Underwater Time in Swimming: Maximising Your Fifth Stroke

The fastest part of every lap happens beneath the surface

The underwater phase after each turn is the fastest a swimmer travels during a lap. After a powerful push-off, the swimmer moves through the water in a streamlined position with no wave drag, no breathing disruption, and momentum that can exceed race pace. This phase has been called the “fifth stroke” because it applies to all four competitive strokes and often determines the difference between good swimmers and great ones.

Underwater time — the duration a swimmer spends submerged after each push-off — is a critical metric that is directly linked to turn quality, breakout strategy, and lap time. Yet until recently, it was nearly impossible to measure outside of a video analysis lab. Swimtraxx One tracks underwater time automatically on every lap, making this advanced metric accessible to all swimmers.

Why the Underwater Phase Matters

Speed Advantage

Immediately after a push-off, a swimmer is moving faster than at any other point in the lap. A strong push-off can propel a swimmer at 2.5-3.0 metres per second, compared to surface swimming speeds of 1.5-2.0 m/s for most competitive swimmers. Every metre of distance covered underwater is a metre covered at a speed that cannot be replicated on the surface.

Reduced Drag

Surface swimming creates wave drag — the resistance caused by pushing water out of the way at the air-water interface. This accounts for a significant portion of total drag, especially at higher speeds. Swimming underwater eliminates wave drag entirely. Combined with a tight streamline position, the underwater phase is the most hydrodynamically efficient part of each lap.

No Breathing Cost

On the surface, each breath creates a momentary disruption in body position and streamlining. During the underwater phase, there is no breathing disruption. The swimmer travels in a perfectly aligned position, maximising the distance gained from the push-off before the first breath is needed.

Competitive Impact

Analysis of elite swimming races consistently shows that the top finishers spend more time underwater than their competitors. In events with many turns (200m and above in short-course pools), underwater distance can account for 20-30% of the total race distance. The swimmers who maximise this phase have a structural advantage that compounds with every wall.

The 15-Metre Rule

World Aquatics rules require that a swimmer’s head must break the surface before the 15-metre mark after every start and turn (Rule SW 6.4 for backstroke, similar for other strokes). This means the maximum underwater distance is 15 metres per lap. Elite swimmers routinely approach or reach this limit, particularly in backstroke and butterfly.

For most competitive swimmers, the challenge is not the 15-metre limit but simply going further than they currently do. Many club-level swimmers break out at 5-7 metres, leaving 8-10 metres of potential underwater distance untapped. Tracking underwater time quantifies this gap and provides a target for improvement.

Underwater Time Across Strokes

Stroke	Underwater Technique	Typical Elite Breakout	Key Consideration
Freestyle	Dolphin kick in streamline	8-12 metres	Transition from underwater kick to surface swimming must be smooth
Backstroke	Dolphin kick in streamline (supine)	10-15 metres	Often the longest underwater phases; backstroke specialists are underwater kick specialists
Breaststroke	Pulldown + dolphin kick + glide	8-12 metres	One full arm pull and one dolphin kick allowed under water (pulldown)
Butterfly	Dolphin kick in streamline	8-13 metres	Strong correlation between underwater distance and event performance

How Swimtraxx One Tracks Underwater Time

Swimtraxx One detects the transition between underwater and surface swimming through changes in the sensor data when the swimmer’s head breaks the surface. The device, positioned at the right temple on the goggle strap, experiences a distinct change in its environment (pressure, movement pattern, and orientation) when transitioning from underwater to surface swimming.

This allows the device to measure how long the swimmer remains submerged after each push-off. The data is recorded per lap and synced to the Swimtraxx Hub app, where swimmers and coaches can review underwater time alongside the other 8 pool parameters.

How to Improve Your Underwater Phase

Build a Stronger Streamline

The foundation of a good underwater phase is body position. A tight streamline — arms extended overhead with biceps squeezing the ears, hands stacked, core engaged, legs together — minimises cross-sectional area and maximises the distance gained from each kick. Many swimmers lose significant speed by having a loose streamline with arms apart, head tilted, or core relaxed.

Practise streamline holds on the wall, streamline push-offs with no kick (measuring glide distance), and underwater mirror work to build awareness of your body position.

Develop Your Underwater Dolphin Kick

The dolphin kick is the propulsive mechanism of the underwater phase (except breaststroke). A powerful, efficient dolphin kick maintains speed after the push-off and can even accelerate the swimmer underwater. Key elements include:

• Kick from the core: The wave motion should originate from the chest and hips, not just the knees
• Moderate amplitude: Big kicks create more drag than propulsion; small, fast kicks maintain streamline
• Consistent rhythm: An even kick tempo maintains speed better than sporadic bursts
• Equal up-kick and down-kick: Both phases should generate propulsion; most swimmers only power the down-kick

Practise Breakout Distance

Set targets for your breakout point. If you currently break out at 7 metres, aim for 8. Then 9. Use the lane line markings as reference points. Over time, extend your comfortable breakout distance incrementally. Swimtraxx One’s underwater time data will confirm whether you are achieving your targets.

Train Underwater Endurance

The limiting factor for most swimmers is not kick strength but breath-holding ability and lactate tolerance during the underwater phase. Include underwater kick sets in training: 8 x 25 underwater dolphin kick with generous rest builds both the kick itself and the comfort of spending extended time submerged.

Optimise the Breakout Transition

Breaking out of the water is a transition that, done poorly, wastes the speed built up underwater. The first stroke should begin before the swimmer fully surfaces, maintaining momentum from the underwater phase into surface swimming. Practise the timing of the first arm pull relative to the head breaking the surface.

Underwater Time and Multi-Parameter Analysis

Underwater time is most powerful when combined with the other metrics Swimtraxx One captures:

• Underwater time + turn time: Together, these metrics define the complete wall phase. A swimmer with a fast turn time and long underwater time has mastered both the rotation and the push-off/streamline phases. A fast turn with short underwater time suggests the swimmer is rushing back to the surface.
• Underwater time + lap time: Does longer underwater time actually produce faster laps? For most swimmers, the answer is yes up to a point, then diminishing returns set in as the swimmer surfaces with too little oxygen. The data reveals each swimmer’s optimal breakout distance.
• Underwater time + heart rate: Extended underwater phases temporarily reduce heart rate due to the mammalian dive reflex. Tracking this pattern helps swimmers understand their body’s response to submersion and can inform race pacing strategies.
• Underwater time + stroke count: Longer underwater time means less distance to swim on the surface, which should reduce stroke count. If underwater time increases but stroke count stays the same, the swimmer may be losing efficiency on the surface portion of the lap.

Why Most Devices Cannot Track Underwater Time

Underwater time requires the sensor to detect the precise moment the swimmer submerges and resurfaces. Wrist-based sensors can detect the underwater phase to some degree, but the wrist enters and exits the water with every stroke in freestyle and backstroke, creating ambiguity about when the swimmer is truly submerged versus simply completing a stroke cycle.

A sensor at the temple, on the goggle strap, does not have this problem. The head is submerged during the underwater phase and surfaced during swimming (except during the brief moment of a flip turn). This clear binary state — head under or head above — makes it possible to accurately measure underwater time from the goggle strap position.

Among consumer swim trackers, underwater time is a rare metric. Garmin Swim 2 and Apple Watch offer limited or no underwater time tracking. FORM Smart Swim Goggles do not isolate this metric. Swimtraxx One provides per-lap underwater time as one of its 9 standard pool parameters.

Frequently Asked Questions

Does underwater time include the start as well as turns?

Yes. The device measures the underwater phase after both the initial start/push-off and every subsequent turn. The first lap’s underwater time captures the dive entry and initial underwater phase, while subsequent laps capture the post-turn underwater phase.

What is a good underwater time?

This depends on pool length, stroke, and swimmer level. As a general reference, elite swimmers in a 25-metre pool might spend 3-6 seconds underwater after each turn, covering 8-15 metres. The key is not the absolute number but the trend: is your underwater time increasing as you work on your breakout skills?

Can spending too long underwater hurt performance?

Yes. If a swimmer stays underwater past the point where their kick speed drops below their surface swimming speed, they are losing time. Additionally, excessive underwater distance creates oxygen debt that can compromise the surface swimming that follows. The optimal breakout point is where underwater speed equals or drops just below surface swimming speed. Tracking both underwater time and lap time helps find this point.