I would go for a different approach. "Muscle" could mean make one´s way by force, so "muscle on
" to me would mean "enquanto os concorrentes continuam a brigar/continuam na briga".
So, the item goes on by showing why McDonald´s is an "outlier" of sorts, it stands out, but then at the bottom of the article it shows how competitors go, their weaknessess and strenghts.http://www.theguardian.com/business/201 ... hake-shack
Muscle - in a "making their way" by force, or forcing their way. is a commonly used term on British/Irish sports news:
Muscle out (forçar a saída de...)http://www.telegraph.co.uk/sport/rugbyu ... elite.html
Muscle past - consegue passar (à força)http://www.irishtimes.com/sport/rugby/i ... -1.2363750
"...in a match that was at times gruesomely physical
but contained moments of high skill."