One of the biggest mistakes I made over the years was treating the dolphin kick purely like a leg extension exercise.
I obsessed over the downkick and how hard I could drive it with my quads and knees.
Naturally, this meant endless reps on the leg extension machine at the gym. Leg up, leg down. Move that weight stack.
But not a lot of great dolphin kicking broke out as a result of this training. Turns out, this was only solving part of the challenge.
When swimmers start pushing their underwater dolphin kick towards max effort, there is an underrated muscle that takes lane four, and it isn’t the quads or calves.
It’s the glutes.
The Faster You Kick, the Harder the Glutes Work
The “fifth stroke” is the fastest and coolest-looking way to move across the pool. And we work hard at improving the dolphin kick because we know that with a splash of mastery we can shave some time off our PBs on race day.
But moving at top speed requires the body to organize itself differently compared to when we are kicking at lower speeds.
For example, a study (Yamakawa et al. 2022) looked at exactly how the hip and leg muscles respond to increased effort. Swimmers did UDK trials at 70%, 80% and 90% in a flume—perfect for not muddying the data with push-off or dive entry velocity.
Researchers measured EMG in the lower body muscles, and predictably, as speed went up, so did muscle activation across the quads, calves, and so on.
But the glute max stole the show:
| Muscle | Activation Increase from 70% > 90% |
| Gluteus maximus | +124% |
| Rectus femoris (quad) | +51% |
| Biceps femoris (hamstrings) | +39% |
| Gastrocnemius (calf) | +37% |
| Tibialis anterior | +34% |
| Gluteus medius | +29% |
| Vastus lateralis | +19% |
| Adductor longus | +8% |
While total lower body muscle activity increased by around one-third, the glute max more than doubled.
Why the Glutes Matter for Dolphin Kick Speed
So why did those big sacks of meat at the back of your Speedos see so much more activity at faster kick speeds?
Because it’s the primary muscle responsible for:
- Hip extension
- Hip external rotation
Hip extension is the obvious one—it powers the big, nasty downward snap of the body wave that drives a fast dolphin kick.
Hip external rotation is a little less obvious, but just as important.
By externally rotating the hips—like turning a doorknob with your leg attached to it—swimmers better position the feet to displace more water with each kick. They can cover more distance with each kick. And even help carve out stronger vortices for increased propulsion.
So it should be no big surprise that external rotation velocity also jumps significantly at faster speeds (Matsuda et al., 2021).
See also: Why Your Dolphin Kick Needs More Hips (and Less Knees)
In other words, the faster you dolphin kick, the faster your hips need to rotate—and the harder your glutes are going to have to work to make it happen.
Weak or fatigued glutes mean that we lose out on the benefits of increased hip extension and rotation velocity, putting a ceiling on how fast you can kick.
(For a more nerdy breakdown on how hip rotation works in the dolphin kick cycle, reference this article.)
The Next Step
The underwater dolphin kick can be maddening as there is a lot going on. Undulation, phase timing, body position, streamline, breath holding, ankle flexibility, and so on.
But the glutes are one of the easier pieces of the puzzle to improve.
If you’ve been focused only on your quads and glutes, peel your butt off the leg extension machine and get after some glute training.
Happy kicking!
ABOUT THE AUTHOR
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
