Introduction

1) decrease resistance;
2) increase propulsion;
3) achieve a combination of the previous two.

Recent improvements made in swimming techniques have above all been in the decrease in resistance and the physical preparation of the athlete.
There can be three types of resistance in water:
frontal, which opposes moving forward and is caused by the water that comes into contact with the front part of the swimmer;
surface friction, which is caused by the resistance of water adhering to the body;
whirlpool resistance, which is caused by the water that is not able to slide behind the poorly hydrodynamic areas of the body.

The position of the body must therefore be the most hydrodynamic as possible, in order to reduce all resistance and in particular frontal and whirlpool resistance.

In swimming, as in other sports, it is important to keep in mind Newton’s third law of dynamics, according to which an equal and contrary action corresponds to every action. This law takes on particular importance if applied to the study of propulsion, which, in water, is given almost exclusively by the arms, above all at high speeds.
In any case, leg kicks can contribute to: increasing propulsion, decreasing resistance or facilitating both.
Always according to this theory, if we try to swim with our head under water, the posterior part of our body will be more submerged as a result. This penalizes the swimmer, consequently also the fact that part of the propulsive energy expressed with the arms will be used to keep the head high. Instead, we must only exploit the speed of the body to emerge on the water line, because the more we are submerged in water, the more resistance we must conquer in order to move quickly.