Hypertrophy is a hot topic nowadays. The fitness industry seems to generally distinguish “hypertrophy” training from strength training, and there are receipts everywhere on how to specialize your training for one or the other. There is a fair amount of peer-reviewed research on the topic, and most theories and claims are based on the findings of these works.

However, there seems to be an alarming lack of deductive reasoning: conclusions have to follow logically from premises. If the findings in this or that experimental study are at odds with deductive reasoning from basic principles, something is not adding up. The prevailing methodology is taking these experimental studies at face value, and leaving deductive reasoning aside, as well as the observations of coaches and trainees for a hundred years. The aim of this article is not to argue the validity or shortcomings of the scientific fitness literature, but to provide the basic principle-based deductive reasoning regarding hypertrophy and how to train for it.

The topic of hypertrophy has been addressed in a few articles on this site, and what I will argue here is in line with what has been recently discussed at length in Starting Strength Radio episodes #219 and #240 (Hypertrophy and You’re Not Doing Hypertrophy, respectively on the Starting Strength Network or listen here). I will start by reviewing what the main function of the muscle is, what hypertrophy is, and how it relates to this primary function, and then I will show how the General Adaptation Syndrome allows us to recognize hypertrophy as a physiological adaptation and helps us to understand how to train for it.

The Primary Function of Skeletal Muscle

The main function of skeletal muscle is to move and stabilize the human body. It does so by applying force to the system of levers that is the skeletal system. To understand this, we need to know what “force” and “lever” are.

Force is an influence that causes an object to change its velocity: to accelerate. Its value is given by multiplying the mass of the object times its acceleration. A lever is a rigid body which is capable of rotating on a point on itself, called a fulcrum. Think about two kids playing on a seesaw for a simple example of how external forces interact with a lever: the force of gravity acts on the first kid, the heavier one, making the seesaw rotate around the fulcrum and lifting the second kid up. The longer the distance between the heavier kid and the fulcrum, the least force needed (the less the heavier kid has to weigh) to move the second kid up.

In order to apply motion to the skeletal system, the muscle contracts – shortens – and applies force by pulling on the tendon, which transfers this force and pulls on the given bone at its attachment point. Rotation follows around the given joint. In mechanical terms, the bone is the lever and the joint the fulcrum. Think about the biceps and forearm: force is applied by the biceps at the attachment point of the tendon on the forearm, the forearm rotates around the elbow (fulcrum), and a resistance located at the hand – a dumbbell for example – is accelerated. The ability to apply force to an external resistance by our musculoskeletal system is what we call strength.

The main function of the muscle is then to generate force by contracting. Force is the physical quantity that measures the action of the muscle on the tendon's attachment to the bone. Force production has to be sustained through varying periods of time, which requires metabolic processes in the muscle to generate and consume ATP (adenosine triphosphate, the molecule that is used as “fuel” for muscle contractions). These metabolic processes are the support for the muscle to perform its primary function, acting as energy supply so that it can contract and produce force against the skeleton.

Muscular Hypertrophy and Strength

Hypertrophy is an increase in muscle size. More specifically, when we talk about hypertrophy we are discussing an increase in the cross-sectional area (CSA) of the muscle. Muscles are constituted of thousands of muscle cells (muscle fibers). Within a simplified model, muscle fibers are made of two main components, sarcoplasm and myofibrils. The sarcoplasm surrounds the myofibrils and contains ions, small diffusive molecules, mitochondria, and other organelles. The myofibrils consist of long chains of myofilaments made of proteins, in particular actin and myosin. These proteins are responsible for muscle contraction: pairs of myofilaments form cross bridges that draw the protein filaments past each other, shortening the myofibrils and contracting the muscle fiber.

Myofibrils are then mechanically responsible for muscle contraction and force production, while the sarcoplasm has other functions, including hosting most of the metabolic processes of the muscle. If it is to increase its force production capacity, the muscle needs to increase the number of myofilaments so that stronger contractions can occur. This means higher numbers of larger myofibrils, and increased sarcoplasm for metabolic and other processes to support the increased contractile potential. As a result, the muscle CSA increases, and hypertrophy occurs.

It is a structural change to the muscle fiber, a persistent transformation that can be accumulated over long periods of time. Note that we are not concerned here with transient increases in muscle CSA related to the accumulation of metabolites, lactate, etc. in the sarcoplasm. Everybody is familiar with “the pump,” which happens when some of this takes place and lasts for a few hours, but there is some evidence that there may well be transient increases in muscle CSA due to training (especially from high volume workouts) that could last days or even a week or two. These changes won’t accumulate over time in the way structural changes do.

It is clear then that increased force production requires larger muscles, but is it at all possible to produce long term muscular hypertrophy in the absence of an improved ability to produce force? According to our model of the muscle fiber, this would basically mean long-term sarcoplasmic hypertrophy in the absence of myofibrilar hypertrophy. The fact is that there is absolutely no evidence of this happening, and most probably never will be. Why would our bodies promote an increase in muscle size in the absence of improved strength? It makes no sense, bigger muscles are heavier and require more force to be moved around, and this wouldn’t be efficient at all. Natural selection is cleverer than that.

Besides, anecdotal evidence doesn’t support it. We all know that athletes in more strength demanding sports are bigger, and athletes in more metabolic demanding sports, which require less force production capacity, are smaller. Powerlifters and strongmen have huge muscles, sprinters or crossfitters have big but not huge muscles, and marathoners have small muscles. We all know that bigger people are generally stronger than smaller people. And we also, crucially, know that a bigger, more muscular version of you is stronger than a smaller version of you.  

Hypertrophy is therefore the mechanism by which muscles increase their force production capacity.

General Adaptation Syndrome

We have established what hypertrophy is, but what makes it happen? For this we need to understand the General Adaptation Syndrome: a model that explains how living systems react and adapt to the environment. The environment is, in this context, a series of physical perturbations that act upon us and produce a response from our bodies that we call stress. This is one of the things our DNA is for: if we are able to recover from this stress, physiological adaptations occur so that the same stressor no longer represents as big a recovery challenge – in its historical context, a threat to our lives. (Stress, recovery, adaptation (SRA) - It’s Time to Stop Talking About "Supercompensation")

Environments change all the time, and always have. Without this mechanism, we wouldn’t be able to adapt to the changing environment and we would die very quickly. A nice example of the SRA cycle is the process by which we get a suntan (The Biggest Training Fallacy of All). The stressor is the ultraviolet (UV) radiation in the light coming from the sun, which produces a stress in our bodies to which we adapt and get tanned. Melanin is produced and our skin darkens, such that next time we can withstand the same volume of UV radiation without it representing a challenging stress.

A very important characteristic of the SRA process is that the stress has to be specific to the adaptation. In order to get a suntan we need to be exposed specifically to UV radiation (any other type won’t work) of sufficiently duration relative to our current level of adaptation, or the stress would not represent a challenge and the adaptation would not be necessary. If you stay in the shade, the solar radiation reaching your skin won’t be of high enough intensity and you won’t get tanned. The important piece of this process is the dosage of the stress, which has to be enough to produce the adaptation but not so much that you can’t recover. The stress in the SRA must be capable of being recovered from.

Hypertrophy as a Strength Adaptation

We have now the tools and ingredients to view hypertrophy in a slightly different light: hypertrophy is nothing but a physiological adaptation. But what is the stress that produces this adaptation? If hypertrophy is the mechanism by which the muscles increase their force production capacity, and the stress has to be specific to the adaptation, then it is clear that the stressor required to produce a hypertrophy adaptation must be something that challenges the ability of the muscle to produce force.

The limiting factor must be your strength, otherwise the stress would not represent a challenge for the actual force production capacity of the muscle, and, like when you tried to get a tan, the adaptation would not be necessary if you were already tanned. We could talk about strength and hypertrophy interchangeably: strength is the functional consequence of muscular hypertrophy, hypertrophy is the physiological adaptation from displaying strength as a stressor for the muscle.

Since hypertrophy and strength are sides of the same coin, the most efficient way to train for hypertrophy is also the most efficient way to train for strength (Bodyparts vs. Movement Patterns). This is big, basic compound movements that work the body in a functional way: squat, presses, deadlift, chins (see Practical Programming for Strength Training, 3rd edition, Mark Rippetoe and Andy Baker, and A New Definition for Strength Training). The phenomenology is quite clear: when you train for strength and get stronger, you also get bigger.

Following logically from our premises, we can infer that performing a heavy single in any of these exercises is theoretically the most specific stressor to the hypertrophy adaptation: it is the most force production-limited stress one can have. But is it enough stress? Lying down in the sun when it is high up in the sky is the most specific stress to get a tan, but if you do it for just two minutes will you get tanned? A heavy set of 5 reps may still be specific enough for hypertrophy – lots of force production required.

Will a set of 10, or 20, or whatever large number be specific enough? These events require less and less force production the further they are from a heavy single, so the higher the number of reps, the less specific and less optimal the stress for the desired hypertrophy adaptation. If instead of lying down in the sun you stayed in the shade, even if you stayed there for the whole day, you wouldn’t have got tanned, as the intensity of UV radiation reaching your skin would have been too low. Lying down in the shade for the whole day is the equivalent to a set of 40000 reps; this is the equivalent of running a marathon, no tan and no hypertrophy at all.

Is training based on heavy singles then the best for hypertrophy/strength? Experience tells us that, for most people, a heavy single is not stressful enough to force an adaptation. We would need to do multiple, lighter by necessity, singles, to accumulate enough stress. This is not practical for several reasons: first, a heavy single depends heavily on skill, this immediately rules out this type of training for novices. Second, novices and intermediates need exposure to multiple reps per set in order to obtain or maintain proper technique, and for the coach to be able to correct the movement in real time. And third, if you need to do 10 singles to produce enough stress, with rest in between, that would be too time consuming for most. Besides, we may well benefit from other physiological adaptations while we train for hypertrophy-strength. In particular, metabolic and cardiorespiratory adaptations that better occur when doing sets of multiple reps.

And here comes the experience of countless coaches and trainees over a long period of time all over the world: sets of 5 is what works best for strength and therefore hypertrophy, for most people (see Fives Build Muscle Better and 5s, Not 10s). A range between 3 and 6 reps, depending on individual characteristics and training advancement, may still be optimal. Experience tells us that a heavy set of 5 is still specific enough to produce the desired hypertrophy adaptation, and the amount of stress is the optimum. If needed, we can use a few sets across.

The heavy set of 5 is the equivalent, when we got tanned, of using the correct intensity of UV radiation for the correct amount of time. The set of 5 for hypertrophy is the sweet spot when we take into account the specificity of the stress to the adaptation, the dosage of stress needed, and the practical aspects of training.

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