This is a concept similar to "metal fatigue"; bending metal a couple of times briefly doesn't do any harm, but continuously doing so will cause metal fatigue, and in severe cases, it could even break.
But now, that's no longer an issue.
No matter how much life force he used up, all he needed to do was sleep, using "[Sleep]+[Nirvana Technique]."
The next day, vitality, energy, life force, blue bar, blood bar, green bar—everything would rapidly refill, and he could even break through a small upper limit.
Thus, Xue Jing had been training almost maniacally for the past few days.
Practicing Coiling Dragon Art, Armor Shaking Skill, and Yellow Grannie Skill under twenty times the normal gravity in the Dragon Thunder, he'd get injured, then ignite his life force to forcibly recover and continue...
For nearly continuous fourteen-hour stretches, he remained in this intense training mode.