So, what basically happens, is that he suffers an injury that causes the Amnesia. Which caused him to not remember anything up until the moment he woke up to head to Japan. He had no clue who he was until his sister (if I'm remembering correctly, it may have been his mother that said it first) said his name. Due to his Amnesia, he had no clue what the world around him was like until his family told him more about what quirks are and what his quirk is. He never lost muscle mass, as Amnesia doesn't affect muscle mass. In this case, it did effect his muscle memory, his memory of his past and his quirk. The reason why he starts out in his Base Mode is because of the fact that he doesn't remember how to use his quirk to it's full potential, which is where that muscle memory loss comes into play. His body's durability is still there, as his muscle memory doesn't change that durability. His sister was merely testing to see if he still maintained that same durability that he had before losing his memory, which it's a given that he was going to regardless.

that doesn't make sense I get that you can forget everything but for your body to forget the muscle memory is impossible since you literally beat it into your body over and over again that builds the foundation for your body so it should not be affected by a head injury sense it's just not understandable what so ever it's like a fish getting hit by a rock and forgetting that it can breathe in water so it's hills close and stop working