it's not synthol, it's PMMA!
synthol won't cause that big of an expansion, people with too much synthol build site lumps and also inflamed sites which is obvious because of discolouration and swelling of subq tissue as well as muscle
Piana has none of that, he has a ridiculously sized sweep to his triceps which is the polar opposite to what his arm structure allowed for before.
no unevenness or inflammation, just expansion.
if he were to try and achieve that size through synthol use he would definitely have oil leakage, lumps, unevenness and inflammation and even then he wouldn't get close to the size he got through PMMA.
he went to Mexico and had several sessions with a renowned PMMA specialist (Dr Casavantes).
initially to help cover up a bicep tear (according to him this is all he used it for!)
but obviously had a brainwave to bring up his sub-par arms as a whole using this method.
PMMA causes collagen formation at the site of injection, over months collagen formed inside (and likely outside) of his muscles.
that's why he has zero vascularity there and has covered them in tattoos to hide this.
it's just mass on his arms through collagen, no muscle expansion (synthol essentially fills the muscle with oil to expand it)
just collagen formation
