Saitama is, indeed, really strong that he can put down any villain, or literally, anyone, with just one blow of his fists. Other than that, he adds in his arsenal his sense of humor which could easily make anyone watching One Punch Man roll on the floor laughing. But how did Saitama reach that level of strength? Where did he get those powers?
How Did Saitama Get his Powers and Become Strong?
Saitama claims it is nothing but a set of 100 sit-ups, 100 push-ups, 100 squats, and a 10km run every single day. He did this so hard that he lost his hair in the process, hence his iconic shiny bald head. Because of his training, he also has good reflexes and timing. In all his fights, his strength is unmatched, he remains undefeated.
Imagine a character so strong he could eliminate any villain or anyone in just one punch as he does that merely for fun? Saitama is exactly that. Other than his training, there must have been something else, right?
He is known to be the fastest, strongest, and most durable character in One Punch Man. He's well versed in hand-to-hand combat with incredibly high accuracy, strength, stamina, endurance, speed and reflex, and senses. He is immune to any attacks based on temperature, and he is invulnerable. If you tell Saitama got all these from his intense training, who would believe he's just a regular guy?
There's a theory about how Saitama is that strong: he is a monster, not just a regular bald guy. Dr. Genus of the House of Evolution suggests that humans have a limiter which, as the namesake, limits the strength a human can possess. Such limiter was put by God himself to prevent humans from crossing the barrier of the "realm without purpose or reason."
In One Punch Man manga chapter #88, Dr. Genus said, "No matter how much effort one puts in, every living being has an intrinsic limit to its growth. Too much power becomes unbearable and overwhelms its host, turning it into a mindless, rampaging monster." This leads to the theory that because Saitama has surpassed way beyond that limit, he is, by definition, a monster.
The limiter could be removed, though, in theory by Dr. Genus and Psykos, the military advisor of the Monsters Association. It could be taken away through various trials. They explained multiple near-death experiences could lead one to break their limiters and emerge stronger after a mental and physical transformation resulting in their monstrous appearance and abilities.
In Saitama's case, his intensive training led to this. Technically, one who undergoes such training is like nearing death. Who wouldn't die in such a hardcore workout regiment? Personally, I think I wouldn't make it past 20 each, then a run of a kilometer would make me want to ice my burning legs.
There already exists evidence of the limiter being broken and pushed beyond with the Human Monster, Garou. He became a half-human, half-monster after he faced the Royal Ripper and Bug God. On the brink of death, he emerged, stronger than ever, and half of his body transformed into a monstrosity. This revealed the true potential of Garou.
However, since him being only half a monster may mean that he only has a broken limiter, it is not entirely torn down into pieces. Half of it still works, therefore, being only half monstrous. If this theory of Dr. Genus holds water, then those who totally broke their limiters would be mindless.
There is, but, a flaw in the theory of Dr. Genus: Saitama remains to be an ordinary human being, bored and into video games. He doesn't go out on a rampage; he even takes the hero stuff as mere fun making.
If Saitama's limiter is still fully intact and still defied all the odd of the theories and laws of reality Dr. Genus has been studying all these years, maybe Saitama is even more than a monster considering all the circumstances, right?
Got to hand it to you, Saitama, you got the strengths and the wits.