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It’s often said that training X muscle more than its antagonist(s) will result in a postural imbalance in favor of the more trained muscle.

Common examples people say:

  • Training the chest muscles too much relative to the back muscles causes hunched posture with protracted scapulas.
  • Training the anterior deltoids too much relative to the posterior deltoids causes rounded, hunched shoulders posture.
  • Overdeveloped spinal erectors and hip flexors and/or underdeveloped gluteals and abdominals causes anterior pelvic tilt posture.

These suggest that training a muscle group under load will make that muscle more contracted in its resting state. Is it true that strength/hypertrophy training makes the trained muscles more auto-contracted or tighter when they’re relaxed?

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Yes, and here's why

Hypertrophy does basically add myosin-actin connections by means of satellite cells fusing with existing muscle fibres and thereby adding the ability to build and sustain more filaments at the same time.

That means that for any given frequency the alpha-motoneurons fire their action potentials at, there are more myosin filaments that "do the job" - the muscle becomes stronger.

Now, there is something called muscle tone at rest. That means that there are always some motoneurons of any given skeletal muscle in your body firing at a given frequency that depends a lot on your stress level.

But here's the thing: if the muscle on one side of the body underwent hypertrophy and has more filaments working, even if the frequency would be identical in rest state, they will "win" against their antagonistic buddies on the other side of the body. The antagonists are going to be stretched, register that, and react with a higher tone: the motoneurons fire in a higher frequency so that the filaments work faster to make up for less filaments working at the same time. This is an autonomous mechanism that adds some "pre-tension" when a muscle is stretched so that it is not ripped apart.

That means that normally, it is not only the trained muscles but their counterparts that have a higher baseline tone in rest and therefore have less capacity to be recruited for functional tasks, which, by the way, makes them harder to train (sic!).

The other problem is that neural connections that are used often become better at their job (what fires together, wires together). And even if the signal is originally the same frequency coming from the brain, the effect can become higher. Also, a steady strain in the mechanoreceptors in your muscle via load means these cells try to "protect" your sinews and muscle fibers by adding more tension by default (detonisation techniques and stretching desensitise this mechanism). All this results in a higher muscle tone in your already stronger muscle, which perpetuates the effect described earlier.

And yes, that can result in permanent postural changes. Those can be further manifested through titine filaments glueing permanently contracted muscle filaments together (called a contracture).

This reasoning is also why we tend not to say "you need to stretch muscle XYZ" for postural change anymore. Instead, you just have to train the other side so that it can hold its own when the baseline frequency at rest hits both of them. Any other strategy is not leading to lasting improvements.

This is kind of hard for some agonist-antagonist pairs in the body but chest and shoulder/upper back are typically not one of them. Glutes and hip flexors as well as hamstrings and quad are usually named because of how they are built and how they are typically used in modern daily life.

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