Science & Biomechanics
Faster Underwaters: Optimize Depth, Glide Timing, and Dolphin-Kick Mechanics
Effective underwater swimming requires more than forceful kicking: depth, kick initiation, and movement frequency all influence performance. Use the research to guide individual testing rather than impose one universal underwater technique (Tor et al., 2015; Lyttle et al., 2000; Ruiz-Navarro et al., 2022).
David Hill, M.S., CPT, CES
Founder of Elite Swim Training, Author, and Swim Coach
November 11, 2026 · 6 min read
1. Depth First: Reduce the Resistance You Must Overcome
Before asking a swimmer to kick harder, examine where the swimmer travels. At the water surface, passive wave drag accounted for 50% to 60% of total passive drag at 1.7 m·s⁻¹. In the same investigation, wave drag fell below 5% of total drag when towing depth exceeded 0.5 m at 1.0 m·s⁻¹ and 0.7 m at 2.0 m·s⁻¹. The relevant depth therefore changed with speed; one fixed depth did not describe every condition (Vennell et al., 2006).
A separate passive towing study compared surface gliding with depths of 0.5 m and 1.0 m across four speeds. Mean total drag was 19.7% lower at 0.5 m and 23.8% lower at 1.0 m than at the surface. At the fastest tested speed, 2.5 m·s⁻¹, the corresponding reductions were 19.8% and 23.8%. These findings support submergence as a drag-management tool, but they describe passive streamlining rather than active dolphin kicking (Tor et al., 2015).
Body dimensions also matter when interpreting depth. Measurable wave drag was eliminated beyond 1.8 chest depths at 0.9 m·s⁻¹ and beyond 2.8 chest depths at 2.0 m·s⁻¹. For a swimmer with a chest thickness of 0.25 m, the latter threshold corresponds to approximately 0.70 m. Treat these findings as reference points for testing, not proof that every athlete should follow an identical underwater path (Vennell et al., 2006).
2. Time the First Kick Instead of Rushing It
The first dolphin kick should be considered separately from the kicks that sustain underwater speed. In a tethered assessment, 16 experienced male competitive swimmers were towed at 0.5 m depth across speeds of 1.6, 1.9, 2.2, 2.5, and 3.1 m·s⁻¹. The investigators identified a velocity window of 1.9 to 2.2 m·s⁻¹ for initiating dolphin kicking after a wall push-off (Lyttle et al., 2000).
Above 2.2 m·s⁻¹, initiating underwater kicking produced active drag forces that exceeded propulsive thrust, increasing deceleration compared with maintaining a passive streamlined glide. The practical distinction is important: movement did not automatically preserve speed. Under those tested conditions, beginning to kick while the swimmer was still traveling too quickly was less favorable than remaining streamlined (Lyttle et al., 2000).
My recommendation is to test first-kick timing rather than prescribe an immediate kick for everyone. Compare an earlier initiation with a later initiation while keeping the intended push-off and underwater route consistent. Use the reported velocity window as a study-specific benchmark, not a universal cutoff: it came from experienced male swimmers in a tethered towing assessment, not from every competitive population or underwater situation (Lyttle et al., 2000).
Mean drag change relative to surface (%) by Water depth (m)
Mean values across towing speeds of 1.6, 1.9, 2.0, and 2.5 m·s⁻¹. Negative values indicate lower total drag than the surface baseline; these were passive streamlining trials (Tor et al., 2015).
Source: (Tor et al., 2015)
3. Increase Frequency Without Assuming Bigger Is Better
Kick frequency deserves close attention. A systematic review covering 15 studies and 412 swimmers identified frequency as the strongest kinematic predictor of underwater velocity, ahead of amplitude and kick length. The reviewed sample included 333 males and 79 females, and reported competitive male frequencies typically ranged from 1.43 to 2.52 Hz. Those values describe the evidence base; they should not become mandatory targets for every swimmer (Ruiz-Navarro et al., 2022).
A flume study of eight national-level male swimmers illustrates the accompanying adjustments. As velocity increased from 70% to 90% of maximum, mean kick frequency rose from 1.46 to 2.11 Hz. Meanwhile, kick length declined from 0.77 to 0.68 m per cycle, and vertical amplitude declined from 0.60 to 0.54 m. Faster swimming in this experiment involved quicker, somewhat smaller kicks—not simply greater excursion or more distance per cycle (Yamakawa et al., 2022).
The same investigation found increases in mean and peak downward toe velocity, along with greater symmetry between peak upkick and downkick toe velocities at the higher speed. For coaching, I would assess frequency alongside amplitude and both kick phases rather than reward tempo alone. These findings describe coordinated changes during faster swimming; they do not establish that forcing a smaller amplitude or a perfectly symmetrical kick will independently improve every athlete (Yamakawa et al., 2022).
4. Use Dry-Land Training With a Specific Transfer Goal
Dry-land work has directly relevant intervention evidence, but the protocol matters. A randomized five-week study compared water-only training with water plus conical pulley eccentric training. Each group contained seven competitive swimmers, and the intervention comprised 14 sessions lasting 30 minutes. This was a defined training approach, not a comparison showing that any form of additional strength work improves underwater swimming (Ruiz-Navarro et al., 2024).
The combined water and dry-land group increased mean underwater velocity by 5.0%, maximum underwater velocity by 6.7%, and mean vertical toe velocity by 6.0%. The water-only group showed no improvement in toe velocity, with a reported change of −0.3%. For toe velocity, the group-by-time interaction was statistically significant at p = 0.035, with partial η² = 0.32 (Ruiz-Navarro et al., 2024).
My coaching recommendation is to define the intended pool outcome before adding a dry-land intervention. If using this study to guide programming, monitor underwater velocity and toe-motion characteristics rather than judging success solely by gym performance. Keep expectations proportionate: the reported improvements followed a specific five-week protocol in a small sample, and they are not guaranteed outcomes for a different exercise selection or athlete (Ruiz-Navarro et al., 2024).
Mean kick frequency (Hz) by Percentage of maximal velocity
Mean kick frequencies for eight national-level male swimmers in an annular flume. Standard deviations were 0.18, 0.26, and 0.33 Hz at 70%V, 80%V, and 90%V, respectively (Yamakawa et al., 2022).
Source: (Yamakawa et al., 2022)
5. Coach's Application
Begin with an underwater assessment organized around separate questions: Where does the swimmer travel, when does kicking begin, and how do the kicks change as speed rises? I recommend recording a baseline before changing technique, then testing one variable at a time. Start with depth and passive alignment, using the towing findings as reference points while remembering that passive drag measurements do not establish an optimal actively kicking depth (Tor et al., 2015; Vennell et al., 2006).
Next, compare first-kick timing options, followed by controlled changes in kick tempo. Ask the swimmer to try a quicker rhythm without deliberately enlarging the kick, then assess whether underwater velocity actually improves. Record kick frequency and amplitude together; do not label a faster rhythm successful merely because it looks busier. This testing approach draws on the glide-to-kick force findings and the observed combination of higher frequency and lower amplitude at faster swimming speeds (Lyttle et al., 2000; Yamakawa et al., 2022).
Finally, use the swimmer's response to select the next training priority. I would retain changes that improve the measured underwater outcome, revisit those that do not, and evaluate any dry-land addition against the same pool measures. The practical framework is to manage resistance, individualize kick initiation, and develop effective kick frequency—not chase one depth, one tempo, or one promised percentage improvement for the entire squad (Tor et al., 2015; Lyttle et al., 2000; Ruiz-Navarro et al., 2022, 2024).
References
- Lyttle, A. D., Blanksby, B. A., Elliott, B. C., & Lloyd, D. G. (2000). Net forces during tethered simulation of underwater streamlined gliding and kicking techniques of the freestyle turn. Journal of Sports Sciences, 18(10), 801–807. https://doi.org/10.1080/026404100419856
- Ruiz-Navarro, J. J., Cuenca-Fernández, F., Sanders, R., & Arellano, R. (2022). The determinant factors of undulatory underwater swimming performance: A systematic review. Journal of Sports Sciences, 40(11), 1243–1254. https://doi.org/10.1080/02640414.2022.2061259
- Ruiz-Navarro, J. J., López-Belmonte, Ó., Cuenca-Fernández, F., Gay, A., & Arellano, R. (2024). The effects of eccentric training on undulatory underwater swimming performance and kinematics in competitive swimmers. Journal of Human Kinetics, 93, 53–68. https://doi.org/10.5114/jhk/175824
- Tor, E., Pease, D. L., & Ball, K. A. (2015). How does drag affect the underwater phase of a swimming start? Journal of Applied Biomechanics, 31(1), 8–12. https://doi.org/10.1123/jab.2014-0081
- Vennell, R., Pease, D., & Wilson, B. (2006). Wave drag on human swimmers. Journal of Biomechanics, 39(4), 664–671. https://doi.org/10.1016/j.jbiomech.2005.01.023
- Yamakawa, K. K., Shimojo, H., Takagi, H., & Sengoku, Y. (2022). Changes in kinematics and muscle activity with increasing velocity during underwater undulatory swimming. Frontiers in Sports and Active Living, 4, Article 829618. https://doi.org/10.3389/fspor.2022.829618