Nothing better than going fast in the water, whether that means getting to the wall first, going a personal best time, or simply leveling up technique in the water.
And while most of our attention is what happens at the surface with our arms, our legs are super busy too, from creating propulsion when full-stroke swimming to getting us in and out of turns, starts, and transitions.
And the two most common kinds of kick during these phases?
The underwater dolphin kick and underwater flutter kick.
They are not created equal when it comes to speed, power, and technical demands. In this little guide, we are going to look at the key differences between the UKD and UFD with the help of a study by Yamakawa et al. (2025), who analyzed both kicks with a group of eight elite male swimmers.
Let’s get to kickin’!
Speed
The underwater dolphin kick is essential for fast swimming and elite swimmers have taken this skill to epic levels. Watching Caeleb Dressel or Gretchen Walsh dolphin kick powerfully and smoothly inspires awe.
So no big surprise, Yamakawa showed that average underwater dolphin kick speed (1.41m/s) was significantly faster than the freestyle kicking (1.21m/s).
The study used hydrodynamic simulations with a model called Swumsuit, demonstrating that swimmers produced higher mean and peak propulsive forces with the dolphin kick.
Range of Motion
One of the striking differences between the two kicks is how the range of motion differs.
Here is how joint movement differed according to the study:
| Joint | UDK ROM | UFK ROM |
| Hip Flexion/Extension | 32.3° | 15.5° |
| Hip Internal/External Rotation | 34.2° | 18.7° |
| Knee Flexion/Extension | 71.5° | 57.1° |
| Ankle External/Internal Rotation | 24.0° | 18.4° |
The increased range of motion in the dolphin kick contributes to undulation and a strong “whip” at the end of the downkick, leading to more propulsion. From the hips to the knees to the feet, there is way more movement happening.
The freestyle kick is a tighter movement, with less trunk movement and way less hip movement.
Hip Roll vs Undulation
The underwater dolphin kick has a lot of hip extension/flexion, but almost no hip roll (just 3.6° on average). Makes sense—hard to roll the hips when both legs are moving up and down at the same time!
The freestyle kick, on the other hand, relies heavily on hip roll (19.2°) which plays a big role in generating force and propulsion. This rolling motion helps swimmers coordinate the alternating kicks and can also help us to control total body roll.
Keep in mind, this study measured hip roll while swimmers were kicking in a streamline underwater. There was no arm motion involved. In full-stroke freestyle, hip roll increases significantly. At sprint speeds, it averages around 40°, and at 400m pace, it climbs to nearly 60° (Andersen et al., 2020).
See also: How to Improve Hip Rotation in Freestyle (and Why It Matters)
Whether working to improve the UDK or the UFK, swimmers should train core stability (Bird dogs, planks, Dead Bugs, etcetera) for increased body control when rolling or undulating.
Muscle Activation
The underwater dolphin kick fires much more muscle than the UFK, especially in the hip flexors, gluteus maximus, tibialis anterior (shin), and gastrocnemius (calves). Increased range of motion and higher power = more muscle activation.
The gluteus medias lit up during the downkick of the freestyle kick. The job? To brake and stabilize, and not necessarily propel.
This highlights another key difference between the two kicks. The dolphin kick is more forceful and dynamic, while the freestyle kick is more controlled and stabilization centric.
Getting the most from both kicks
Whether you are trying to fine tune your breakout (dolphin kick right to the surface) or improve kicking technique or simply want to add more boom to your kicks, here are some ideas for tailoring your training approach.
For the underwater dolphin kick:
- Emphasize power and strength. Resisted kicking, vertical kicking, and plyometrics to boost lower-body power.
- Increase hip and ankle mobility. Add dynamic ankle and calf stretches and mobility flows to unlock stiff ankles and “catch” more water with your feet.
- Work that undulation. Undulation is the foundation of a thundering dolphin kick, so work on mastering the timing and coordination of a smooth undulation.
For the freestyle kick:
- Train hip roll mechanics. Work that hip roll control with drills like side kicking, three strokes-six kicks drill. When swimmers get tired, the legs “die” and hip roll increases to compensate for loss of speed (Psycharakis and Sanders, 2010).
- Anti-rotation core exercises. Strengthen your obliques and hip stabilizers with a mix of rotational and anti-rotation exercises to improve control during kicking.
- Use coordination drills. Overkick drill and blasters (kick fast in a streamline for 10m and then transition to full-stroke sprinting) help connect and coordinate the freestyle kick with the arm stroke.
The Bottom Line
The underwater dolphin kick is the go-to for faster propulsion in the water. But with great power comes greater demands of mobility and strength.
The freestyle kick, while less speedy, is still super important as it powers full-stroke freestyle—especially when sprinting per Strzala et al. (2021)—and is used for a smooth transition from dolphin kicks to surface swimming.
Both kicks are essential, and understanding the differences between the two will help you make smarty-pants decisions for more targeted training in the pool and dryland.
Happy kicking!
Level Up Your Underwater Dolphin Kick
Olivier Poirier-Leroy is a former national-level swimmer, 2x Olympic Trials qualifier, and author of several books for swimmers, including The Dolphin Kick Manual: The Swimmer’s Ultimate Guide to a Fast Underwater Dolphin Kick.
The book is a beastly 240+ pages of actionable insights and research into elite dolphin kicking technique and performance. It details everything from mastering undulation to vortex recapturing to structuring a dryland program for dolphin kicking success.
The Dolphin Kick Manual combines evidence-based insights with a collection of 20 ready-to-go sets and a 6-week Action Plan to help swimmers set a course for dolphin kicking success.
Train smarter and kick faster.
👉👉👉 Learn more about The Dolphin Kick Manual
