Action reverse in kicking
Physics says that every move in any direction must be matched by a movement in another direction. This is called momentum. The technical formula for this is m1 X v1 = m2 X v2. Imagine that your legs weighing 10 kilo and you want to achieve a speed of 5 meter / second with the foot, then this becomes m1 X v1 = 10 X 5 = 50.

Assume that your arm weighs 5 kilo. Then you must move your hand in 10 meter / second for the result to be the same as for the leg. m2 X v2 = 5 X 10 = 50. In short, it means that you can get higher speed of your kick if you move your hand quickly in the opposite direction. The calculation is not entirely simple because other factors come into play.

The calculation is only fully correct if you would have been in space with no gravity there. In reality, affecting, for example, friction between the foot on the ground and the ground – higher friction means that the kick can be faster. Other factors affecting the speed of the kick can be the wind speed and as previously mentioned gravity.

Action reverse in practice
Throw your arms and move the shoulders in the reverse direction to the kick, in order to get a more powerful kick. Remember that you drop your guard if you follow this example which means that you expose yourself to a greater risk of being hit by a possible counter attack.