Bondage or Sadomasichism

When does bondage change from being just restrained to becoming sadomasichism (spelling?). I’ve read several forum posts about people adding pain to their bondage experience. They seem to have crossed the line into sadomasichism. I enjoyed a little pain in my first time but some people seem to be pushing it to extremes. If you need several days to recover from your bruises or injuries would that then be considered too extreme?