Keeping One's Figure - Myostatin Regulation

Background:

I am working on a super soldier transhuman/posthuman concept for my fictional world. It takes the base human genome then uses genetic engineering to augment, delete, enhance, add, and otherwise modify the resultant creature. I am aware that genetic engineering is mostly speculative at this point but would like some imput on various ideas in my head.

Specifically I am working with the idea of a super soldier who is designed to look a certain way (either as a standard model "clone" or a specific design choice by their designers).

Because humans who are athletic, fit, and active tend to build muscle, grow, put on weight or otherwise change shape and deviate from their pre-athletic selves (assuming they are well-fed) I was wondering if there was a way to limit physical changes that accrue from physical activity to keep them looking like their intended original design despite the physical activity they engage in.

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Question:

Could genetic engineering alter a human body's myostatin production to inhibit excess muscle growth beyond a certain pre-set point?

Specifically could someone's body be designed in such a way that they cannot build excess muscle. They could train and perform athetic feats, their muscles would tear and reform, but because of the presence of Myostatin they would reform "exactly" the same size/density of muscle as before and thus so long as they eat enough to maintain their peak form, they would not go over that genetically programmed limit.

In essence I am pretty sure there is a limit to how large muscles can grow in humans, and was wondering if genetic engineering could artificially set a specific limit to give a human a certain desired look despire how much physical activity they do.

I would like to stick to hard-science, but given that genetic engineering is in its infancy, I understand some things are speculative.

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Additional Info:

The following are additional thoughts and ideas that come to mind related to the topic. They are possible future questions, but not the main question of this post.

I have read in several places, about the opposite effect being present in nature. Two examples that come to mind are the Belgium Blue Cow and the Bully Whippet dogs are prime examples of repressed or lack of myostatin.

They are discussed in this question another user asked:

Humanoid Race with Surpressed Myostatin Gene - Appearance?

Another reason for my question is to see if it could be genetically engineered such that two identical twins, one who is very athletic and sporty, and the other who works out often but only lightly, could still end up with the exact same build, because both were at the peak of this artificially imposed muscular cap.

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Celebrity Super Soldier - How Strong Could a "Realistic" Humanoid Be Designed to Be?