Gravis had been punching boulders for over a day by now. It was exhausting, and Gravis had wanted to take a break multiple times already but always forced himself to do one more. At some point, punching "just one more time" had become ingrained in him. He just continued to punch without stopping.
This whole experience was something that he had never gone through. He never had to train since everything always came naturally. His physical strength increased with pills. His Realm increased by simply pushing lightning into his mind. His battle-strength increased by fighting. His Will-Aura also increased by fighting. Nothing had ever needed him to physically train like this before.
The only time when something similar happened was when he was training Martial Arts with William, but that had been different. He only had to learn the movements then. It hadn't involved pushing his body to his limit.