Foundational Concepts for Understanding Hypertrophy Posted on 14 Jan 16:29

Justin Swinney - January 14, 2020

     As of late, the word hypertrophy has gained popularity in the fitness industry.  Unfortunately, a majority of this new popularity is from online trainers and coaches with poor comprehension. Their lack of interpretation led to incomplete explanations, which fueled unnecessary assumptions by individuals in search of an operational definition.  The perplexity was recently brought to my attention by a conversation involving the odd but confident use of the word hypertrophy.  The unexpected statement describing their workout program, “I am doing a hypertrophy workout program.  It has heavy 3 RM strength days and light 15 to 20 hypertrophy days.” and I said, “Wait, hold on… Hypertrophy workout program with strength days and rep days.  Let me take a step back and ask what do you mean by “hypertrophy” workout program?”.  The next few seconds were silent and then the unexpected reply, “hypertrophy, you know, like a bodybuilder, more reps, to get pumped up and grow muscles.” I replied, “Well, since you know it means muscle growth, that is primarily correct.  How did you learn the meaning of hypertrophy?”.  The statement that I have heard numerous times before, “I found the workout on and Googled it.”.  I started to sense a little bit of uncertainty in the tone and said, “Would you like me to explain the term hypertrophy? I can provide a few specifics, and perhaps the information helps you in some way.”. 


     While some trainers have a thorough understanding and articulate their perspectives with clarity, other trainers attempt to gain attention by creating a mere intellectual mirage.  Individuals can’t know the author’s rationalization behind the search mediated material.  Collectively speaking, without prior specialized education in exercise physiology, it is nearly impossible to identify any limitations demonstrated with a conversation.  In general, the population spends the majority of their time at work or socializing with friends and family, not reading clinical studies, literature reviews, and textbooks.  Considering the time constraints, individuals rely on a trainer or an online fitness personality to provide them with practical perceptions of relative exercise information. The purpose of this article is to filter potential misunderstandings and provide a base description of the word “hypertrophy.”  Then build upon that base and delineate the specific categories of skeletal muscle hypertrophy.  Finally, a summary of the information to aid in developing foundational concepts and helping confirm the understanding of skeletal muscle hypertrophy. 


     The word hypertrophy first appeared in the mid-19th century. The combination of the English term “hyper-“ denoting “beyond” or “exceeding” and the Greek term “-trophia” denoting “nourishment” was used in the medical literature to describe an observed adaptation of excessive growth (1.). The aforementioned etymology of hypertrophy supports the operational definition, growth from the increased size of cells. As a refresher from Biology 101, Cells are the smallest independently functioning unit of our biological system. Multiple cells make tissues, and multiple tissues make organs. Multiple organs make organ systems, and the symphony of organ systems is an organism. Humans are multicellular organisms with numerous pathways and feedback loops to react and adapt to stressors for survival (2.). In the context of this article, we focus on the adaptations of muscle tissue, more specifically skeletal muscle tissue, and not venture into the discussion of cardiac or smooth muscle tissue.


     Skeletal muscle’s integral connections of neuromuscular, hormone, energy, and nutrient balance is an amazingly complex subject. Modernization of equipment and tools used in the scientific process has produced more than an increase in the volume of clinical research. These new technologies have added complexities and richness to the collected data, accompanied by more intelligent and updated interpretations.  In recent literature, collected proteomics was able to significantly support previous thoughts regarding the existence of various types of skeletal muscle hypertrophy (3.)  While we still don’t have enough clinical literature to know the exact relationship between resistance training programming variables and their effects on the development of specific types of skeletal muscle hypertrophy.  We have enough confidence in supporting the idea that skeletal muscle hypertrophy is not as simple as an increased cross-sectional area.


     Some of the underlying terms can be confusing, but I provide definitions and practical descriptions throughout the article to prevent misunderstandings. I begin by building upon the cellular definition, and skeletal muscle hypertrophy is the increase in skeletal muscle mass or volume. For accurate comprehension, it is necessary to reinforce the distinction between mass and volume to clarify the concept of muscle density. Mass is the measure of the amount of matter in an object, usually measured in grams (g) or kilograms (kg) and volume is the measure of the amount of space that a substance occupies. Density is the measurement that compares the amount of mass to the amount of three-dimensional volume.  If the muscle tissue increases in density, then the mass (weight) increased, and the volume stayed the same or decreased. If the muscle decreases in density, then the volume (three-dimensional space) increases, and the mass stays the same or decreases.  The SAID principle (specific adaptations to imposed demands) dictates these hypertrophic responses. Meaning, the specific stimulus imposed upon the skeletal muscle provides an experience of stress that demands a unique adaptation to efficiently tolerate similar future demands (4.). It is intriguing to contemplate the multitude of resistance training variables that modify the categorical response from skeletal muscle (5.). (Training Variables discussed in future work.) Considering that muscle tissue has the ability to individually modify its structure and composition (mass, volume, density), it is necessary to correctly highlight skeletal muscle’s hypertrophic categories: (1.) myofibrillar, (2.) sarcoplasmic, (3.) connective tissue. (6.) For this article, I provide a clear evidence-based description for each hypertrophic adaptation.


     Since connective tissue is rarely acknowledged in the discussion of hypertrophy, we begin by describing its importance. Skeletal muscle is wrapped in an extracellular matrix of connective tissue, fibrous fascia, that provides a structural framework from origin to insertion, creating tendons that attach the muscle to its bony attachment sites. The connective tissue contains nerves that carry central nervous system information to direct the muscles to contract and produce force, and the nerves also relay information back to the central nervous system for the brain and spinal cord to understand the current state of the muscle. It also contains blood vessels to supply nutrients and dispose of muscular metabolic waste products (7.). Muscle cells have a cylindrical shape and are referred to as muscle fibers (muscle cell = muscle fiber). These cylindrical-shaped fibers can be as short as ½ an inch or as long as 20 inches. (8.) Muscle fibers are rarely the entire length of the muscle and are typically arranged in a series end-to-end or overlapping each other in parallel. There is a specific organization of muscle cells to properly transmit their force of contraction laterally to the adjacent fibers. The phenomenon of lateral force transmission occurs between fibers through another type of fibrous fascia.  Muscular fascia is mainly composed of collagen fibers with some elastin fibers. Briefly, each muscle fiber is surrounded in its own fascia called endomysium, and those muscle fibers are divided into organizational bundles called fascicles, which is surrounded in another fascia called perimysium. Finally, the entire muscle is surrounded by a layer of fibrous fascia called epimysium. (9.) All three of the fascial layers blend and attach the muscle to bone. Muscular fascia extends beyond the origins and insertions, dividing specific muscles into groups known as fascial planes (fascial planes are groups of muscles enveloped by a thin aponeurotic sheet of fascia and bordered by the intermuscular sept). As this information has demonstrated, the fascial connective tissue plays an integral role in the structure and function of muscle tissue, which is why it is appropriate to provide this glimpse of kinesiology, for accurate visualization of the components of skeletal muscle in the discussion of hypertrophy.  


      Next, we venture into discussing the fraction of skeletal muscle hypertrophy that is the most directly related to the increase in force production capacity. Myofibrillar hypertrophy is the increase in size or number myofibrils with an increase in the contractile units, sarcomeres, and contractile force generation. Myofibrils are contractile units that lie in parallel and extend end-to-end on the long axis of the muscle cell. The myofibrils contain even smaller contractile units called myofilaments. Myofilaments are composed of thick and thin filaments in a repeating pattern that is responsible for muscle contraction. The repeating pattern of thick and thin myofilaments is called a sarcomere. The sarcomere is known as the functional contractile unit of a muscle fiber. The sarcomere’s thick filament is primarily myosin, and its thin filament is primarily actin. They also contain regulatory proteins troponin and tropomyosin (10.). Please note, this is a very brief description of a contractile unit and is used to give relevance to the sliding-filament theory of a muscle contraction. “Contraction requires activity between two major protein filaments in the sarcomere: thick filaments of myosin and thin filaments of actin. According to the sliding filament theory, the interdigitation of these two filaments is the mechanism of force generation” (11.). The muscles contract as the myosin heads extend out and bind to the sliding actin filament. The process of sarcomeres shortening and cross-bridges forming generates the force of the contraction. (Note: on average a thick filament contains approximately 200 to 300 myosin molecules)  The increase in myofibrillar hypertrophy is significant for an individual that wants to improve a strength related skill. This relationship between myofibrillar hypertrophy and strength is widely known in the strength and conditioning community. When discussing periodization and programming, most strength and conditioning coaches categorize certain training variables with their resulting hypertrophy. Since sarcoplasmic hypertrophy is more voluminous and less related to force production, it is not the primary goal of adaptation in strength sports. But myofibrillar hypertrophy is directly related to the increase in force production capacity and strength. If an individual experiences myofibrillar hypertrophy, then the individual most likely will get stronger. But we must remember, if the individual experiences an increase in strength, then they may not have experienced myofibrillar hypertrophy. Myofibrillar hypertrophy may have a causal relationship with strength, but strength doesn’t necessarily have a causal relationship to myofibrillar hypertrophy. (Note: Strength can increase by improving neural function, enhancing movement skill, mastery of lifting technique, and more.) Hopefully, this small slice of information is enough to mentally digest myofibrillar hypertrophy and prevent confusion in the upcoming section featuring sarcoplasmic hypertrophy.


     The last type of hypertrophy discussed in this article is sarcoplasmic hypertrophy. Historically, the term sarcoplasmic hypertrophy has been described as an increase in the fluid of the muscle that is non-functional and non-force producing type of hypertrophy (12.). Recently, Haun et al. provided a thorough description as “a chronic increase in the volume of the sarcolemma and/or sarcoplasm accompanied by an increase in the volume of mitochondria, sarcoplasmic reticulum, t-tubules, and/or sarcoplasmic enzymes or substrate content.” (13.). Furthermore, it is a prerequisite to have a basic understanding of the ribosomes, nuclei, mitochondria, proteins, glycolytic enzymes, metabolic enzymes and a host of other intracellular components included in the category of sarcoplasmic hypertrophy to discern the potential for practical application in periodization and programming (periodization and programming will be featured in future work) for hypertrophy.  Briefly identifying a few elements mentioned above, within the sarcoplasmic membrane-type complex, is a network of t-tubules perpendicular and parallel to the sarcolemma. Located adjacent to both sides of the perpendicular t-tubules is terminal cisternae. The combination of two terminal cisternae and one t-tubules is referred to as a triad. The triad invaginates the sarcolemma and delivers the factors for a muscle contraction. “Excitation-Contraction coupling requires a highly specialized membranous structure, the triad, composed of a central T-tubule surrounded by two terminal cisternae from the sarcoplasmic reticulum.“ (14.). The t-tubules store voltage-gated Na+ and voltage-gated K+, which participates in conducting an electrical signal (action potential) and the terminal cisternae that serve as reservoirs for calcium ions (Ca2+) used in muscle contractions (15.). Even though this is only a fraction of information regarding the underlying mechanisms and elements in the category of sarcoplasmic hypertrophy, maybe it aids in organizing thoughts about skeletal muscle hypertrophy.


     This article highlights the importance of a thorough collegiate background in the studies of human science (anatomy, physiology, biology, chemistry, kinesiology, and more) for anyone who attempts to consider themselves evidence-based, data-driven, or scientifically motivated. In an attempt to give a basic understanding of skeletal muscle hypertrophy and not bore you with too many of the minute details, I briefly touched on a few of the critical elements in each section. Skeletal muscle hypertrophy can be classified into three distinct categories of (1.) myofibrillar, (2.) sarcoplasmic, and (3.) connective.  Each category features specific roles in skeletal muscle hypertrophy, but they all work incongruence to achieve the same overall goal.  In summary, this topic is an incredible phenomenon that I have been obsessed with my entire life, and I hope I have provided you with enough foundational information to begin your understanding of skeletal muscle hypertrophy.


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 (3.) Haun CT, Vann CG, Osburn SC, Mumford PW, Roberson PA, Romero MA, et al. (2019) Muscle fiber hypertrophy in response to 6 weeks of high-volume resistance training in trained young men is largely attributed to sarcoplasmic hypertrophy. PLoS ONE 14 (6): e0215267.

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(13.) Haun, Cody T et al. “A Critical Evaluation of the Biological Construct Skeletal Muscle Hypertrophy: Size Matters but So Does the Measurement.” Frontiers in physiology vol. 10 247. 12 Mar. 2019, doi:10.3389/fphys.2019.00247

(14.) Al-Qusairi and Laporte: T-tubule biogenesis and triad formation in skeletal muscle and implication in human diseases. Skeletal Muscle 2011 1:26.  doi:10.1186/2044-5040-1-26 

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