

In One Piece, Luffy has a technique called "Gear 4th", which allows him to
transform into an enhanced version of himself, further bolstered by Haki. This
technique has a time limit, and when time is up, Luffy reverts back to his
normal form, and cannot use Haki for 10 minutes. This is mentioned in Chapter
728.

Unlike other shounen series, Haki is an ability that is fueled by the user's
willpower. It is not dependent on their physical strength or stamina. This is
further proven when Sanji was able to still use his Haki normally, even when
trapped in Nami's body during the Punk Hazard arc.

So my question is, how is it possible for a person -especially a person as
strong willed as Luffy- to run out of willpower? Is willpower something that
can just "run out" like that?


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one-piece
How is it possible to run out of Haki?


I think it wasn't that he ran out willpower, but remember when Luffy was
fighting Rob Lucci in the Impel Down Arc, after gear 3 he grew smaller. He
said the reason that happened was because his body needed balance, therefore,
his body had a major drawback after a major boost which was his body shrinking
after growing large. My theory is that the 4th Gear is like this and since he
used large amounts of Haki, his body needed balance, therefore, his body
couldn't handle any Haki after that as the drawback.

P.S. To answer your question, no, you can't just "run out" of willpower.


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That episode/arc was the first to introduce such a Haki limitation concept. In
fact, the [wiki page](https://onepiece.fandom.com/wiki/Haki) on Haki cites
that set of episodes to state that:

> Despite their great capabilities, Haki is not limitless as it can be
> depleted from overuse, rendering the user unable to use it for a set period
> while it regenerates.

Also taking from that page the general definition:

> Broadly speaking, there are two types of Haki available to everyone, given
> the proper training, but there is a third type that only a certain group of
> "chosen ones" are said to possess. In simple terms, Haki is an ability to
> sense and predict spiritual energy (Kenbunshoku), use life force as physical
> reinforcement (Busoshoku), and, for the rare "chosen ones", overpower
> enemies' willpower with your own (Haoshoku).

Thus it is only Haoshoku or Conqueror's Haki that is directly tied in to their
willpower. An example of this would be:

> In Chapters 923 and 924 of the Wano arc, we see that Luffy's willpower
> manages to make some weaker soldiers faint even though he was knocked out by
> Kaido. Kaido himself says that Luffy has been glaring at him.

As you said, none of the Haki forms are specific to a user's body. So Sanji
was able to use them despite being swapped with Nami. However, that isn't to
say they don't have any limits at all.

> In the WCI arc, Luffy comments that Katakuri will deplete his Kenbunshou
> haki if he drags the fight long enough.

* * *

Now as for Luffy's drawbacks specific to Gear 4th, it is because of overuse of
the Busoshoku Haki. As mentioned on the [wiki
page](https://onepiece.fandom.com/wiki/Gomu_Gomu_no_Mi/Gear_Fourth_Techniques):

> When utilizing Gear Fourth, Luffy makes extensive usage of his Busoshoku
> Haki by coating his arms, legs and much of his torso in it to the point that
> they turn black from hardening.
>
> Combined with Luffy's rubbery composition and his compressed muscle
> structure, Gear Fourth both adds explosive power to Luffy's attacks and
> grants him enhanced defensive capabilities far beyond what he can
> demonstrate with or without Gear Second or Gear Third

The cause for Gear 4th explosive power is not the Haki alone, but how Luffy
compresses his rubber body. Similar to how he increases his speed by pumping
his blood faster in Gear 2nd. This means his physical body is under a lot of
strain, as Rayleigh mentions when Luffy showcases these abilities to him.

> Once this limit is reached, Gear Fourth automatically deactivates, leaving
> Luffy exhausted and barely able to move. He is also left unable to use Haki
> for ten minutes after using this technique as it quickly depletes his Haki
> reserves. The technique also greatly taxes Luffy's stamina, as using it just
> once leaves Luffy so physically drained in the aftermath that he can barely
> stand and fight, even after he has regained his Haki and mobility.
>
> Akin to Gear Second, using Gear Fourth causes Luffy's metabolism to increase
> rapidly. In fact, after Luffy stuffed himself to a bloated size in his fight
> with Charlotte Cracker, using Gear Fourth once made him burn off the massive
> amount he had eaten in a manner of minutes

The exhaustion and side effects are so great that he can't properly stand and
fight even _after_ regaining Haki and mobility. However, it should be noted
that Luffy's ability to bear these drawbacks, or maybe reduce their effect, is
improving as seen during his fight with Katakuri where he manages to run
around and dodge a bit even after his first attempt fails.


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