As this thread is full of pseudoscience, I might as well chip in. Here's my take on Gear 2nd and its potential side effects:
1. In Gear 2, luffy rapidly shortens and expands his legs to act like a bike tire pressure pump increasing blood pressure and flow through the cadiovascular system. The only thing stopping his heart and arteries from bursting is the elasticity of his blood vessels.
2. Luffy hyperventilates to maximize oxygen absorption in the lungs. Thanks to the extreme elasticity of his lungs, he is able to achieve an unheard of level of efficiency in his breathing.
3. Through these two factors, Luffy is able to bypass one of the fundamental restrictions on physical endurance in mammalian biology. During strenuous activity, mammals become unable to deliver sufficient oxygen to their muscles to keep up with the rate at which it is being burnt. The muscular cells then switch to much less efficient anaerobic respiration which has the nasty byproduct of lactic acid (the cause of the burning sensation in your muscles when they are overworked).
4. So, because luffy is only using aerobic respiration to power his actions, he can achieve extremes of physical endurance and stength.
5. One problem in this are the two byproducts of aerobic respiration: Carbon Dioxide and Heat. Luffy somehow releases the carbon dioxide through his skin creating the mist effect. The evidence of the increased heat from the super powered metabolism caused by gear two is the red sheen to his skin.
6. The multiple negative side effects of this process should be obvious:
-The increased blood pressure on his brain would cause immediate stroke in any normal person.
-By never switching to anaerobic respiration, he surpasses the physical limitations of his muscles, tearing them apart.
-His increased body temperature should give him all side effects of an extreme fever, virtually cooking his body from the inside.
-If at any point he tries to push this effect too far, he runs immediate risk of bursting his heart, resulting in immediate death. He was fairly close to this with the jet gatling.
-Overcooking his muscles to that extent could render them into jelly, permanently handicapping him.
So, to answer the OP's question, he runs the risk of permanently damaging his body through excessive strain or immediate death through coronary failure.
Aldrich is right though that this is a common theme in shounen where the hero surpasses their physical limitations at the risk of destroying their body. The hero never tends to die from it though… although it would make a fitting end for the pirate king.
im going to have that as my sig from now on.
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