The underwater phase has long been a discipline in its own right within swimming. After the start and turns, performance depends not only on the power of the dolphin kick, but above all on how movement is transmitted from the upper body through the hips, knees and feet.
A study published in 2026 examined this joint kinematics and so-called intersegmental coordination during underwater undulatory swimming.
One wave rather than isolated joints
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The interesting idea is that an efficient dolphin kick is not an isolated knee movement. It is a continuous wave in which several body segments have to be coordinated in time.
The better this coupling works, the more effectively movement can travel backwards through the body without creating unnecessary drag surfaces. Differences between performance levels therefore cannot be reduced simply to kick rate or one joint angle.
Why “more knee” is often the wrong instruction
In practice, underwater technique is often corrected through individual visible errors: too much knee bend, feet too low, hips too stiff. Such cues can be useful, but they are incomplete if the real problem lies in timing between segments.
Side-view video is therefore especially valuable. Coaches can observe where the wave begins, how it travels through the body and whether individual segments react too early or too late.
Underwater work becomes more measurable
The study does not provide a simple recipe for the perfect dolphin kick. Instead, it shows that movement quality should be viewed as a coordination problem.
For day-to-day coaching, this means measuring more than kick count and 15-metre time. The structure of the movement also matters. Two swimmers may achieve the same underwater time – but with completely different efficiency and therefore different reserves for improvement.
Do not look only at the 15-metre time
In addition to underwater time itself, it can be useful to examine speed immediately after surfacing. A spectacular dolphin kick has little value if the first surface-swimming cycle then collapses. Good underwater technique must generate speed while also allowing a clean transition.
SCIENTIFIC SOURCE
https://pubmed.ncbi.nlm.nih.gov/41297385/