UPDATE:-
Adding more frames making motion bit smoother but still jaggy :B

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This is the result how it looks. I think I will take couple commission for this animated Bicep Flex Icon. The price will be $30 and will increase depend on how complex your character. The veins is optional, I can put it out if you don't want the veins during flexing.

So, if you want one, send me note with your character references