In all relationships I have been in, there has always been an almost unspoken clause on any command - only if you think it is safe. This is mainly because, in online LD relationships a Domme cannot be there in person to assess the risk of any activity (they can know more or less what might be safe for them in thier situation but not necessarily if that same thing is safe for the sub) and obviously don't want to risk safety of the sub. And this does carry over to r/l in a sense, though in this case there is more of an element of trust as the Domme is there to monitor the situation and so it depends more on how much you trust the Domme...

Consent is all. There is no legal restriction on you that forces you to submit, only your devotion and trust of the Dom/me and your acceptance of the rules of the lifestyle. You are responsible for your own safety and if a Dom/me looks like they are stepping over the line with regards to safety, you have a right to talk to them about this and maybe renegotiate. Situations change all the time - your health may get worse, meaning some activities are no longer safe, for example. In these cases, a Dom/me needs to adapt to that and learn to keep the negotiation process open all the time - even if you are a 'slave' who has no hard limits.

Following on from that thought... hard limits...

A slave (who therefore may not be allowed to have any hard limits under the definition some use) has a health problem that precludes certain activities... do these activities now count as 'hard limits' or are they kept as 'things the Dom/me could do, because the slave has no hard limits, but will not do out of respect for the slave's state of health'?