Ninjas in Naruto run with their hands at the back,
which is very different from normal anime character running.
Why is that? Is it because of their too much running speed or the wind force that their arms tend to be at their backs? Don't they lose balance when they run like that? Is there any explanation for this?
Based on research...
...Ninjas frequently were portrayed running with The Airplane Arms.
Ninja are also frequently portrayed running this way (with the arms swept farther back, like the wings on an F-14 Tomcat), though they now tend to prefer the Ninja Run. Samurai also run in a similar manner, usually while keeping one hand grasped onto their katana. In a more (apparently) realistic series this may have something to do with the runner having a reduced profile and thus being harder to hit with ranged weaponry, but it's generally just the Rule Of Cool.
One forearm crooked in front of the body, apparently to block attacks (two fingers held in front of the mouth
optional but prevalent).
The other arm behind the back, ready to pull out a bladed weapon.
Another speculative guess would be better aerodynamics. Also they move very quickly at times and it is said that swinging your arms make you tire faster and give you bad form, so it could be to save energy.
To reduce the profile, leaning forward the way they do and keeping the arms straight makes for a harder target to hit and lower to the ground so its more difficult to see moving from cover to cover, which is a Ninja's main objective; to move stealthily.
1.5 This also gives them the perfect posture to leap forward for whatever reason since most attacks from behind are hard to block compared to a forward attacks which will have a reasonable amount of time to see and intercept them.
An easier/quicker access to weapons, which are attached to the thighs and back.
Increased aerodynamics. Running at a fast speed like most Ninja do creates a lot of drag, the more aerodynamic the posture the better. I can also think of several balance issues that come from running face first like that, the arms held behind the body fix these balance issues.
Can achieve a faster speed. Technically... With all the weight moving forward like that it acts like a lead weight on the front of a toy racecar. The force above and ahead of the main pivot of the legs increases the speed. Inversely this makes things harder to control, but, given that Ninja are basically Jesus and can walk on water and vertical surfaces like walking on flat ground, it makes sense that they have amazing leg strength/balance and can stop on a dime relatively easily.
Improves correct sneaking posture. Most Ninja are taught to be stealthy, but learning a toe heel posture that emphasizes stealth takes a lot of practice and is hard to maintain. Leaning forward as they do while running forces the body into a toe heel posture.
5.5 This early academy training idea also has a good grounding on why most adult Ninja in later chapters use the "one hand behind and one hand forward with a hand sign" running style. They've already learned the correct posture to run and sneak with, now they can keep a hand forward, ready to cast the Jutsu they learned during their time as Genin, alongside their proper sneak/running posture.
When they keep their arms behind, they are streamlined.
They penetrate through air faster and air resistance is reduced, and hence it boosts their speed.
It also provides easy accessibility to weapons.
Actually, I once saw a bit about this in a Japanese TV show. It comes from the way samurai used to run before meeting the English. Samurai ran into battle using only their legs, keeping the upper body stable and in a ready stance. The arms back thing was, I believe, an extension of this having to do with a belief that it cut down on air resistance. (like using the same technique to run through water) But one thing that happened when the English arrived is that each saw how the other ran and, as men will do, competed. The Western style of running is harder to do with a weapon in hand but is more efficient and therefor faster. The eastern style has no swinging of the arms, which allows larger weapons (swords) to be carried safely, but also requires the shift in weight to occur in the body rather than the arms, making it less efficient and slower.
Perhaps it's because the weapons they'd use to handle an ambush are drawn from their thighs and behind their backs. I'd think it would also give them a uniform running profile so that there is no 'best moment' to attack them. If you run with your arms swinging there will be moments when your hands are completely out of position. Especially if you have, say, a left thigh shuriken holster and a right shoulder sword draw then you will constantly be creating a moment where you can easily draw neither weapon due to arm swing. Thirdly it allows you to draw and throw weapons hidden in your sleeve without the delay of needing to swing your arm back.
I suppose it could also be an artifact of chakra assisted movement. A proper calisthenic running style might conflict with getting the best results from a run that relies more on sticking with chakra and explosively releasing it for a 'push' rather than the rounder motion of running based off pure athleticism. Perhaps it also helps their moment to moment balance by keeping their center of gravity near constant rather than allowing it to sway, that would theoretically allow for faster, tighter reactions and evasions.
Less air resistance, faster speed. Used a lot when designing jets, planes, cars. You want something to be faster, you either make it more powerful or have the resistance against it cut. Speed was important to ninjas so wearing tight clothes and running in an aerodynamic position would make them faster. That is in addition to obvious benefits that people have stated (a hand at the ready to draw a blade...etc).
Ever wonder about the shape of submarines or torpedoes? Its cause its hydrodynamic. Faster speed due to less resistance from the water. It works the same for air resistance. NASA used to have a great page on aerodynamics but obviously it is no longer available (can't even be crawled due to robots.txt for the site).
It obviously doesn't ACTUALLY make you run faster. If it did, you would see it in the Olympics.
Any additional aerodynamics isn't made up for by the loss of swinging momentum of the arms and the awkwardness that causes.
There is a reason people swing their arms when they walk/run, even though it uses significantly more energy to do so.
Indeed, I thought perhaps the reason you would see this run is that maybe it used less energy and helped for distance, but that is unlikely too, because it is probably more work to keep balance overall---and there are people who can basically run so long that what they have to stop for is sleep anyway.
Because putting your hands behind your back while running makes you go faster. Because it's weighing you down in the back but your weighing yourself down in the front, So it's like your trying to keep your balance but the front has more weight than the back so when your running your also kind of leaning forward at the same time which makes you run faster giving you more speed. That's why ninja do this so they have more speed.
The reasoning for this is most likely because it looks cool and fits what westerners thinks of when they think of ninjas. There may be some reasoning with weapons but I do not know of any. However it has nothing to do with aerodynamics, physics or speed.
Sprinters use their arms because it makes them faster. Leaning forward while running does not make one faster it actually slows you down because you are in front of your bodies natural center of balance.
It also does not aid in long distance running. Humans have evolved for bipedal movement because it is more energy saving and efficient way of moving because of the center of gravity and swinging of the arms while walking and running. This is why humans were able to prosper in our early developer because while other quadrupedal animals exhausted themselves while fleeing humans where able to run longer distance, though at a slower rate than other animals to tire them out.
A mixture of obscuring your profile and having you hands closer to the branches on your side, should you have to pivot/swing away without wasting chakra on a shadow clone or substitution jutsu. I think.