I've read and heard from several bodybuilders (Arnold, big Youtube guys) that the pectorals will only grow in proportion to the upper lats. Both must be trained equally to see growth.

  • What is the scientific explanation for this? How do the muscle fibers in the pecs know how the muscle fibers in the back are doing?

  • Does this rule apply to all pairs of opposing muscles?

Granted, the pecs and upper lats are not mechanically opposing, but are physically placed opposite of each other. Examples of other opposing muscles include bicep/tricep, quads/hamstrings, tibialis anterior/gastrocneumus. Arnold said that the tricep should be two-thirds of the upper arm to be aesthetically pleasing, so the proportion requirement could not always be 1:1 for all muscle pairs. He said that was an aesthetic requirement, not a physical requirement, but this doesn't rule out that a minimum physical requirement of some other proportion exists.

  • Your muscles know, because shortening one stretches the other. There are structures that will recognize these stretches and signal your central nervous system ;-)
    – Ivo Flipse
    Mar 29, 2011 at 18:17
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    I know that it's definitely possible to increase muscle mass out of balance. For instance, guys who put too much emphasis on biceps and not enough on triceps end up with 'monkey arms'. And, if you do weighted or high resistance abdominal workouts without balancing with a sufficient back workout your abs will consistently pull your body out of alignment causing chronic lower back pain (been there before, it wasn't fun). This doesn't really apply to the limits of how much a specific muscle can grow though. Mar 29, 2011 at 18:49
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    @Evan Plaice - this should probably be an answer... I'd see if I could find a source to back it up though as well. Mar 30, 2011 at 13:57
  • @md5sum Yea... the 'find a source' part is the problem. I'm talking mostly from what I've seen/experienced in the past. No actual written proof. Mar 30, 2011 at 17:48

3 Answers 3


To the best of my knowledge this statement is not true. I can think of several possible explanations for why this is such a prevalent "docterine" and espoused by such successful body builders:

  1. It's not a scientific statement and thus doesn't mean "grow" literally. In body building size is only one factor. Proportion and shape are also very important. While working on your chest, your pecs will grow, however if they grow in mass not in proportion to your back they will not appear to grow, instead they will start to pull your shoulder forward as the imbalance progresses and they increase in size while occupying the same amount of lateral space.

  2. In coaching, you generally don't give your athletes "facts", you give them "cues". The difference is that Facts are true but may not communicate to the athlete effectively what they should be focusing on, and "cues" may or may not be true but they are designed to invoke the athlete to execute the desired behavior/performance.

  3. This may be something they learned from experience which doesn't mean it's invalid but it's far from being a scientific truth.

I will say that if an extreme imbalance develops that very well may trigger physiological mechanisms that inhibit growth, but I think that would have to be extreme. I think that in the context that this statement is used regularly it is more about coaching/encouraging balance.

  • About #3: Arnold disputed many scientific findings with his personal experience. For example, science says the ribcage cannot expand, but he believed pullovers could expand it. In the end, who are you going to believe? Isn't Arnold also a scientist? He made a hypothesis, carried out the experiment, and saw the results. But the scrawny scientist is just sitting behind his desk making conjectures, without carrying out the experiment.
    – JoJo
    Apr 3, 2011 at 3:41
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    I don't think that is a fair characterization of science. There are good scientists and bad ones, good methods and bad ones, and it's not perfect. Without getting into a lengthy discussion about scientific process I will say that we are constantly proving possible, things that we previously thought were impossible. The point I was alluding to is that anecdotal evidence that supports the statement is going to be insufficient because there are so many other factors that can't be ruled out without a controlled study.
    – matt
    Apr 3, 2011 at 4:28

I don't have scientific studies to support this; however, Dave Tate (power lifter) has observed that you get a big bench with strong lats. His reasoning is that you need both a stable platform, and to engage your lats during the lift. This helps lift heavier.

When carrying this over to some of the theory that I've read from body builders about how to build bigger muscle, you need to be able to do more work (volume, time under test, etc.). The take away is that lack of strength in your lats will limit the amount of work you can do on your chest speaking in practical terms.

Seile's Thesis on General Adaptation Syndrom (GAS) basically states that a living organism responds to stress by adapting to that stress to resist it better the next time it is introduced. Essentially, in order to cause the adaptations that build larger muscles, you need to invoke the types of stress that cause that to happen. If you are limited because of muscle imbalances, you can't induce the necessary stress.

Another major concern has to do with the risk of injury. Severe muscle imbalances put the lifter at a greater risk of injury. If you injure yourself so you can't lift for an extended period of time, you will never be able to get the bigger chest you desire.


I'm not sure how you would research this :) You would have to have a group train, for example, only pecs (and no lats) for a long enough time to observe this phenomenon; the duration is problematic, not to mention convincing someone to do it. Then I feel it's also not really 100% ethical since as a researcher in the field you would probably be aware that intentionally creating a strength imbalance increases the risk of injury.

As far as less scientific evidence, I've seen a lot of talk about this problem of having much stronger pecs than lats for example, so I feel that this invalidates the theory, since it describes a completely opposite situation (that of unwanted strength imbalance) as being very prevalent. I am presenting this as anecdotal, but I am fairly certain that there is some medical and scientific work to support this.

Finally, I think that what applies at their level (Arnold etc.) is not necessarily applicable for the general population. When you are Arnold's size (I'm of course thinking of him during the years in which he was competing), there are probably tons of little details like this that you can observe, because you've already grown so much and are now trying to push your body even more.

Finally, I feel like this doesn't really answer your question and I'm sorry for that, but I guess that what I am trying to say it that I'm not sure if it can be answered :) I hope at the very least that my answer can be a good start for more discussion.

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