In One-Punch Man #5, while watering his cactus, a mosquito swoops in and tries to bite Saitama’s hand. Annoyed, Saitama swipes at the bug but misses. He swipes again, apparently striking the bug dead only to see it once again fly away. Increasingly frustrated to the point of rage, Saitama swings furiously at the bug only to have it evade his strikes. Finally, it lands on Saitama’s temple, to which he responds with a well-placed smack to the side of his head. Shockingly, not only does the mosquito evade the strike, but Saitama, himself is “no worse for wear”. While the scene provides a humorous introduction to the villain mosquito girl, it also begs the question; can Saitama withstand his own punch?
Saitama’s strengths and abilities are simple but effective. If he gets close enough to punch an opponent with his fists, his “one-punch” will knock them out. So far, he’s used these skills with devastating results. Indeed, a list of those who have succumbed to the “one-punch” includes a cavalcade of incredibly powerful monsters and villains. Despite such accolades, Saitama’s abilities have become a source of frustration, even for himself. His enormous strength means no villain can truly challenge Saitama in One-Punch Man. However, as the scene with the mosquito suggests among the numerous opponents who have been hit by Saitama, the one person who seems most able to withstand the irresistible force of his one-punch technique is Saitama himself.
From a perspective based purely on physics, Saitama should be able to survive his punch. Indeed, the dynamics that allow a fist, arm, torso, and legs to be able to generate significant force outwards to an opponent are not present to support as significant force inwards towards one’s own body, especially if the target of the strike is one’s one head. That is, even as super-powered as Saitama is, he would not be able to produce that same punch he gave Centichoro on his face or body. It might be painful, but it would be survivable.
Second, there is the fact that most people seem to have a natural tendency to avoid actions that intentionally harm themselves. In other words, even if Saitama would be able to physically deliver the same punch, he inflicts on an opponent on himself, he would mentally be unlikely to deliver it with the same force. In other words, no matter how much he might want to knock himself out, in seeing his fist coming at his face, forehead, or torso, his mind would likely institute some degree of reactive control such that the strike would not be at its full capacity. As they attempt to kill the mosquito demonstrated, automatic self-control would prevent Saitama from hitting himself with the same force he would use on an opponent.
Third, there is the mental aspect of his style. While he can knock out most people, some opponents have been able to withstand his punches. The capacity to withstand, however, is not based on their own capacities, but rather on Saitama’s feelings towards them. That is, if Saitama cares about them or believes that giving them his full force is unwarranted, then he will not strike them with his full power. Assuming that Saitama loves himself, he would also be unlikely to use his full capacity to strike himself.
Finally, there is the fact that outside of his punching power, One-Punch Man is one of the most durable superheroes in the world. This comes from Saitama’s rigorous, albeit simple, work-out regimen. Indeed, he’s withstood the best that his opponents have given him, and rarely comes away with a scratch. Accordingly, there is no evidence suggesting that he would be physically unable to withstand even his own attack in One-Punch Man.