First of all, mbkiller, your avatar is the most offensive gag inducing thing ive ever seen. I love it. I wish I could blow it up so that i could see the folds of skin more clearly (to see if they house any cheese)
Second, scar tissue, specifically that which is formed in between the muscle layers in areas that repeatedly injected is thickening of connective tissue that holds our muscles together. If you cut your skin real bad the skin that heals there will not be of the same consistency, it will be less organised but much thicker in density due to the different type and orientation of collagen fibers that come after the healing process. Same thing happens in the muscle tissues that we inject. the injection itself cuts and serrates muscle tissue and surrounding connective tissue. Im not sure if the chemicals that go in have any influence of creation of scar tissue, but I 'm sure if they are caustic enough they will. Regardless, the muscle tissue reforms/heals pretty normally unless its a big boo boo , which injections arent, but the connective tissue always heals a little funny, meaning with repeated insults, it will heal much thicker and denser than before. some people are of course more prone to scar tissue thatn others, ie keloids in african americans. some people also have problems with collagen synthesis which greatly hampers their healing from wounds and bone fractures.
Thats a lot of shit, i hope some it answers you question