I think it is well accepted that beef round, which comes from a working part of the cow, is thus tough and needs to be handled properly. From various sources, I've read that beef rounds (some say all three top, bottom, eye, some say just the bottom round) have lots of connective tissue and braising is a good choice of method for cooking them.
However, it seems that the braised round cuts feel a lot different from chuck, shank, or ox-tail, which really seems to get fork tender and gives unctuous mouthfeel, and still feels quite tough.
That said, is the source of toughness of round cuts same as that of chuck, shank or oxtail, which is a large amount of collagen that will eventually turn into gelatin? If beef rounds do have lots of collagen, then is the perceived toughness due to drying out easily due to lack of fat, which the other cuts seem to have plenty of?
Another thing that confuses me is that some sources say the telltale signs of collagen are thick muscle fibers (easily seen on beef rounds) and multiple muscle groups held together by natural seams. (easily seen on chuck or shank)
Are those two factors, the thick muscle fibers and bunch of muscle groups held together, going to play out differently when braised, which one giving a better mouthfeel than the other?