Shifting Tension

Is it possible to put more stress on certain parts of a muscle?

My last few articles were quite controversial:

• A chest specialization article which included exercises to develop pectoral width and thickness.

• A rant against strength coaches who claim that bodybuilders should train like athletes.

• An article where I slam the idea of bulking to gain muscle size.

Controversy is good; it stirs up discussions and an exchange of ideas. However, sometimes people will peruse through one of my articles and immediately make up their mind one way or another without taking time to actually read and digest the whole thing. This leads to some misinterpretation of what I'm saying.

So with that in mind, please read this article in its entirety and think it through before making up your mind either way. Once this is done, feel free to expose your point of view and discuss it at will.

A Word of Warning

This article will also be quite controversial, maybe more so than anything I've ever written. It'll show how muscle subdivision is much more complex than previously thought, and that it is possible to put more growth stimulus on certain parts of a muscle via proper exercise and training method selection.

However, don't take this message the wrong way. While you can put more mechanical stress/growth stimulation on certain portions of a muscle, it's impossible to entirely isolate one portion in particular.

What If...

1. What if uber-vixen Jessica Alba sees you walking down the street and comes to the sudden realization that you're the exemplification of what a true male should be? She then proceeds to complete a practical review of the whole Kama Sutra with you... twice.

2. What if I told you that it's possible to put more growth stimulus on certain parts of a muscle, enabling you to improve weak points that you believed would be your doom forever?

What do these two sentences have in common? Easy, they both sound too good to be true. Impossible even! Yet one of these two is actually possible. No, not the one involving Miss Alba, sorry. I'm talking about number two: you can selectively put more growth stimulus on certain parts of a muscle. Don't believe me? Then read on!

Physiology, biomechanics, and anatomy have greatly helped us understand how the human body works and, in our case of interest, how we can build muscle and increase muscular strength. Exercise science has helped explain the reason why certain training techniques are better than others, and it has also directed us toward interesting new avenues when it comes to stimulating growth.

However, it's not all good.

Problems may arise when we look at some scientific knowledge in isolation, accept it as dogma, and then stop looking further into the subject. This could become even more problematic when the knowledge we use as our basis for understanding muscle recruitment and growth is outdated. The subject of this article will deal with such an occurrence, namely the possibility of putting more growth stimulus on certain parts of a muscle.

We most often base our knowledge of muscle stimulation on the gross anatomical division of a muscle group. When you train a muscle, all parts of it are activated and work equally hard. According to this dogma, it's impossible to put more growth stimulus on certain parts of the muscle.

Well, I'm about to demonstrate that this is just not the case! In fact, it is possible to emphasize certain portions of a muscle over others and therefore place more growth stimulus on it.

The Levels of Division

Everybody is a scientist nowadays. A ton of guys who are into weight training will be quick to either buy an anatomy/biomechanics textbook or visit a website with that info. They often don't have an in-depth foundation in these sciences. As a result, they see an anatomical drawing of a muscle and automatically assume there aren't many subdivisions within a muscle; thus, it must be impossible to emphasize certain regions.

That's a prime example of a little science being worse than no science at all! They don't understand that most anatomy/biomechanics textbooks use very simplified drawings of the muscles, and that often authors have been using the same drawings for years and years.

The fact is, there are more subdivisions than what can be shown in the scholastic textbooks. We'll focus on the four levels of subdivisions that have the greatest impact on our bodybuilding/muscle mass gains.

Level 1: Muscle heads – A lot of muscles are divided into separate portions or "heads." While these heads are part of the same general muscle group, they're actually semi-independent or independent and can have various functions. For example:

The chest includes: the sternal head of the pectoralis major (the big portion of the chest); the clavicular head of the pectoralis major (what's often called the "upper chest"); and the pectoralis minor (which is underneath the pectoralis major).

The delts include: the anterior head of the deltoid (front delts), the lateral head of the deltoids (side delts), and the posterior head of the deltoid (rear delts).

The quads include: the vastus lateralis (outer portion of the quads), the vastus medialis (the teardrop next to your kneecap), the rectus femoris, and the vastus intermedius.

The hams include: the short head of the biceps femoris, the long head of the biceps femoris, the semitendinosus (the long head and semitendinosus are the "meat" of the hamstrings), and the semimembranosus.

The biceps include: the short head of the biceps brachii (inner portion of the biceps) and the long head of the biceps brachii (outer portion of the biceps).

You get the picture.

Level 2: Fiber layering – Studies have demonstrated that slow-twitch muscle fibers predominate in the deep portions of muscles while fast-twitch fibers predominate in the superficial portion of muscles. (Acosta et Roy 1986; Bredman et al.1990; Knight et Kamen 2005)

Level 3: Neuromuscular compartment – This might be the most complex level of the subdivisions, but a lot of research backs it up. Neuromuscular compartmentalization refers to the division of the muscle fibers within a muscle group and their grouping in said muscle. Basically, muscle fibers are "bundled up" regionally into functional sub-units.

I'll get to the more complex scientific stuff later on, but let's start with an analogy which explains this theory. Imagine that one of your muscles (or one head of the muscle) is a city and that the highway leading to that city is the motor nerve (the motor nerve is what sends the motor command from the CNS to the muscle).

Now according to the "old" muscle recruitment theory, the command sent by the motor nerve would activate all portions of the muscle equally. In other words, it would be like the highway only having one exit toward the city. This theory has been proven to be erroneous. It was first postulated because "most studies of human muscle have not taken into account the possibility of partitioning beyond the grossly visible anatomical divisions." (English et al. 1993)

In real life, it's been shown that the motor nerve (highway) branches out into many smaller nerves which actually innervate different "sectors" of a muscle. See the picture below. (Be gentle, it took me hours to create!)

As you can see, the city (muscle) is divided into three separate portions. (These aren't "muscle heads." The city should actually be seen as a muscle without separate heads or a specific head. Partitioning means that there's a level of subdivision even beyond heads.) There's one big highway (motor nerve) leading to the city and this highway branches out into three separate exits (secondary motor nerves) each leading to one neighborhood of the city.

So as you can see, it's possible to send more traffic to one part of the city, just like the CNS can send a stronger activation signal to certain regions of a muscle.

Now, we can mix the first level of division (heads) and the third level (neuromuscular compartments) to further illustrate the point. We'll once again use our city as an example.

This map shows a region which has four towns, all in close proximity to each other. As you can see, the highway leads to each of these towns and some of them actually have more than one exit leading to it (not representative of the actual number of neuromuscular compartment of the muscles).

So, more traffic can go to one city or another. Within each city more traffic can also be sent to different neighborhoods, just like when you're performing an exercise for the quads, more stress can be placed on one of the four portions of the muscle as well as on individual sections of each portion of the muscle group.

Another way of explaining this level of division is the electrical current within a house: it all comes from the same source, but once it enters the house it's divided into several directions going to all the rooms. We can thus light up one room without having to light up the whole house. It's also possible to turn on only one electric apparatus in a single room. Same thing holds true with our muscles.

Okay, back to our scientific chit-chat! The analogies above give us an overview of the neuromuscular compartmentalization theory of muscle architecture.

So to recap, each muscle is innervated by a single motor nerve. As this nerve enters into the muscle, it branches out (divides itself) and goes to different regions of the muscle. (English et al. 1993) Therefore each of these regions, while synergistic (in most cases), are actually independent of each other. Why is that? Because of the function of the muscle as well as its fiber type distribution.

If you look at the anatomy of a muscle you'll notice that not all fibers are running in the exact same direction. Take the pectoralis major, for example. We can see several different lines of force gradually going from inward and up to inward and down.

This is a good indication that the muscle is divided into different compartments. The muscle is built this way to allow for a better control of movement: "Differential activation of motor units in different compartments may be a means of regulating the direction of the overall force application." (Bonasera et al. 1992)

Our pectoralis major plays an important role in bringing the arm toward the midline of the body. If we're trying to bring our upper arm from a "high and lateral" position (beginning of a crossover exercise for example) to a "low and medial" position (end of a crossover exercise), doesn't it stand to reason that the fibers which are actually oriented in that particular direction will play a larger role in the movement than the fibers going inward but up? I'm not saying that only the lower portion of the pectoralis will contract, but it will play a bigger role in the movement.

If we were to perform an exercise where the "height" of the arm doesn't change but where you're bringing it toward the midline of the body, the line of force will be parallel to the floor. So doesn't it also stand to reason that the fibers oriented along that line will perform more of the work?

Finally, if you were to perform a crossover from the low pulley, starting with the arms "low and lateral" and lifting them "high and medial," the line of force would be angled upwards and inwards. So for one last time, wouldn't it be logical to assume that the fibers oriented along the line of force would be more mechanically advantaged to perform the work and thus would receive more stimulation?

Fiber type is also important in neuromuscular compartmentalization. Understand one important physiological fact: an alpha motoneuron (the secondary motor nerve that's branched out from the main motor nerve) innervates muscle fibers of only one type. (Burke et al. 1973)

In other words, a secondary motor nerve (the exit from our map or the electrical wiring going to one room) will be linked to only one type of muscle fiber (Type I, Type IIa, or Type IIb). This kind of distribution allows the body to better modulate the amount of force to produce. Since we've already seen that the superficial portion of a muscle is predominantly fast twitch dominant and that the deeper portions are the opposite, this once again shows us that each muscle is actually divided into more independent compartments.

In truth, if you forget about the old-time dogma of muscle structure and look at it from a logical standpoint, you'll see that muscle subdivision is not only possible, it's a fact and it's necessary for proper mechanical functioning. However, I know that a lot of guys live to make fun of us writers and are quick to point out the flaws in our articles, so here are some tidbits from the literature on the subject:

1) "Variability in force direction within one and the same muscle has been suggested to be an important function of muscular compartmentalization." (Herring et al. 1979)

"Motor units in complex muscles have different lines of action and may have different functions." (Turkawski et al. 1998)

This means that the subdivision of each muscle into different subunits with varying lines of force allows the muscle the capacity to produce force in more directions.

2) "On the basis of muscle architecture, three regions were identified within the semimembranosus. Semitendinosus comprised two distinct partitions arranged in series. In the biceps femoris long head, two regions were supplied via a primary nerve branch divided into two primary branches. Biceps femoris short head consisted of two distinct regions demarcated by fiber direction with each innervated by a separate muscle nerve." (Woodley and Mercer 2005)

This means that the hamstring muscle group can actually be divided into nine different regions instead of the four originally thought. This is a prime example of subdivision within a certain muscle group.

3) "Electromyographic data indicates that there is a selective recruitment of different regions of a muscle that can be altered, depending on the type of exercise performed. ... muscles adapt in a regional-specific manner." (Antonio 2000)

This explains that not only are the muscles divided into specific regions, but that one can put more growth stimulus on one particular region via a change in exercise selection.

4) "Motor units may not be randomly distributed throughout the muscle but confined to localized sub-volumes." (Hammond et al. 1989)

5) "Just because something goes against what you personally believe doesn't mean that it's not true." (Christian Thibaudeau, 2006)

Level 4: Fiber segmentation – Each fiber within a muscle runs from its origin right to its insertion. In other words, every muscle fiber spans the entire length of a muscle, right?

Wrong! Recent research has shown the opposite to be true in most cases. Muscle fibers are actually segmented and joined with other muscle fibers/segments to form a long ensemble that will eventually link both attachments together. Both ends of the connected muscle fiber segments are joined together via a collagen bridge or myotendinuous junction (the latter serving to pass tension from one segment to the next). (Hijikata et al. 1993, Young et al. 2000)

Furthermore, it seems that each strip/segment of a muscle fiber is selectively linked to its own set of motor axons. (Paul 2001) This means that not only are muscle fibers really divided into shorter segments aligned in series, but that each segment has its own source of activation.

The following image will help us understand all this. Understand that the picture is just an oversimplification, but for our objective it'll be sufficient.

This certainly doesn't mean that we can isolate only a few of those segments via exercise selection. However, it does show us that depending on the range of motion, anatomical position, and line of action of a movement, it's possible that certain segments will be placed under a greater tension and thus will undergo a greater amount of damage, which in turn could cause more hypertrophy to occur on certain portions of the muscle.

Anecdotal Evidence

Anecdotal or "in the trenches" evidence is often dismissed because some say it lacks scientific validity. While this is certainly true, it doesn't mean that real life observations are worthless. Certainly when such evidence is analysed in a broader context which also includes scientific data, we can get a more precise portrait of what's really going on.

So in that spirit I ask the following questions:

1. When you perform a certain exercise (let's say for chest), you initially get sore. But after a few workouts using the same exercise you don't get sore anymore. Then you change the exercise for a new one and suddenly you get sore again. Why?

If it were impossible to put more stress on certain portions of a muscle, then that would mean that all exercises for a certain muscle group would hit all the muscle fibers equally. If that were true, then changing the exercise for a muscle group wouldn't cause more soreness because we'd continue to hit the same muscle fibers equally. Yet this doesn't happen.

2. Why is it that different exercises get you sore in different portions of a muscle group? If the whole muscle was working equally, then muscle damage would be uniform and thus soreness would always occur at the same place. Localized DOMS might indicate localized muscle micro-trauma which would indicate that different exercises can indeed put more stress on certain portions of the muscle.

Obviously, if taken by themselves, these would be weak arguments, but when you add the bulk of scientific evidence then I think it becomes obvious that it is possible to shift more tension to certain parts of a muscle.

Why Is It Hard to Shape a Muscle Then?

That's the obvious question. If it's possible to put more growth stimulus on certain parts of a muscle, how come it's so hard to see dramatic results in the shape of the muscle?

The first reason is quite simple. The process of building more muscle is a very slow one. Under the best possible circumstances, one can add around 0.25 to 0.5 pounds of muscle tissue per week. Obviously these gains will occur all over the body (provided you work all of your major muscle groups) so it may take a while until you gain enough muscle that it can make a visual difference.

Not to mention that even if we can put more tension on certain portions of a muscle, it doesn't mean that we can isolate that portion. It may receive a bit more growth stimulus but it'll take a while for some "reshaping" to occur. Does that mean that we should forget about trying to hit different portions of the muscle? Not necessarily. It means that if you decide to use this avenue you must stick with it long enough to see results.

The second reason is that while we can put more tension on certain portions of a muscle, we're still limited by our genetic/structural makeup. So even if it is possible, with time, to enhance some parts of a muscle group, we can only do it within our body's structure.

Practical Applications

The following tables illustrate how exercise selection can be used to put more stress on the different portions of the pectoralis major muscle. It doesn't mean that the exercise will isolate the portion mentioned, only that it will put a bit more emphasis on that section of the muscle.

In future articles I'll detail exercise selection for other muscle groups.


Hopefully after reading this article you'll understand exactly how and why it's possible to put a bit more growth stimulus on certain portions of a muscle group. I'll say it again though: you can't isolate a portion of a muscle. In fact, with regular exercises you can't even isolate one specific muscle entirely.

However, it is possible to shift a bit more physiological stress (thus place a greater growth stimulus) on certain portions of a muscle. This is due to the four levels of muscle subdivision, which go way beyond the gross anatomical division previously thought to be true.

Please understand that this article in no way means that you should drop big heavy compound movements in favor of a "shaping" program. It simply means you can correct certain weaknesses by selecting the proper assistance exercises over time. The base of your training program should still revolve around the basic lifts.

Another controversial article? We'll see.

Christian Thibaudeau specializes in building bodies that perform as well as they look. He is one of the most sought-after coaches by the world's top athletes and bodybuilders. Check out the Christian Thibaudeau Coaching Forum.