The title is one of my longest running jokes/barbs/snappy answers to the inevitable client plea, "This is hard!"
Training is supposed to be hard.
Is it supposed to make you vomit? No. I don't care what you saw on "The Biggest Loser," it truly is NOT okay to train so hard that you vomit. Even if you are morbidly obese. (Because it seems as though humiliating obese people on television is the fashionable custom.)
Is it supposed to cause sharp pain, numbness, dizziness, heat stress, or anaphylaxis? No.
Does the good personal trainer take into account your activity history (or lack thereof) when deciding how hard is appropriate to push your intensity level on any given day? Yes.
Will the good personal trainer help you adjust your intensity level, taking into account the body you brought into the studio with you on any given day, as it reflects your sleeping/eating/hydration/stress load/activity habits over the last 24 to 48 hours? Yes.
Can the good personal trainer read your mind? No.
Your expressions and reactions are quite readable to the experienced personal trainer, but you may be expert at minimizing your discomfort or masking pain. The good personal trainer will frequently ask you for feedback about how easy or challenging an exercise is, while watching for signs of failure of form. Please, give candid feedback.
You can coddle yourself (er, um, come on), you can cripple yourself (clearly not recommended), or you can challenge yourself (oh, yes, pick me! pick me!).
Move more, move well, and move with deliberate intensity. And over time, you'll move better. :-)