sure, a safe word- that's important.

honestly though, there are other indicators of someone being in too much pain. I work as a massage therapist, and you always get the guy who comes in and wants really deep pressure. he will never tell you that he is getting too much, wehn I know my elbow is against his ribs or something.

there is a limit to what the body can take without damage. so yuo watch for shortness of breath, of the body tensing, of hypremia- reddness is ok, but if the tissue gets too red, you are going to produce a hematoma- a bruise. maybe bruising is ok- but i'v gotten a few over the years folks wanted me to seek medical attention for.

I guess what i am saying is that you need to pay very close attention to your sub, take them a little bit at a time, know their tolerance levels, learn their body.