Massage has been shown to reduce neuromuscular excitability by measuring changes in the Hoffman's reflex (H-reflex) amplitude. A decrease in peak-to-peak H-reflex amplitude suggests a decrease in motoneuron excitability. Others explain, "H-reflex is considered to be the electrical analogue of the stretch reflex...and the reduction" is due to a decrease in spinal reflex excitability. Field (2007) confirms that the inhibitory effects are due to deep tissue receptors and not superficial cutaneous receptors, as there was no decrease in H-reflex when looking at light fingertip pressure massage. It has been noted that "the receptors activated during massage are specific to the muscle being massaged", as other muscles did not produce a decrease in H-reflex amplitude.
For starters, you bear in mind the things described above that tend to cause ugly pain, and you avoid that kind of therapy like the plague. Then you look for some clues that painful pressure is okay. Here are at least three reasons why unpleasantly intense pressure might be therapeutic — “bad pain,” but not ugly. In each of these situations, it might be acceptable to tolerate sensations so intense and painful that the only thing about them that is pleasant is the part where it stops.