Ok, hear me out, I was just thinking about an article in Nature I read in the past titled "Scrotal asymmetry in man and in ancient sculpture"$^{\dagger}$ and more recently an entire medical textbook detailing everything about hands wherein the author accounts the different types of blood vein topology categories there are in humans, and he touches on handedness too. The part about handedness stuck with me, and I began to notice (looking in the mirror, etc.) that my right neck muscles are slightly larger than the left ones, but so is my friend's who is left handed. I thought this might be due to handedness not translating to chewing behavior (chewing on the right or left predominantly). Anyway, all this is building up to my main question which is this:

Is there something about our anatomy (organ placement, vein topology, etc.) that makes certain symmetric-pair muscle groups unavoidably asymmetrical or is it just handedness or the environment?

$\dagger$ Heard about it here: https://www.youtube.com/watch?v=Xyquv6SsLWg&t=0m56s

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    $\begingroup$ perhaps related: besides environment and genetically programmed phenotypes, there also is developmental noise (e.g. scroll down to "developmental noise" in the genetics textbook ncbi.nlm.nih.gov/books/NBK21842 to see a nice example of differences between left and right eyes of fruit flies) $\endgroup$ – tsttst Nov 17 '17 at 5:08
  • $\begingroup$ So cumulatively random molecular events during embryogenesis then? $\endgroup$ – user36920 Nov 17 '17 at 17:11
  • $\begingroup$ Without any evidence, or excluding genetically encoded handedness, or environmental influences, I would be reluctant to say that the specific case with the human muscles that you noticed would be due to developmental noise; However it would extend the scope of possibilities that you sketched out, and might be a further possibility. (Irrespectively, developmental noise indicates that it is quite difficult, and sometimes impossible, to create two identical structures - if one would look close enough) $\endgroup$ – tsttst Nov 18 '17 at 0:53
  • $\begingroup$ I think it is too difficult to know. There are so many factors influencing development - such as developmental noise as mentioned - as well as environment and genetics. If you were to scientifically investigate this, you would need to control all of these things, which is simply impossible. We would not be able to differentiate between asymmetry caused by environment or anatomy, so most likely we just won't know what might be causing this. $\endgroup$ – GrumpyMammoth Nov 21 '17 at 1:52

There are many variables that could affect the answer to this question. Such as what activities you partake in.

For example, I am a hockey player at the he varsity level in high school. I have noticed that my entire right side is more toned, muscular, and flexible (especially my right latisimus dorsi and deltoid). It just so happens that I rely on my right side for energy and strength more than my left when it comes to shooting pucks and just playing the game.

Typically what we see in humans is the dominant side of their body will tend to show more hypertrophy than the non-dominant side. When you look at a human anatomically, their dominant side shoulder will be bigger and will rest lower when relaxed. Even small things like sleeping habits can effect the visual difference of you symmetry. I will agree though that it is odd that your friend noticeably has more hypertrophy in his neck muscles on one side but is dominant on the other side. This could be due (but not limited) to small habits not taken into account. Like I mentioned before, sleep habits, any motion with the neck in a specific way, chewing could maybe be a factor, or (maybe not likely) a genetic mutation.

This is an interesting topic and I urge you to maybe look at some more people for evidence that dominance has a role in it. 2 people is maybe not enough evidence to really back up a claim you're going to make.

Hope this helped!



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