Streamline and underwater swimming explained
Why underwater phases matter
After a start or turn, swimmers can travel faster underwater than at the surface for part of the transition. Streamline, body alignment and kick timing therefore play a major role in modern racing.
The 15m boundary
In freestyle, backstroke and butterfly, current World Aquatics rules require the swimmer to surface by 15 metres after the start and after each turn. That makes the underwater phase both a performance tool and a regulated part of the race.
Stroke differences
Butterfly and freestyle commonly use dolphin kick underwater, while backstroke uses its own legal underwater phase. Breaststroke has a different pullout sequence and should not be grouped under the same 15m explanation.
Safety boundary
Underwater performance should not be turned into prolonged breath-hold or hypoxic training advice. Competitive underwater phases are short, rule-bound and normally practised under supervised training conditions.
Internal links
Link to starts, turns, sprint swimming, butterfly, backstroke, breaststroke pullouts and race analysis. Rule limits remain refreshable.