Why do martial artists "shout" in the execution of a technique?

If one has sufficient vocal training, and can project ones voice, this technique can indeed be used to disrupt the concentration of a many opponents.

Here what I mean is being able to vocalize with sufficient direction and volume that the opponent can feel it in their inner ear. This is particularly effective in a closed space, which creates resonance at certain frequencies—trained actors know how to "tune their pitch" to a given room for maximum resonance, and it becomes an unconscious reflex. (In my school, because of my vocal training, my shouts were acknowledged as being more effective than that of other students. When we had a strong fighter who also studied opera, he was likewise able to disrupt attention with a shout, which provided some validation.)

I've never seen value in using with with a strike, but instead, before a strike because you can often get people to wince, which creates an exploitable advantage.

You can compound this effect by sometimes striking after a shout, sometimes not, to mentally unbalance the opponent, since deception is the core of strategy!

Besides the inner and outer manifestations of power and focus which are mentioned in the other answers, the yell also has other uses which haven't been mentioned. In two styles, the kiai (japanese) or kihap (korean) have context in sport rules:

In Kendo, the attacker must yell the target: men (head), kote (wrist), do (side, waist, body), and tsuki (throat)

In Kumdo, the attacker must yell the target: mori (head), sun-mok (wrist), and hori (side, waist, or body)

When striking, you must yell the attack. Failure to do so, or yell the wrong target, means the attack does not count. If you do not yell it loudly enough, the judge may not hear it, especially if your opponent does it loudly at the same time as you.

Its as simple, go bk to Bruce Lee, the reason behinf the shout of a heavy aggresion of anthem that is called in certain arts is to use your individual, toatal force as vital as possible, with the breath, that is in the very point of the positioning attack, the as you phrase scream, is a vital use of internal and your physical force in the point of skill being used , example when a person scream ,what is he using on his internal emotion force?Got it.... Maximum force in a vital but positive extreame attack. This skill is only used, if your opponent is coming towards you in a killing motion of moves or the fight has come to a deadly point where by your life is to be safed..

What i Know is - human brain receives one signal at a time. We take our strike as a light signal and shout as a sound signal. If the brain received sound signal the hit will serve more dangerous to the opponent. Just like a hit to a person who is not alert towards the move, strike becomes more dangerous than to whom who is aware of the strike.

Secondly it is a natural process producing sound force to pull or push a more heavy weight, we use the same force to deliver the strike.

Thirdly animals scream, shout & Grunt naturally when they are scared and try to scare the opponent by high sound.

One of my partners this weekend reminded me of another reason that it is important to exhale during execution of a technique. If you are hit, or hit the ground (take a fall) while holding your breath, you're going to (a) hurt more and (b) take longer to get up.

I can't quantify the difference, and I would recommend against the experiment . Sadly even though I remember (not pleasantly) the falls I've taken while holding my breath, I still have to work on it every couple of years - it is a bad habit that comes back.

I'd say it's a waste of energy if you aren't using it to scare an opponent. When striking, kicking or blocking it is important to exhale to relax the body so that you can get more power into the strike. This should however be done more or less silently.

People bench pressing also does this to get more power in to the press, most of them without screaming or making to much noise as you want the force to go into your arms not out through your mouth.

This question is too broad to give it an exaustive answer, because (as you can see from the answers) each martial art gives its interpretation of the kiai.

Speaking of Karate Shotokan, the kiai is performed to stress the power on a technique (for kihon and kata) and to intimidate the opponent (for kumite, and also for validating a successful blow during tournaments).

It is worth noting that the kiai is not performed by "shouting" (after some shout in succession you lose your voice); but instead is performed by a powerful contraction of the abdomen.

A Kihap is basically a battle cry, carried over into modern times. But there are actually some very positive benefits.

1. It strengthens your core by tightening your abdominal muscles. This translates into more power in your motion and increased defense. The Kinetic Chain, also called Kinetic Linking, is the way your legs can pass their power through your torso and into your arms. To be effective every part of the chain must work as one unit.

2. It makes you breathe. Heavy physical activity requires energy, the cellular process in your body to generate energy (ATP or Adenosine Triphosphate) requires oxygen. There is another process that doesn’t require oxygen, but it generates much less ATP.

3. It can help you focus. With lots of repetition your mind and body can be programmed a certain way. National Geographic has a show called Fight Science; in one episode, SWAT team members could control their heart rate even with highly elevated levels of adrenaline.

4. It startles your opponent. To me this one is a side benefit, but still valuable. It’s possible to avoid the fight completely by dropping back and letting your kihap out; might make them think twice.

It doesn’t really matter what sound you make, you can achieve benefits 1 and 2 by remembering to breath properly when striking. Some people make a "Tsst" sound when striking; as long it comes from the abdomen it should still provide benefits.

I don’t believe letting out a long blood curdling battle cry is beneficial when striking. Your kihap needs to be very quick, one, maybe two syllables max. The reason I’m saying this is because if you have a long kihap when striking your jaw is in a vulnerable state, it’s open. A tight jaw will help guard against dislocation or outright breaks.

Another negative might be that you signal your intent if you kihap too soon in your strike against a trained opponent, or signal the end of your attack. For example, in class we kihap at the end of the pattern, which means the attack is over. A large component of fighting is information warfare, you're reading your opponent's weaknesses, trying to conceal your own, and conceal your intentions.

So there is something to this ancient activity. It doesn’t summon the power of the spirit world to let you shoot a fireball out of your hands, but it gives you more power, a better defense, and might scare your opponent off.

There are two kind of shouts, long and short ones.

Long shouts:

The shouts you're referring, Kiai, are a way to increase intensity, and give the practitioner an easy meaning to give a direction to his/her KI.

When speaking about Ki direction, do a favour to yourself, and refrain to think in esoteric terms.

Ki direction really means direction of everything: mind and body intentions. A shameless and (really) strong scream practically forces your mind and body to focus on one direction and one thing. It's physiological.

A cheap trick to know what we're talking about is to look at something in front of you and scream very strongly at it and then turning your head and screaming very strongly at something in another direction (say 90° left). Pay attention to actually look what the objects/persons you're screaming at.

When you do that strongly and shamelessly enough, you should feel subtle differences in your body position. That is: the scream effectively directed your body here or there.

Summing up: it's an easy (when done right) way to achieve something otherwise difficult.

Since it works when you're screaming hard, though, it is difficult to practice in many environments. If you're a novice and screaming less than your maximum then you're doing it wrong and it will serve no purpose.

Short shouts:

They're just a mean to contract the diaphragm, and coordinate breath and body actions to improve breath endurance and marginally augment the power of the strikes.

All the answers about aiding concentration and releasing your inner power are good, but mostly for striking moves.

On the Judo side of the house, the Kiai mostly manifests itself as a loud and determined grunt. That said, I have heard a few roars during some particularly impressive standing work ... generally where a smaller player gets a larger opponent airborne. A roar like that is often followed by standing applause from the spectators if the technique worked!

There are a few reasons that they claim is why they do this:

• They claim that it may scare off the attacker
• It is supposed to alert other people
• and lastly, it is supposed to put more power behind your technique

I can understand the mentality behind this, if your attacker doesn't want to attract attention, shouting will attract attention and may scare them away.

When it comes to the last one, I can understand that too. it does in fact help the power of the technique. Personally though, being that I do Krav Maga, and ICS, I prefer using a technique where you breathe out quickly with each punch making sort of a quick ssshhh sound. This particular technique is also used in MMA, Muay Thai, Boxing, and other similar fighting styles.

The shouting, called kiai, has multiple purported purposes. I would note that I'm posting these not out of a necessary belief in them, but out of explanation as they were taught to me:

1. The forced and trained rapid exhalation of breath. This can be used as both focus (by focusing on breath, one is less inclined to focus on the fear of failure when faced with the thought of breaking something apparently hard), and as a method of rapidly exhaling carbon-dioxide from the body to increase oxygenated blood flow to the extremities.
2. The external (outer or omote) harmonizing of ki energy (気合 - kiai), as opposed to the internal (inner or ura) harmonization of ki energy (合気 - aiki). [NB: This is not something I believe, so this is best left to someone else to explain familiar with the concepts.]
3. The expulsion of intent. Kiai acts as a declaration of your fighting spirit, your internal desire to prevail in those circumstances. This can be for intimidation, self-reassurance, rallying (the war cry was essentially a form of kiai), etc.

Something that many people may never learn: the kiai need not be loud. In some arts, the kiai is taught as a voiceless projection, called kage no kiai.

In addition, some arts offer specific forms of kiai, for example (from the Bujinkan; I have no record from which school, I simply have it in my notebook as kihon, but the proper term is more likely kotodama)

• ei! - A growling shout, meant to accompany an attack; intended to force the opponent to lower his guard momentarily.

• toh! - A heavy shout, meant to accompany a counter; intended to make the opponent believe he has left an opening.

• ya! - A boisterous shout, meant to respond to a series of blows; intended to dishearten the opponent, making him believe that you believe you're already victorious (may tie in to folk beliefs that a battle is decided the moment two adversaries first see each other).

It's said the kage no kiai taught in the Bujinkan is a harmony of these three shouts, voiceless or in a low hum, assuring the warrior of victory. This might be viewed as a form of self-assurance, helping to maintain a form of inner calm or mushin.

In addition to Rob Z's answer, kiai force you to breath. This is a good thing, especially when you are in a randori/competition situation where one tends to hold their breath.

A proper 'spirited yell', or 'power exhale' or what-have-you actually is meant to activate the very core muscles, the abdominal band, and the tantien, one of the three main chi centers and the place where the body stores chi). This allows you to release more energy and, well, be successful in your technique. When breaking boards, it's useful so you don't hurt yourself. In a competition, it's to impress the judges, so forget that. In a form, in practice, you are training yourself for specific ways to release this energy (kick, uppercut, elbow, etc).

Depends on the style, but for the purposes of most Japanese martial arts, the "shouts" are Kiai and serve a couple different purposes:

1. Contracts the diaphragm and chest which can allow you to take a hit better.
2. Puts extra "energy" behind the strike as it causes you to focus on the moment of impact.
3. Shows "spirit" when in competition.
4. Shows where strong strikes would be during kata.
5. Can catch opponents off guard if they are anticipating it or don't know what to expect.

Depending upon the situation they can also act as a battle cry in some ways but I haven't heard that explanation for them given at the dojo.