stretching directly cause muscle growth? strongerbyscience.com. 2022, Aug 31. 
9. Warneke K et al. Influence of long-lasting static stretching on maximal strength, muscle thickness and flexibility. Front Physiol. 2022 May 25:13:878955. 
10. Kassiano W et al. Greater gastrocnemius muscle hypertrophy after partial range of motion training performed at long muscle lengths. J Strength Cond Res. 2023 Sep 1;37(9):1746-1753. 
11. McMahon GE et al. Muscular adaptations and insulin-like growth factor-1 responses to resistance training are stretch-mediated. Muscle Nerve 49: 108–119, 2014. 
12. Beardsley C. Stretch-mediated hypertrophy = sarcomerogenesis. patreon.com. Oct 2, 2023 
13. Beardsley C. Stretch-mediated hypertrophy. patreon.com. Mar 25, 2024. 
14. Warneke K et al. Physiology of stretch-mediated hypertrophy and strength increases: a narrative review. Sports Med. 2023 Nov;53(11):2055-2075. 
15. Wolf M. Why stretch-mediated hypertrophy is overhyped. strongerbyscience.com. Dec 23, 2024.