I noticed in Muay Thai Teep kick, they always recommend to keep same side hand is down. You will notice one hand is up, and another hand is down in picture. Why do they recommend this? In other martial arts, Karate, Krav Maga, they recommend hand stays up. Is there a reason for this?
From what I can tell from the discussions between Muay Thai-ists(?) that I've seen/known, this is done for the same reasons as dropping the hand back is done in the roundhouse (again something which a majority of karate, kickboxing, etc schools seem to avoid). These reasons are an increase in torque/power, improved balance, and in some cases an element of "we've-always-done-it-ism."
There's a minimal argument for power generation here (in the same way the "hikite-for-power-generation" people in karate are demonstrably wrong), but the balance argument is a sound one in my opinion. Bearing in mind a single leg is about 1/5th of your total body weight, having it out in front of you will create a moment force at your hip of about 132N (average man) and shift your center of gravity forward away from your body. Having one arm further back (about 1/20th of your body weight) will mitigate this to a degree, especially in combination with a slight
From personal anecdotal experience as a karateka, I both kick and teach the roundhouse kick with the hand back, because from experience it allows people (me included) to get into a better kicking position and therefore (because they are better balanced and positioned) kick harder. I am sure there is a biomechanical reason for this but I don't know it. I cannot see there being a similar biomechanical advantage in the linear technique of the front kick/teep.
My personal opinion would be swing the hand back in the roundhouse kick because the benefits to the kick outweigh the risks, but not to do so in the front kick/teep because I don't see a benefit to this.
In summary, the most common reasons are:
- Increase power
- Improve balance
- "We've always done it"
The validity of these reasons may vary.
A small addition to exactly-right Rob's answer.
Such move is not always welcome - some people advising to avoid it for better defense. And for, say, middle-kicks they are right - it can be "eaten" to close the distance - and if your closer hand is down, you are in trouble.
But in this particular situation your head is just out of your opponent's reach - with inclining your torso back you making it even more out of reach.
So, in case of teeps, it definitely better to increase power - just because you're losing nothing increasing it.