Why it matters
In HIT, intensity is defined by how close you come to momentary muscular failure. A set that stops short — because it burns, or because a rep count is reached — is submaximal. The stimulus for adaptation is the effort near and at the failure point. Without it, the set is just exercise.
What it is not
It is not pain, soreness, or fatigue in general. It is a specific, objective point: the bar (or handle, or pad) stops moving through the range despite your full effort. Reaching it requires controlled form; an uncontrolled, bounced, or jerked rep is not a true repetition and does not count toward failure. Train to failure safely →