The way you phrase things on the site, I was left with the impression that you were in fact stealing the content while at the same time asking for donations, which is morally questionable at best.
I had to dig around in their copyright section before I found this:
> Some content may be used free of charge without prior consent. The .gif files, text, videos which can be found on youtube.com and muscle information can be used with the MuscleWiki branding and with links back to musclewiki.com.