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Infinite Isometrics - Henneman’s Size Principle: The Law of Bodybuilding

Isochain Single Leg Deadlift Isometric Hold

For many decades, a ferocious argument raged in bodybuilding circles:

Which is better for building muscle? Using heavy weights for short sets, or light weights for longer sets? Or somewhere in the middle?

It’s unrealistic to just label this "a debate"—it has always been, in fact, the biggest issue in all of bodybuilding ideology. In many ways, this argument defined different eras of bodybuilding. Each position has had its own adherents. The "heavy" camp claimed that strength training and muscle-building overlap—that all the great champs, from Arnold to Coleman, have been hugely strong as well as supremely muscled. At the other end of the spectrum, there have always been devotees of the "muscle pumping" cult--going right back to the 40s and 50s (when it was called "muscle spinning")—who felt that bodybuilders training like strongmen were just idiots at risk of an injury. Of course, you can find numerous athletes and coaches who sit somewhere in the middle.

The quarrel strongly continues in gyms and on web forums around the world to this very day—despite the fact that this question was already definitively answered by an American physiologist in the 1950s. That physiologist was Dr Elwood Henneman, and the most important law in muscle recruitment is named after his research.

Before we can discuss exactly what Henneman’s work can teach us about our training, it will be helpful to understand some basics about muscle composition.

Muscle Composition

Your muscles are not comprised of a homogenous group of cells, or fibers. There are numerous different fiber types in voluntary muscle. At the most basic level, we have red muscle fiber and white muscle fiber. Red muscle fibers are smaller, weaker, and are intended for endurance work—holding posture, walking around, etc. White muscle fibers are larger and have evolved for heavy force output. They are big and strong, but fatigue more quickly.

This is true for many species. If you have ever cooked or carved a chicken, you will notice the difference between the dark meat and the white meat; the dark meat (for example, the thighs) are the muscles the chicken walks around on all day. These are the red fibers. The white meat (for example the breast) is used for high force output; for example, an explosive take-off.

Humans—like our avian cousins—also possess differing degrees of red and white fibers in different muscle groups. The soleus of the calf, for example, is high in red fibers; the latissimus of the back contains more white fibers. But most of our muscles have some kind of admixture of different types. In fact, there are different kinds of fiber types in-between the red and white.

Diagram of Red and white muscle

The important thing to know about different fiber types is that the ones towards the red end of the spectrum have evolved to be fatigue resistant. They are dense in myoglobin and mitochondria which allow them to perform aerobic activities—they utilize a lot of oxygen. As a result, when we train these red fibers, they tend to adapt by becoming more aerobically efficient. Think of a marathon runner. His or her muscles don’t get bigger through training, they just get better and better at their basic job, which is resisting fatigue.

By comparison, the fibers towards the white end of the spectrum are anaerobic. They evolved not to resist fatigue over a long period, but to express large levels of force over a short period. They don’t use oxygen and so they are not dense with myoglobin and mitochondria. Instead, they have their own private banks of chemical energy. This is one reason why white fibers are larger than red fibers. When we train these muscle cells, they don’t primarily become more efficient—they just expand their little banks of chemical fuel. This is what happens in bodybuilding; you train the muscles briefly by expressing force, and these individual cells become bigger over time. This makes the entire muscle larger.

Franco and Casey Biceps

In this short preamble, we have the key to bodybuilding: to become bigger, you must recruit and exhaust these larger, stronger fibers. These are the ones with the adaptation potential to get bigger.

So how do we train these fibers? This is where Henneman’s principle comes in.

Henneman's Size Principle

Remember that the smaller (red) fibers are weaker, but very slow to fatigue; the larger (white) fibers fatigue rapidly, and take a while to recover—they have a much shorter battery life. For this reason, the body’s motor recruitment software is ultimately based on managing its resources—it will always favor the red fibers in activity if it can. This is your body’s "default mode"—using smaller, red fibers.

Larger fibers are only ever brought into activity when the smaller fibers cannot perform the task expected of them. There are only two cases when they might not be able to perform a task:

CASE A: When superior levels of force are required, which smaller fibers can’t generate.

CASE B: When the smaller fibers are already fatigued.

In these two cases, fiber recruitment happens from smallest to largest. If smaller fibers can provide the force/are not fatigued, they are used. Progressively larger fibers only get drafted in if the forces are too high for smaller fibers, or if there are no smaller fibers left unfatigued. This, in a nutshell, is Henneman’s size principle.
Hennemans Size Principle Diagram Motor Unit Recruitment
Muscle fibers are recruited in size order—smallest to largest.
Larger fibers are ONLY recruited if smaller fibers are too weak or tired for the job.

Application to bodybuilding

So—back to the question at the beginning of this article; which is better for building muscle—heavy weights, moderate weights, or light weights?

If you have fully grasped the above paragraph describing Henneman’s size principle, you will already know the answer. They are all right.

The goal of bodybuilding training is to recruit, and exhaust, the larger, white fibers. The heavy lifter is successful because he or she uses superior forces to recruit, and exhaust, the larger fibers. (See CASE A, above.)

The lighter lifter using more reps/time-under-tension is also successful, because those extra reps will gradually fatigue smaller and medium-sized fibers and force the larger fibers to be recruited and trained. (See CASE B.)

Hypertrophy can happen in a variety of ways. You can target your larger, white fibers with very heavy loads, training them that way. But you can also use lighter loads over more time—this will begin by working smaller muscle fibers, but as they become fatigued and "switch off", progressively larger fibers are recruited, until you finally reach the large, white ones that have the growth potential. Or you can use a balance of forces and volume somewhere in the middle. These are all just different equations of time-under-tension (one end of the spectrum involves more tension, the other more time) but the product is exactly the same: muscle growth.

It might seem counterintuitive to state that smaller loads can build muscles just as well as very heavy loads, however, it’s true: light loads can become as effective in training as heavy loads—provided we exhaust the smaller fibers first. This is the caveat of using lighter loads; to ensure that you are recruiting larger fibers, the smaller and medium fibers must be exhausted first. A few reps with light weights won’t do this, as the smaller fibers aren’t "used up". You have to train to failure.

We all have some experience of this aspect of the size principle in everyday life. Have you ever been forced to hold onto something fairly light—like shopping bags—for an extended period of time? You will have noticed that, as the time mounts up, the light loads begin to feel "heavy". We even say things like "this is getting heavy!" Of course, the load hasn’t changed in weight. What has happened is that our smaller fibers have become exhausted, and can no longer support the load; the larger fibers are recruited, and as a result the light load has the same recruitment effect as a heavy one.
Farmers Walk
Physiologically, any load becomes "heavy" if you carry it long enough.

This isn’t just theory—it’s been tested and verified on numerous occasions. One definitive study was completed by the Exercise Metabolism Research Group of McMaster University in 2012.1 In the study, the researchers had young, healthy males train with both heavy weights (80% of max) and light weights (a mere 30% of max) in different sessions. The resulting protein synthesis (i.e., growth) in the trained muscles was then measured via sophisticated biopsy/histology plus magnetic resonance imaging techniques. The researchers discovered that the same amount of growth occurred with both amounts of weight—provided the subjects trained to failure. The larger, white fibers were equally recruited with heavy and light weights. (Interestingly, although the loads used didn’t affect the growth response, the number of sets did—three sets produced about twice the growth of a single set. Something to bear in mind.)

This explains why different bodybuilders have traditionally exploited such a wide variety of set and rep schemes, certainly compared to other athletes. There are successful bodybuilders who lift very heavy; successful bodybuilders who lift relatively light, but with more volume; and all kinds of athletes who fall somewhere in between. In bodybuilding, there really is "more than one way to skin a cat"—as long as you work hard.

Isometric Bodybuilding and the "Goldilocks Zone" for Muscle

So—if heavy, moderate and light forces all build similar amounts of muscle—which should you be using in your isometric training?

Firstly, we must establish that there is such a thing as "too heavy" for bodybuilding. Extremely high loads will recruit the large fibers you want, but since you can’t sustain maximum forces for too long, they won’t exhaust them of their chemical resources, which is the trigger for muscle growth. Heavy—for hypertrophy—does not mean maximum loads, but loads you can hold for at least 3-6 seconds.

There is also "too light". Once you are holding loads light enough to sustain for over 45-60 seconds, your cardiovascular system begins taking more punishment than your muscular system, and this will naturally curtail your performance and results.

So, for isometric bodybuilding, we want to be working with a "Goldilocks Zone"—neither too heavy, nor too light—somewhere between 6 seconds and a minute. This is still a pretty large zone…so which part of it should we be aiming for?

The science is quite explicit here—you should ideally train in every part of this zone. Just not during the same training cycle.
Training Effect Chart
From The Ultimate Isometrics Manual, p. 276. MVC = Maximal Voluntary Contraction.

All forms of training—including heavy, light, and moderate—result in diminishing returns (also called accommodation). Over time, your body adapts to the stimulus, and then results begin slowing down. This effect is more pronounced the more powerful your training is—the quicker you adapt, the quicker diminishing returns set in. Since isometrics are the most effective form of resistance training, isometric athletes have to be very aware of accommodation, and must train around it by altering their training program every 6-8 weeks.

There are numerous ways of altering your program—you can change exercises, training angles, volume, frequency, etc. Another fundamental variable you can play with is intensity: the forces you are expressing. For this reason, athletes should be actively changing their relative loads every cycle or two: sometimes use very heavy holds for just 6-10 seconds; sometimes use holds comprising 20-30 seconds; sometimes use lighter loads for longer, up to a minute. Doing this consistently will keep you in the "Goldilocks Zone", while still actively avoiding accommodation throughout your training career.

This approach is also useful because different loads have different benefits, beyond hypertrophy; heavy loads increase strength, lighter loads are easier on the joints and increase endurance, etc. There is no reason why you can’t reap all these benefits in your training, over the medium-term.

Three principles to take from this article:
  • You can grow muscle using a wide range of isometric holds; heavy holds work, as do moderate and light holds. They are all equally effective if applied correctly.
  • When using lighter loads, larger fibers are only recruited and trained when smaller fibers are already exhausted. For this reason, you must train to the point of muscular failure when using lighter and moderate loads. The lighter the load, the more important this is.
  • Don’t get stuck in a rut working with specific force levels; every training cycle or two, mix it up. Sometimes go heavy, sometimes light, sometimes in-between. This will help you avoid diminishing returns in your muscle gain efforts. 

It can be easily seen why Henneman’s Size Principle has been called the "Law of Bodybuilding", as it teaches us how to recruit the large muscle fibers responsible for hypertrophy, or muscle gain.

There is, however, is a twin law of physiology which could be called the "Law of Strength"—as it explains how muscles become stronger, not via hypertrophy, but through increased efficiency. This principle is known as Hebbe’s Rule, and we will be discussing it in the next article in this series.
  1. Mitchell, C. J., Churchward-Venne, T. A., West, D. W., Burd, N. A., Breen, L., Baker, S. K., & Phillips, S. M. (2012). Resistance exercise load does not determine training-mediated hypertrophic gains in young men. Journal of applied physiology (Bethesda, Md. : 1985), 113(1), 71–77.