The premise of One Punch-Man is that its protagonist Saitama is stronger than anything and anyone he might face. Because the manga started as a parody of the shonen genre, the reason why Saitama is so powerful has never been explained. However, one secondary character actually proposed an explanation that makes perfect sense in the context of the most recent developments in the manga.
At the beginning of One Punch-Man, Saitama was an unemployed young man with a nihilistic attitude. He did not have a purpose in life and believed he would never find one, nor fit into society. After a chance encounter with a monster (also called «mysterious beings» in the manga), he remembers his childhood dream of being a hero and starts training every day. His «training» is actually nothing spectacular, just 100 push-ups, 100 sit-ups, and 100 squats, along with a 10 km run, but he did it every single day. After three years, he is so powerful that he can defeat any monster with a single punch. He then starts his career as a pro hero, but due to his excessive strength, nothing is a challenge for him, meaning that he still struggles to find motivation or excitement.
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There is no way that Saitama's training explains his power: other pro heroes in the manga have trained in more gruesome ways and are nowhere close to his level. However, there is one theory that could explain it, proposed by Dr. Genus, a former evil scientist whose experiments were destroyed by Saitama. After witnessing the hero's power, Genus theorizes that Saitama was able to remove something he calls «limiter». After
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