Video Analysis of Underwater Kick
By Genadijus Sokolovas, Ph.D., Global Sport Technology Inc, FINA Expert
Underwater kicking is the fastest swimming stroke. Elite level male sprinters can swim the first 15 m of the race in about 5 sec. That’s almost 3 m/sec speed! No swimmer on the surface can swim so fast. There are at least three reasons why swimmers can reach higher speed underwater:
- When kicking underwater, swimmers are minimizing frontal drag by keeping arms in streamline position.
- By engaging strong body muscles, swimmers can kick underwater strongly in both directions – up-kick and down-kick.
- Our Swim Power studies show that up-kick is much stronger underwater than on the surface.
Video analysis of elite level swimmers may reveal the reasons behind fast underwater kick. Twelve years ago, we started to use video analysis with our unique Swim Power technology to understand how elite level swimmers generate speed after dives and turns. Currently, our Swim Power tests are available to all swimmers around the world. Swimmers from 125 countries, including 65 Olympic Champions and 90+ medalists, have been tested in the last 12 years. Thanks to FINA, these tests are offered at international events, such as FINA World Championships, FINA Development Programme, and FINA Swimming Coaches Clinics.
Keeping arms fully extended in streamline position is very important to reduce frontal drag. Swimmers should squeeze the head and keep arms strong and stable. Water is 784 times more dense than air. That’s why even small motion with arms creates significant drag, which reduces swimming speed underwater. The fastest swimmers are keeping arms very stable and strong.
Phases of Underwater Kicking
Video analysis with Swim Power tests shows that underwater dolphin kick includes three phases: 1) bending the knees; 2) down-kick; and 3) up-kick.
1. Bending the Knees
Based on Swim Power tests, even elite level swimmers may lose 40% and more speed when bending knees underwater. There is strong correlation between bending knees and loss of swimming speed: the more swimmers bend knees, the more speed they lose. When swimmers bend knees too much, knees below the body and shins above the body create additional drag (see picture 1). To minimize drag at this phase, swimmers should bend knees less and kick more from the body.
Picture 1. Bending knees during the underwater kick
It is interesting to compare swimmers underwater kick with fish or dolphins. Fish, dolphins or other fast creatures don’t have big joints, such as knees. Instead, they have many small joints in their tails, which creates less frontal drag. As result, they are not losing swimming speed as much as swimmers. Humans should learn from fish and dolphins how to use more body and less knees to kick underwater.
Down-kick is the most powerful phase of the underwater kick. Elite level swimmers increase their speed up to 70% during this phase. The fastest rate of speed increase is the beginning of the down-kick motion, when swimmers push more water back. At the end of the down-kick, swimming speed starts to drop down (see picture 2).
Picture 2. Speed drops at the end of down-kick.
It happens for two reasons: at the end of down-kick swimmers move water more down than back, also feet/toes below the body drag larger amount of water. To minimize drag at the end of down-kick, swimmers should avoid kicking too deep. Kicking at small amplitude would create less drag and allow maintaining higher speed.
Video analysis with Swim Power tests shows that the up-kick can be as powerful as the down-kick. In fact, some really fast underwater swimmers can kick up stronger than down. Normally, it is related how deep the down-kick is: the shallower down-kick, the higher speed swimmers reach on the up-kick. Up-kick is initiated from small hips motion up. If it is done correctly, lower back, gluteus, hamstrings and other muscles may generate a very strong up-kick (see picture 3).
Picture 3. High speed at the end of the up-kick.
Drills To Improve Underwater Kick
We are creating hundreds of swimming drills to improve swimming technique, including underwater kick. Below are a few examples of drills that can be used to develop fast dolphin kicks.
- Keeping soft and moving arms is one of the typical mistakes that prevents swimmers from kicking fast underwater. Stable and strong arms position can be developed by kicking in the wall with fully extended arms. Kicking in the wall on the side gives more room to kick strongly in both directions – down-kick and up-kick.
- Bending knees too much reduces swimming speed significantly. To learn how to kick more from the body and less from knees, swimmers may kick with pull-buoy between ankles. In this case, it will be almost impossible to bend knees too much. As result, swimmers will learn how to kick correctly.
- Many swimmers don’t have a strong up-kick. That may be changed when kicking more with shoes. Shoes or other resistance will force swimmers to develop stronger muscles that will eventually improve the up-kick. This drill has to be done carefully swimming short distance. Longer distance and duration of this drill may lead to small muscle injuries. Once swimmers will be stronger, they may increase distance and duration of this drill.