You know those eternal discussions. Philosophical questions like: “Is there life after death?”. Cultural questions like: “How long will Black Pete remain part of Sinterklaas”. In the gym, completely different discussions keep coming back. One of them is about whether you can train the inside of the chest separately or not.
Check the following link for all chest exercises
The middle part of the pectoralis
In this discussion, you’ll find different camps. Traditionally, they were divided into two camps.
On one side, bodybuilders and others who “felt and saw” that they used the inside more with certain chest exercises.
On the other hand, people who, based on the study of anatomy, argue that this is not possible at all.
Personally, I’ve been on both sides.
However, a third camp is emerging. Advocates of the proposition that the inside of the chest can be emphasized separately and that this can be explained anatomically and physiologically.
The anatomy of the pectoralis major
The discussion revolves around the pectoralis major and its parts.
Anatomically, it can be divided into three parts based on various points of origin.
Muscles function by pulling from different sides, called origin and insertion.
Origo is the point closest to the heart, and insertion is the further point where the muscle ends.
However, “a muscle” is made up of different muscle fibers that sometimes begin and end in different so-called heads.
The different fibers and heads can have different attachment points where they are connected to bone.
The pectoralis major is anatomically often divided based on the three different points of origin.
- Pars clavicularis: Starts from the collarbone
- Pars sternocostalis: Starts from (the top of) the sternum
- Pars abdominalis: Starts from (the bottom of) the sternum and the rectus sheath (simply put: top of the abdominal muscles).
Functionally, however, the last two are seen as one, namely the “bottom”, while the fibers that originate from the collarbone are called the “top” chest.
Importance of fiber direction
In the image, you can see that all three (or both, depending on the subdivision you use) end in the upper arm.
Due to the different points of origin, they pull from different angles on the same point, resulting in different movement paths of the arm.
If you push straight forward or diagonally downward from a standing position, it is mainly the “bottom of the pectoralis” that is used for this.
If you push diagonally upward from a standing position, it is mainly the fibers of the “top pectoralis” that are activated and pull the upper arm forward and upward.
The pulling of the muscle is done by the so-called filaments in the muscle fibers that can slide over each other to shorten and lengthen when the load is relieved.
“Inside chest training”
You may already see that there is no distinction between the “inside” and “outside” of the chest here.
They all “start” on the inside, so there is no distinction there. You can point out the inside, but it is not a separate, functional, part.
At least, that’s the prevailing theory and at the same time the source of discussion.
Camp one: “The inside chest does not exist”
An example often given by supporters of this proposition is the chain.
Imagine you are pulling on both sides of a chain.
Can you then ensure that more load is placed on one link than on other links?
No, you can’t. The pressure is distributed over all links.
Muscles grow partly in response to load and microtrauma caused by it. Small tears in the muscle fibers that become stronger and larger through recovery.
If you can’t determine which links of the chain are most stressed, then you can’t determine which ones will grow in the case of muscles either.
If you look at the chest, you can choose “which chain to pull on” (the top, middle, or bottom fibers), but not which link you stress more.
I often give a different example for comparison than a chain because the force in muscles is generated by the links themselves and not by the person pulling on them.
Imagine people with outstretched arms, hand in hand in a chain on an ice rink.
They represent one muscle fiber where the people are the filaments.
When they hear over the speaker system that they should pull their elbows into their sides as hard as possible, the chain becomes smaller as they are pulled toward each other over the ice.
In this comparison, the load on each person is 100% because they are all pulling at full strength. After all, they all hear the announcement and all react by pulling.
In this comparison, the loudspeaker system represents the nerves that give the muscles commands from the brain.
Camp two: “I feel the local effort and see the result”
In this camp, there are both experienced, old-school bodybuilders and beginners. For decades, they have described feeling more stress on the inside chest with certain exercises.
The most popular exercise to “feel” the inside chest is the cable crossover.
However, this camp seemed to be slowly dying out because it could not be anatomically explained how this exercise would extra stress the inside.
The people from camp one therefore argue that the feeling of heavier stress on the inside comes from the fact that the entire chest muscle is pushed inward and accumulates there
Resulting in a feeling of local stress.
The partitioning hypothesis
As far as this discussion is concerned, interesting studies have appeared since the early 1990s that seem to confirm earlier, fairly revolutionary, theories.
These point to a new way of thinking about the control of muscle fibers by nerves.
Think again of the people on the ice rink.
Now, however, the speaker system is not working, and they have all been given a mobile phone with their own number. Now, not everyone gets the same command at the same time, but for example, you can call the first 5 people in the chain with the command to pull their elbows into their sides while the rest stay with arms outstretched.
If you keep giving this command to the same five people, they will go home tired at the end of the day while the rest are still fairly rested because they didn’t have to pull.
The partitioning hypothesis suggests that nerves have separate branches for different parts of the muscle fiber so that they can receive separate commands and act separately.
Studies have appeared showing the separate control from the nerves, but also studies that have actually shown more growth in one part of a muscle fiber than in the other part (1,2,3,4)
What does this mean for chest training in practice?
Little to nothing, to be honest.
It’s food for thought for a fun discussion in the gym, but of little practical value.
For muscle mass, variation in training is needed to keep providing the muscles with the necessary stimulus.
Even if it were undoubtedly established or assumed that you could train or emphasize the inside of the chest separately, it would still not make much sense to put a lot of emphasis on the exercises designated for this.
Due to the law of diminishing returns, the chest muscles will become so accustomed to these exercises that the result will become smaller and ultimately even smaller than with exercises that are not so much focused on the inside.
So you will still have to vary your exercises.
References
- English AW, Wolf SL, Segal RL. Compartmentalization of muscles and their motor nuclei: the partitioning hypothesis. Phys Ther. 1993;73:857–867.
- Price, T., et al. (2005). Biceps brachii regional growth in response to 12 weeks of resistance training. Med Sci Sports Exer. 37:S131.
- English AW, Weeks OI. Compartmentalization of single muscle units in cat lateral gastrocnemius. Exp Brain Res. 1984;56(2):361-8.
- ANTONIO, JOSE, 2000: Nonuniform Response of Skeletal Muscle to Heavy Resistance Training: Can Bodybuilders Induce Regional Muscle Hypertrophy?. The Journal of Strength and Conditioning Research: Vol. 14, No. 1, pp. 102–113.
