Someone in massage therapy told me that muscles require some chemical reaction to relax, and that magnesium helps with that.

Looking into it, every source online I've found states that muscles relax on their own. On one hand, this makes sense because flexing requires energy and our arms would fall to our sides without it. On the other hand, it doesn't make sense because sometimes muscles hold "tension" (knots) even when they aren't flexed.

I've also read that muscles use ATP only to move, but then have a chemical bond to hold in place (kind of like a ratchet system). At the same time, running out of ATP will lead to relaxation. This would indicate ATP is required to keep a muscle contracted.

Conversely, I've read that magnesium helps with blocking calcium, a part of muscle contraction. This would indicate that calcium is part of some sort of chemical bond. Chemical bonds tend not to not consume energy to remain bonded.

Other sources state that "muscle tension" is less about the fibers and more about other things (fascia). That may be the answer to what a "knot" is.

Overall, I'm very confused about what keeps a muscle contracted, and what relaxes it.

If a muscle fiber was contracted in a vacuum, would it stay like that? Is it chemically bonded in that state? Or is it somehow held in place by the expenditure of ATP? Why would the absence of ATP lead to fiber relaxation?

EDIT: thanks to @BryanKrause, the "physiology" section on Rigor Mortis contains details which seem to imply the existence of permanent chemical bonds that require a reaction to release (https://en.wikipedia.org/wiki/Rigor_mortis)



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