The underwater dolphin kick is the part of swimming that almost nobody practises and almost everybody loses time on. Swim a 200 individual medley in a 25m pool and you push off eight walls. Give away half a second on each and four seconds are gone before anyone looks at your stroke. I raced the 200 individual medley at two Olympic Games, so the three seconds after the feet leave the wall have taken up much of my life.
Most adult swimmers treat the underwater as a pause: push, glide, surface, swim. It is the phase where the physics are most in your favour, and the cheapest time in the pool to find.
Why is the underwater dolphin kick faster than swimming on the surface?
Because under the water you stop making waves, and making waves is expensive. A review of the kinematics of the underwater undulatory swimming cycle puts it plainly: during that phase "swimmers achieve higher velocities than during surface swimming due to lower drag resistance".
The advantage is big enough that the sport capped it. The same review notes: "For this reason, the Federation Internationale de Natation (FINA) rules limit the duration of the underwater sections to the 15 m mark from the starting or turning wall." A rule exists because the thing it limits works.
How deep should I be when I dolphin kick?
Deep enough to escape the surface, and no deeper. A three-dimensional CFD analysis of the effect of depth on drag during the streamlined glide reports that "Hydrodynamic drag decreased with depth, although after 0.75m values remained almost constant", and the authors draw the practical line themselves: "performing the underwater gliding (and the underwater dolphin kicking) more than 0.75 m depth will not be to the benefit of the swimmer." The same paper cites earlier work finding "there is no significant wave drag when an adult swimmer is gliding at least 0.6 m underwater."
So there is a band where the surface stops charging you rent, and below it you gain nothing while paying for a longer trip up. That is a model of a glide rather than a kicking human, so treat the depth as a ballpark. It still kills the two habits I see most: surfacing almost immediately, and diving so deep that the second half is spent climbing.
Should the dolphin kick come from my knees or my hips?
From neither on its own. The kick starts in your chest and arrives at your feet, and the review above describes it exactly: "The body wave movement travels caudally from the torso to the toes, increasing in amplitude at each body segment in a whip-like action." Caudally means head to tail. Your feet are the end of the whip, not the handle.
A study of trunk and foot kinematics in underwater undulatory swimming found where the difference lives: "The range of motion of the lower waist was greater for the faster swimmers than for the slower swimmers", and "Both the maximum flexion and extension angular velocities of the lower waist and maximum extension angular velocity of the chest were greater for faster swimmers than for slower swimmers." Thirteen swimmers is a small group, but it matches what I see from the deck. A knees-only dolphin kick is loud and moves almost nothing. That wave is the one that drives butterfly, which is why fly swimmers arrive with a usable underwater already built.
Should I kick faster or bigger to go faster underwater?
Faster and smaller, not bigger. When eight national-level men were measured across increasing speeds in a study of kinematics and muscle activity during underwater undulatory swimming, "kick frequency increased with increasing swimming velocity", while "kick length decreased with increasing swimming velocity" and "Kick amplitude was lower in the 90%V trial than in the 70 and 80%V trials". What rose instead was speed at the end of the whip: "Mean downward toe velocity, mean upward toe velocity, and peak upward toe velocity increased with increasing swimming velocity."
There is a catch. In the comparison between faster and slower swimmers, "no group difference was found for kick frequency", while "The toe vertical velocity during upward and downward kicks and horizontal displacement per kick were greater for the faster swimmers than for the slower swimmers." Read them together: tempo is how you change gear within your own kick, not what separates a good underwater from a poor one. That comes from foot speed and distance per kick, and both come from the trunk.
Do I need flexible ankles to dolphin kick well?
You need ankles that are not stiff, and past that point more flexibility stops paying. Ten swimmers were tested with their ankles taped, stretched and left alone in a study of ankle joint flexibility and undulatory underwater swimming speed. With plantar flexion restricted, "Swimming velocity and kick efficiency significantly decreased during swimming with restricted PF". With it increased by stretching, "Swimming velocity and kick efficiency did not differ between normal and increased PF."
The authors' conclusion is the useful part: "Particularly swimmers with low or average maximum PF angles may benefit from a long-term ankle joint flexibility program to improve their UUS performance." Ten swimmers is a hint rather than a law, but it draws a sensible line. If you cannot point your toes much past straight, ankle work is worth your time, and the mobility routine in my post on the freestyle kick applies here. If your ankles already move normally, spend the time on the trunk instead.
How far should I go underwater off each wall?
As far as you can hold a real kick, and not one metre further. The 15m limit is what the rules allow, not a target every swimmer should hit. An underwater that dies at 10m and leaves you gasping is slower than a clean 7m and a strong break into stroke. No study can tell you where your line sits, because it moves with your fitness.
Test it: swim 6 x 25m from a push, surfacing at a different marked point each time, and take your time and stroke count for each. The distance that gives the best time without wrecking the first few strokes is your distance this month. The swim pace calculator gives you target times, and my guide to swimming without getting tired covers pacing the session around them.
An underwater dolphin kick set that actually transfers:
- 8 x 15m underwater kick from a push, on your back with arms at your sides, so you can feel whether the wave starts at your chest
- 6 x 25m dolphin kick on your side, three kicks each side, which exposes a knees-only kick immediately
- 6 x 20 seconds vertical dolphin kick in deep water, hands out of the water, the most honest test of your upkick there is
- 8 x 25m as underwater kick to a fixed mark then full stroke, holding the same mark every repeat
- 4 x 50m freestyle with one rule: every wall gets the same number of kicks, counted
- Fins once a week at most, since they make the wave easy and let your trunk stop working
Outside the current supported analysis: underwater dolphin kicks. SwimRead is a swimming video review app, currently in beta. It supports short, side-view freestyle recordings, shows which automatic measurements are available, and keeps a history and chosen training focus for each swimmer. It does not grade technique or establish that a specific error was detected. Video stays on the phone; technical measurements and body-landmark coordinates are sent as described in the Privacy Policy. Join the beta waitlist for an invitation as places open.