The Watt Beneath the Water

Freestyle biomechanics made simple enough to coach and precise enough to measure

Underwater stroke power is the rate at which your hand does work on the water, combining how hard you push and how fast your hand moves. ¹ In front crawl performance, that “work rate” is shaped by how your torso rolls, how your shoulder pulls, and how your elbow finishes the stroke. ¹

Most swimmers grow up hearing cues like “catch more water” and “finish the stroke.” Useful words, but fuzzy. This paper by Kudo and colleagues sharpens the picture by treating the underwater stroke like a power story, not just a shape story. ¹ The spark for awareness often comes on a day when you feel fit, yet the clock refuses to move. That is the moment you start wondering whether speed is missing strength, missing timing, or missing a better way to measure what your hands are really doing underwater.

The study followed trained male swimmers during maximal front crawl and computed upper body motion and the hydrodynamic forces acting on the hands. Then it calculated power during the underwater stroke as the dot product of hand force and hand velocity induced by segmental rotations of the thorax, upper arm, and forearm. ¹ It is a clean idea with big coaching value. If speed is the output, stroke power is one way to look at the engine.

What makes this performance lens so gripping is how concrete it becomes. The authors found two power peaks, one in the pull phase and one in the push phase. Average underwater stroke power was strongly associated with swimming speed. ¹ That does not mean power is the only thing that matters in swimming. It means power is measurable and closely linked to outcome, which is exactly the kind of signal coaches love.

The Curiosity Dive

Four research backed payoffs and the places the science asks you to stay humble

If your intention to explore this paper is practical, you are in luck. It does not just say “rotate more” or “pull harder.” It breaks down where the power comes from, when it shows up, and which joint actions matter most in each stroke phase. ¹ Below are four science backed benefits you can take from this work, supported and sometimes challenged by other peer reviewed research.

Power becomes a coaching language

The first benefit is clarity. The paper introduces power generated by underwater strokes as a key factor for assessing front crawl performance within stroke phases and shows a strong relationship between average power and speed. ¹ That aligns with research showing that hand speed and propulsive force relate to swimming velocity during maximal front crawl. ² Put simply, “freestyle stroke power” is not a vibe. It is a measurable performance signal that can anchor feedback when technique talk gets too abstract.

Stroke phase analysis gets sharper

The second benefit is phase honesty. This study reports two power peaks across pull and push and identifies different segmental rotation contributions in those windows. ¹ That fits well with research showing that different phases of the front crawl stroke cycle relate differently to whole body speed and propulsiveness, with the push phase often propulsive and the pull phase often slower. ³ If you coach “underwater pull and push” as one blended block, you may miss what needs fixing. If you coach them as distinct windows, you can aim your cues with less noise.

Segmental rotation timing becomes trainable

The third benefit is that segmental rotation stops being a vague instruction and becomes a timing problem you can actually work on. In this paper, shoulder adduction and thoracic roll are major contributors to underwater stroke power, with elbow extension also important in the push phase. ¹ Prior work has shown upper trunk rotation contributes meaningfully to hand velocity in front crawl and is associated with stroke frequency. ⁵ Other research has measured how torso twist and roll velocities change with sprint pace and argues that training specificity improves when dry land work challenges the torso to reproduce race relevant rotational demands. ⁷ This is a big deal for “segmental rotation timing.” It gives you a reason to film, mark, compare, and iterate instead of guessing.

Power needs guardrails

The fourth benefit is a protective one. This paper makes it tempting to chase power harder every session. But other studies warn that more propulsion is not always the direct path to the fastest swim. For young sprint swimmers, swimming velocity can reflect an interaction of anthropometrics, kinematics, and kinetics, and the highest propulsion was not necessarily responsible for the fastest velocity. ⁴ Even within hand based measures, faster swimmers do not necessarily show a higher propulsion ratio, which hints that efficiency and direction matter, not just force magnitude. ² Think of it this way. Underwater stroke power is a spotlight, not a crown. It helps you see what is happening, but it does not replace the full front crawl picture that includes drag, rhythm, breathing, and fatigue.

One more science note that supports the paper’s focus on kinematics is hydrodynamics. Unsteady flow around the hand can increase hydrodynamic forces compared with steady assumptions, showing why acceleration and direction changes can matter. ⁶ That is part of why modern front crawl performance analysis keeps returning to the same theme. Your shape matters, and your motion through the water matters just as much.

The Personal Pattern Studio

Turning research into your cues without turning training into a science fair

The motivation for personalization is simple. Two swimmers can look similar from the deck and still produce very different underwater stroke power because their timing and joint contributions differ. ¹ The paper even shows that different segment rotations contribute differently across pull and push, which is a strong reminder that one cue rarely fits everyone. ¹

Here is the practical move. Keep the paper’s core concepts, then personalize the feedback loop. Use video and notes to locate your own stroke phase analysis, then test one change at a time. If you want structure, compare strokes side by side so your eyes stop lying to you. If you want an always available thinking partner, build a personal coaching loop that keeps your cues consistent across weeks.

Below are conversational AI prompts designed to increase engagement while staying faithful to what the research can support. They are meant to help you translate freestyle biomechanics into one clean action, one clean feeling, and one clean review.

Act like my swim biomechanics coach. Based on the idea that underwater stroke power comes from hand force and hand speed driven by torso roll, shoulder motion, and elbow action, ask me a few questions about my pull and push phases. Then give me one cue for the pull and one cue for the push that I can test in a short set. ¹

Help me run a simple stroke phase analysis after practice. I will paste my notes and what felt easy or hard. Turn it into a short summary with one likely cause linked to segmental rotation timing and one experiment for next session. Keep it high school simple and avoid guesses that cannot be supported. ¹

I want to focus on thoracic roll timing without over rotating. Give me a dry land warm up idea that matches the idea of training specificity for torso rotation demands, then give me one pool cue that keeps the same rhythm in the water. ⁷

Help me avoid chasing power blindly. Using research that suggests the highest propulsion is not always the fastest swim, ask me what event I race, what my stroke frequency and stroke length feel like, and what breaks first under fatigue. Then suggest one way to keep underwater stroke power high while protecting efficiency. ⁴

As you use prompts like these, you will naturally collect semantic keyword variations that also help your thinking stay organized. You are not just “fixing freestyle.” You are working on underwater stroke power, propulsive hand force, thoracic roll timing, segmental rotation timing, and front crawl performance in a way that is specific enough to repeat.

The Living Loop

When training meets lifestyle through art, science, technology, and design

The customizable purpose of this paper is not to turn every swimmer into a lab project. It is to give your training a design spine. Measure what matters, test one change, and keep the loop sustainable. That is where art, science, technology, and design can sit in the same room.

Here are lifestyle takeaways that keep the work grounded and repeatable without burning you out.

1. Build an artist’s rehearsal before you swim. Pick one technical image for the day such as a clean shoulder adduction path or a calm thoracic roll rhythm. Visualize it once, then swim it. This keeps cues simple and reduces mental clutter. ¹
2. Use technology like a mirror, not a judge. Film a short set, then review only one thing tied to stroke phase analysis. In the pull, ask whether your motion supports hand speed without rushing. In the push, ask whether elbow extension helps finish the stroke without collapsing your line. ¹
3. Design your week like a product sprint. One theme for the week, one cue for the day, one short review after practice. That rhythm makes it easier to build consistency and makes personalization feel light instead of overwhelming.

Done well, this becomes a sustainable loop. Technique becomes a creative craft. Data becomes a friendly guide. And performance becomes the byproduct of better questions, not just harder effort.

FAQ

Is underwater stroke power the same as swimming power in a gym test

Not exactly. This paper focuses on power generated by the underwater strokes during actual front crawl, computed from hand force and hand velocity induced by segment rotations in the water. ¹ Gym tests may relate, but they do not measure the same underwater mechanics.

Does this paper prove that changing thoracic roll will make everyone faster

No. It identifies which segmental rotations contribute to underwater stroke power in the tested swimmers and shows that average power was strongly associated with speed. ¹ It does not test a training program that changes thoracic roll and then measures long term improvement.

What is a safe way to apply these ideas if my shoulder gets cranky

Use the paper as a cue filter rather than a push to do more volume. Focus on clean timing and controlled range, film short efforts, and build specificity gradually. Research on torso and shoulder roll highlights that movement patterns and demands change with speed, which supports a careful progression mindset. ⁷

References

¹ Kudo S, Yanai T, Matsuda Y. Power generated by underwater strokes in front crawl swimming and its kinematic contributing factors. Journal of Biomechanics. Two thousand twenty five.

² Koga D, Tsunokawa T, and colleagues. Relationship between hand kinematics, hand hydrodynamic pressure distribution, and hand propulsive force in sprint front crawl swimming. Frontiers in Sports and Active Living. Two thousand twenty two.

³ Psycharakis S G, Coleman S G S. Which phases of the stroke cycle are propulsive in front crawl swimming. Research Quarterly for Exercise and Sport. Two thousand twenty four.

⁴ Morais J E and colleagues. Understanding the role of propulsion in the prediction of front crawl swimming velocity and in the relationship between stroke frequency and stroke length. Frontiers in Physiology. Two thousand twenty two.

⁵ Kudo S and colleagues. Forwards backwards hand velocity induced by the upper trunk rotation in front crawl strokes and its association with stroke frequency. Journal of Sports Sciences. Two thousand twenty one.

⁶ Takagi H and colleagues. Unsteady hydrodynamic forces acting on a robotic hand and its flow field. Journal of Biomechanics. Two thousand thirteen.

⁷ Andersen J T, Sinclair P J, McCabe C B, Sanders R H. Kinematic differences in shoulder roll and hip roll at different front crawl speeds in national level swimmers. Journal of Strength and Conditioning Research. Two thousand twenty.