Understanding the Effects of Training on Underwater Undulatory Swimming Performance and Kinematics

! 2! Understanding the Effects of Training on Underwater Undulatory Swimming 1 ! Performance and Kinematics 2 ! 3 ! Jesús J. Ruiz-Navarro1, @Ruiz_NavarroPhD , https://orcid.org/0000-0002-0010-7233 4 ! Marta Cano-Adamuz1, 5 ! Jordan T. Andersen2, @AndersenScience , https://orcid.org/0000-0002-3424-9205 6 ! Francisco Cuenca-Fernandez1, @Cuenca_Fernandz , https://orcid.org/0000-0003-2942-4862 7 ! Gracia López-Contreras1, https://orcid.org/0000-0002-0488-8356 8 ! Jos Vanrenterghem3, @ScienceJos, https://orcid.org/0000-0002-1682-8430 9 ! Raúl Arellano1, @R_Arellano_C , https://orcid.org/0000-0002-6773-2359 10 ! 11 ! 1- Aquatics Lab, Department of Physical Education and Sports, Faculty of Sport Sciences, 12 ! University of Granada, Granada, Spain. 13 ! 2- Sydney School of Health Sciences, Faculty of Medicine and Health, University of Sydney, 14 ! Sydney, Australia. 15 ! 3- Department of Rehabilitation Sciences, KU Leuven, Leuven, Belgium. 16 ! 17 ! Corresponding author: Arellano, Raúl . Aquatics Lab, Department of Physical Education and 18 ! Sports, Faculty of Sport Sciences, University of Granada, Carretera de Alfacar, without 19 ! number, 18011, Granada, Spain. Email: [email protected] 20 ! 21 ! 22 ! Funding Information: 23 ! ! 3! This study was supported by grants awarded by the Ministry of Economy, Industry and 24 ! Competitiveness (Spanish Agency of Research) and the European Regional Development 25 ! Fund (ERDF); DEP2014-59707-P ‘SWIM: Specific Water Innovative Measurements applied 26 ! to the development of International Swimmers in Short Swimming Events (50 and 100 m)’, 27 ! by the Ministry of Science, Innovation and Universities (Spanish Agency of Research) and 28 ! the European Regional Development Fund (ERDF); PGC2018-102116-B-I00 ‘SWIM II: 29 ! Specific Water Innovative Measurements: Applied to the performance improvement’ and the 30 ! Spanish Ministry of Education, Culture and Sport: FPU17/02761 grant. This article is a part 31 ! of an international thesis belonging to the Program of PhD in Biomedicine (B11.56.1), from 32 ! the University of Granada, Granada (Spain). The authors acknowledge the participants who 33 ! selflessly participated in the study. 34 ! 35 ! ! 4! Abstract 36 ! In swimming, the underwater phase after the start and turn comprises gliding and dolphin 37 ! kicking, with the latter also known as underwater undulatory swimming (UUS). Swimming 38 ! performance is highly dependent on the underwater phase; therefore, understanding the 39 ! training effects in UUS and underwater gliding can be critical for swimmers and coaches. 40 ! Further, the development of technique in young swimmers can lead to exponential benefits in 41 ! an athlete’s career. This study aimed to evaluate the effects of a training protocol on UUS and 42 ! underwater gliding performance and kinematics in young swimmers. Seventeen age group 43 ! swimmers (boys =10, girls =7) performed maximal UUS and underwater gliding efforts 44 ! before and after a seven-week training protocol. Time to reach 10 m; intra-cyclic mean, peak, 45 ! and minimum velocities; and gliding performance improved significantly after the training 46 ! protocol. The UUS performance improvement was mostly produced by an improvement of 47 ! the upbeat execution, together with a likely reduction of swimmers’ hydrodynamic drag. 48 ! Despite the changes in UUS and gliding, performance was also likely influenced by growth. 49 ! The findings from this study highlight kinematic variables that can be used to understand and 50 ! quantify changes in UUS and gliding performance. 51 ! 52 ! Key words: swimmers, velocity, assessment, Statistical Parametric Mapping, biomechanics. 53 ! ! 2! Introduction 54 ! Underwater undulatory swimming (UUS), also known as ‘dolphin kick’, is a technique used 55 ! by swimmers to propel themselves forward after the start and turns of the freestyle, butterfly, 56 ! and backstroke events. In UUS, the swimmer adopts a streamlined position with the arms 57 ! outstretched and held together over the head while performing body undulations (Arellano, 58 ! Pardillo, & Gavilán, 2002; Connaboy, Coleman, Moir, & Sanders, 2010). Each kick cycle 59 ! comprises a complete downward (downbeat) and upward (upbeat) movement of the lower 60 ! limbs created by a sinusoidal wave that travels caudally along the body. During competition 61 ! the underwater distance is limited to a maximum of 15 m from each wall in freestyle, 62 ! butterfly, and backstroke events (FINA, 2013). With the exception of the dive at the start of a 63 ! race, the underwater phase of the start and turn represent the fastest parts of the freestyle, 64 ! butterfly, and backstroke events, making UUS one of the most influential variables on race 65 ! performance (Mason & Cossor, 2000). 66 ! Maximisation of propulsive impulse and minimisation of resistive impulse are key variables 67 ! when assessing technique to optimise swimming performance (Connaboy, Coleman, & 68 ! Sanders, 2009). Propulsion in UUS is generated by producing a ‘body wave’ that increases in 69 ! amplitude as it travels caudally along the body (Gavilan, Arellano, & Sanders, 2006; 70 ! Ungerechts, 1983), resulting in a leg-dominated technique (Higgs, Pease, & Sanders, 2017). 71 ! Resistive impulse is greatly affected by wave drag, a resistive force produced by the transfer 72 ! of kinetic energy from the body to the water. The wave drag represents 50–60% of the total 73 ! passive drag force at the surface in swimming; nevertheless, as depth increases the wave drag 74 ! decreases noticeably (Vennell, Pease, & Wilson, 2006). This fact results in the potential for 75 ! higher swimming velocity in UUS than in surface swimming. 76 ! ! 3! Before executing UUS in the start and turns of a race, swimmers glide in a streamlined 77 ! position underwater away from the wall. In addition to one’s ability to perform UUS, a 78 ! swimmer’s underwater gliding capacity likely plays an important role in swimming 79 ! performance. A more streamlined body position in underwater gliding helps to minimise 80 ! hydrodynamic drag (Arellano, 2010), which could improve the performance of starts and 81 ! turns without increasing physiological cost (Naemi, Easson, & Sanders, 2010). In this regard, 82 ! swimmers would likely benefit from a training protocol aimed to improve UUS and 83 ! underwater gliding abilities. 84 ! The optimum age for learning swimming technique ranges between 7 and 12 years old 85 ! (Navarro, Oca, & Castañón, 2003), for this reason previous studies have investigated the 86 ! effect of specific training protocols to improve UUS and underwater gliding in young 87 ! swimmers (Collard, Gourmelin, & Schwob, 2013; Helmy, 2013). In a study conducted in 88 ! swimmers aged 9-10, Collard, Gourmelin, & Schwob, (2013) observed greater improvements 89 ! in 25 m freestyle times in a group that received UUS-specific training, comprising undulation 90 ! drills incorporated daily into a standard swimming program, than a group that received a 91 ! standard swimming program only. The distance covered underwater was larger (6.50 vs. 4.91 92 ! m) and improvements in 25 m freestyle time were greater (0.94 vs. 0.36 s) in the UUS- 93 ! specific trained group than in the control group. Similarly, Helmy (2013) observed 94 ! improvements in underwater gliding performance, measured as the time to cover 8, 10, 12.5, 95 ! and 15 m, in swimmers aged 11-13 after a 12-week combined program of land and aquatic 96 ! exercises designed specifically to improve underwater gliding performance. 97 ! In the aforementioned studies, the UUS and underwater gliding performance were assessed as 98 ! time to cover a given distance (Collard et al., 2013; Helmy, 2013). The information provided 99 ! was helpful to evaluate the overall performance; however, the factors underlying this 100 ! performance enhancement are unknown. It is possible that the improvements reported in 101 ! ! 4! underwater gliding by Helmy were a consequence of technique changes, strength gains 102 ! associated with normal growth, or a combination of these factors. Thus, to extend beyond the 103 ! information provided by the assessment conducted in the studies by Collard and Helmy and 104 ! colleagues, kinematic data could be used to better understand the biomechanical factors 105 ! underlying overall UUS and gliding performance improvements after a period of training. 106 ! A variety of kinematic parameters have been used to assess UUS performance, such as 107 ! maximal and minimal velocity, or kicking frequency (Arellano et al., 2002; Atkison, Dickey, 108 ! Dragunas, & Nolte, 2014; Higgs, Sanders, Pease, 2014). Maximal velocity is achieved near 109 ! before finishing the downbeat, while minimal velocity is achieved at the end of the upbeat 110 ! when the knees reach peak flexion (Arellano et al., 2002). Increases in maximal or minimal 111 ! velocity would produce improvements in average velocity of the kick. Kicking frequency has 112 ! been proposed as one of the most important factors that can be modified to improve UUS 113 ! velocity (Arellano, Pardillo, & Gavilan, 2003; Arellano et al., 2002). A comparison between 114 ! age group swimmers and national and international swimmers showed main differences in 115 ! kicking frequency: the best swimmers were able to reach higher frequencies with similar 116 ! amplitudes to achieve better performance (Arellano et al., 2003; Arellano et al., 2002). 117 ! Calculations of these kinematic variables can be done with a single 2D camera or linear 118 ! potentiometer and minimal data processing time, making them accessible to sports scientists 119 ! and coaches. 120 ! A variety of tools can be used to evaluate underwater gliding performance. For instance, a 121 ! new ‘TorsoShape’ tool has been developed to better understand the effects of torso 122 ! morphology on resistive drag (Papic, Mccabe, Naemi, & Sanders, 2019; Papic, McCabe, 123 ! Gonjo, & Sanders, 2020) as a corollary for underwater gliding ability. The Hydrokinematic 124 ! method is another measurement tool that can be used to predict the exact time that underwater 125 ! undulatory swimming should be initiated (Naemi & Sanders, 2008). Moreover, in addition to 126 ! ! 5! measuring the distance covered during a glide (Helmy, 2013), simple measurements of 127 ! gliding performance have been proposed, such as distance reached as the swimmers slows to 128 ! 2 m/s and 1 m/s and the time until which forward movement stops (Arellano, 2010). These 129 ! simple time and position variables allow a quick and detailed quantitative description of 130 ! underwater gliding. 131 ! The scarcity of knowledge about the effects of training on UUS and gliding kinematics in 132 ! young swimmers led us to implement a skill-specific training protocol aimed to improve UUS 133 ! and underwater gliding performance. The purpose of this study was to evaluate the 134 ! performance and kinematics changes after a period of training in young swimmers. It was 135 ! hypothesised that UUS and gliding performance would improve following a period of training 136 ! using our protocol. 137 ! Methods 138 ! Participants 139 ! Seventeen age group swimmers, ten boys and seven girls (11.6 ± 0.2 and 10.6 ± 0.4 years, 140 ! 1.47 ± 0.01 and 1.45 ± 0.04 m of height, 39.2 ± 1.4 and 38.2 ± 3.6 kg of body mass, and 1.50 141 ! ± 0.01 and 1.48 ± 0.05 m of arm span, respectively), volunteered to participate in the current 142 ! study. All of them were under the supervision of the same coach at five training sessions per 143 ! week and had at least two years of competitive swimming experience. The protocol was 144 ! explained to the swimmers and their parents, who were informed about the benefits and risks 145 ! of participating in the current study prior to signing an informed consent form. The study was 146 ! conducted according to the Code of Ethics of the World Medical Association (Declaration of 147 ! Helsinki) and The University of Granada Ethics Committee approved the protocol (project 148 ! reference: 852). 149 ! ! 6! Experimental Approach 150 ! A ‘pre/post testing’ design was conducted with an intervention carried out over eight weeks 151 ! during the second macrocycle of the season, which started at week 22 of the annual training 152 ! cycle. The first week of the intervention comprised two familiarization sessions with the 153 ! pre/post testing procedures that were used to evaluate UUS and underwater gliding 154 ! performance during the first week (PRE) and after seven weeks (POST) of UUS- and glide- 155 ! specific training. Swimmers were asked to refrain from intense exercise the day before and 156 ! the day of testing and to abstain from caffeine and stimulants (e.g. energy drinks) during those 157 ! days. 158 ! Swimmers followed the training program set by their coach throughout the study. Standard 159 ! methodologies were used to compute and categorise swimming training load using a five- 160 ! zone system (Mujika et al., 1996). The swimmers trained in zones 1, 2, and 5, which 161 ! corresponded to general swimming, basic endurance, and speed, respectively. Swimming 162 ! training load was calculated for each week and expressed in the total volume completed (km) 163 ! and arbitrary training units (T.U.), which was quantified as: 164 ! 𝑇. 𝑈. = %%(𝑘𝑚!" ∗𝑖𝑓 !") + (𝑘𝑚!# ∗𝑖𝑓 !#) + (𝑘𝑚!$ ∗𝑖𝑓 !$) + (𝑘𝑚!% ∗𝑖𝑓 !%) + (𝑘𝑚!& ∗𝑖𝑓 !&) 165 ! 166 ! Where km represents the sum of the total volume swum in kilometres in the respective zone 167 ! (z1 = zone 1, z2 = zone 2, z3= zone 3, z4 = zone 4, and z5= zone 5) and if was the respective 168 ! intensity factor for each zone: ifz1 = 1, ifz1 =2, ifz1 = 3, ifz1 = 5, and ifz1 = 8 (Mujika et al., 1996). 169 ! The progression of swimming training load (determined by total volume completed and T.U.) 170 ! was monitored over 11 weeks from the first week of the second macrocycle until the end of 171 ! the eight-week intervention (Figure 1). 172 ! ! 7! (Insert Figure 1 near here) 173 ! Experimental Setup 174 ! Height (m) and body mass (kg) were measured using a stadiometer (Seca 799, Hamburg, 175 ! Germany) and arm span (m) was measured with measuring tape. In order to test the reliability 176 ! of height and arm span measurements, swimmers were measured by two independent 177 ! researchers. The standard error between researchers was calculated as 0.012 m for height and 178 ! 0.011 m for arm span. The final anthropometric values were the mean of the two independent 179 ! measurements. 180 ! UUS and underwater gliding were assessed in a 12.50 m long x 5.94 m wide x 1.20 m depth 181 ! swimming pool (water temperature = 27 ºC, humidity = 60%). This pool enabled to securely 182 ! place a vertical barrier fixed to a platform (Supplementary File 1), which allowed placing 183 ! touchpads of an electronic timing system (ALGE-TIMING, TP1890C Anschlagplatte, 184 ! Lustenau, Austria) to the start wall and to the vertical barrier. This system allowed to 185 ! electronically measuring the time to cover 10 m in the UUS trials. Horizontal velocity during 186 ! USS and underwater gliding were registered using a speedometer cable (linear transducer, 187 ! Heidenhain, D83301, Traunreut, Germany) attached to the swimmer’s hip via a belt. 188 ! Training Protocol 189 ! The skill-specific training protocol comprised three 30 min sessions per week conducted 190 ! during regular training sessions. The training protocol was designed according to 191 ! recommendations of swimming drills designed for teaching youth swimming (Guzman, 2017; 192 ! Lucero, 2015). Exercises were divided into five groups: ‘body awareness’, ‘gliding’, ‘gliding 193 ! + propulsion’, ‘propulsion’, and ‘speed’. ‘Body awareness’ exercises were performed on land 194 ! and all other exercises were performed in the water. ‘Body awareness’ and ‘gliding’ exercises 195 ! ! 14! performance, measured by changes in Ttime between PRE and POST, were likely the result 336 ! of regular swimming training, the UUS and gliding skill-specific training protocol, and 337 ! growth. Our findings contribute to a better understanding of the changes in UUS and 338 ! underwater gliding technique underpinning these improvements. 339 ! There are two potential reasons for the performance enhancement observed. The first 340 ! possibility is related to propulsive and resistive forces. When the propulsive forces are higher 341 ! or lower than the resistive forces (i.e. hydrodynamic drag) the body is accelerated or 342 ! decelerated and therefore swimming velocity increases or decreases, respectively (Vilas-Boas, 343 ! Fernandes, & Barbosa, 2011). The larger improvements in Upeak (9.1%) and Umin (40.4%) 344 ! were therefore a consequence of changes in both the propulsive and resistive forces or only in 345 ! one of them. As we did not measure hydrodynamic forces, we cannot elucidate whether or not 346 ! propulsive and resistive forces during UUS changed after the training protocol. 347 ! The second probable explanation is related to the downbeat and upbeat execution. While 348 ! Upeak is obtained near the end of the downbeat, Umin is obtained at the end of the upbeat 349 ! (Arellano et al., 2002). The upbeat is important for UUS performance since its successful 350 ! execution can be challenging, setting the fastest swimmers apart from the rest (Atkison et al., 351 ! 2014). Hence, the larger improvement observed in Umin compared to Upeak is of great 352 ! interest. Swimmers achieved higher velocity during the beginning of the downbeat and 353 ! during the whole upbeat (Figure 5). Therefore, the fact that the velocity was significantly 354 ! higher during the complete execution of the upbeat indicates that UUS performance 355 ! improvements presented here were mostly produced by a better execution of the upbeat. 356 ! Arellano and colleagues (2002, 2003) suggested that increasing kicking frequency could 357 ! improve UUS performance; however, in the current study, while kicking frequency did not 358 ! change significantly, improvements were observed in UUS performance and kinematics. 359 ! ! 15! Indeed, performing just several maximal trials at different kicking frequencies can provoke 360 ! kinematic changes at their preferred kicking frequency; without affecting maximal UUS 361 ! velocity (Shimojo, Sengoku, Miyoshi, Tsubakimoto, & Takagi, 2014). In other words, 362 ! training may induce changes in UUS kinematics without affecting the kicking frequency. 363 ! Therefore, our results suggest that swimmers might have improved their ability to utilise the 364 ! same kick frequency more effectively after the training. 365 ! Underwater gliding performance can be determined by two factors: initial push-off velocity 366 ! and hydrodynamic drag, where the latter decelerates the swimmer (Lyttle, Blanksby, Elliot, & 367 ! Lloyd, 1998; Novais et al., 2012). The push-off velocity did not improve after the training, 368 ! which may suggest that swimmers did not enhance the impulse during the push-off (Lyttle & 369 ! Mason, 1997). Nevertheless, since the push-off velocity did not increase and T2, T1, and 370 ! T0.15 were significantly improved, it can be postulated that swimmers reduced their 371 ! hydrodynamic drag. Hydrodynamic drag may have been reduced by improvements in the 372 ! ability to hold a more streamlined body position (for example, from the ‘body awareness’ and 373 ! ‘gliding’ exercises). Moreover, since push-off velocity correlated with Ttime in POST and not 374 ! in PRE, the swimmers likely improved their ability to utilise the push-off velocity more 375 ! effectively in POST than in PRE. These findings support the need to measure different 376 ! aspects of underwater gliding to accurately evaluate performance. 377 ! The gliding kinematics measured here are easily collected and relate strongly to UUS 378 ! performance (Table 2), which makes them appropriate for age group swimmers or daily 379 ! assessment. From a coaching perspective, the time taken to reach surface swimming velocity 380 ! is likely to be the variable of greatest interest since swimmers should start kicking prior to this 381 ! velocity to avoid slowing below surface swimming velocity. In the current study, the velocity 382 ! for T2 was chosen because it is similar to swimming velocity achieved in sprint racing (i.e. 2 383 ! m/s equates to 25 s for a 50 m race). On the other hand, T1 may be more suitable to use with 384 ! ! 16! swimmers who are not capable to reach higher velocities while swimming. Furthermore, 385 ! while swimmers inevitably start kicking before slowing to T0.15 and thus this variable may 386 ! not be as applicable as T2 or T1, the T0.15 measurement provides an understanding of a 387 ! swimmer’s ability to maintain their body position and whether coaches should focus on core 388 ! stability, which is vital in swimming due to the unstable nature of the water environment 389 ! (Willardson, 2007). 390 ! Improvements observed in the current study might have been influenced by swimmers’ 391 ! training and growth. While UUS performance changes were not correlated with height, 392 ! weight, or arm span changes, this does not necessarily mean that growth had no effect on 393 ! performance, because the combined effect of the change in height, weight, and arm span 394 ! might have had an influence on the outcome. Hence, the findings are limited by the lack of a 395 ! control group. Yet, the assessment conducted here will allow coaches to identify the effects of 396 ! training on their swimmers. This assessment will aid to identify weaknesses in specific 397 ! components of UUS and underwater gliding that can be used to better plan future training and 398 ! therefore achieve higher performance. Other skill-specific training protocols, such as 399 ! resistance training, may be complementary to the exercises used in the current intervention. 400 ! Future research should be conducted to better understand how muscle strength and technique 401 ! training interact to induce enhancements in UUS performance. 402 ! Conclusion 403 ! The detailed assessment of UUS and underwater gliding kinematics in the current study 404 ! contributes to the understanding of training effects on youth swimmers by showing individual 405 ! changes. Our results showed that after a period of seven weeks, swimmers improved their 406 ! UUS and gliding performance. The UUS performance enhancement was mostly due to an 407 ! improvement in the upbeat execution. Since push-off velocity did not change, swimmers may 408 ! ! 17! have improved their ability to hold a more streamlined body position, which could have 409 ! provoked a reduction in hydrodynamic drag that led to improvements in gliding performance. 410 ! The strong correlations between most of the kinematic variables with performance suggest the 411 ! use of this assessment protocol in future studies. 412 ! Disclosure Statement: 413 ! The authors have no conflicts of interest to report. 414 ! References 415 ! Arellano, Raul., Pardillo, S., & Gavilan, A. (2003). Usefulness of Strouhal Number in 416 ! Evaluating Human Under-water Undulatory Swimming. In IX International Symposium 417 ! on Biomechanics and Medicine in Swimming (pp. 33–38). Saint-Etienne. 418 ! Arellano, Raúl. (2010). Swimming technical training [Entrenamiento técnico de natación]. 419 ! Royal Spanish Swimming Federation. 420 ! Arellano, Raúl, Pardillo, S., & Gavilán, A. (2002). Underwater Undulatory Swimming: 421 ! Kinematic Characteristics, Vortex Generation and Application During the Start, Turn 422 ! and Swimming Strokes. In Proceedings of the XXth International Symposium on 423 ! Biomechanics in Sports (pp. 29–41). Caceres, Spain. 424 ! Atkison, R. R., Dickey, J. P., Dragunas, A., & Nolte, V. (2014). Importance of sagittal kick 425 ! symmetry for underwater dolphin kick performance. Human Movement Science, 33(1), 426 ! 298–311. 427 ! Cohen, J. (1988). Statistical power analysis for the behavioural sciences (pp. 20–27). 428 ! Hillsdale, NJ: Lawrence Erlbaum Associates. 429 ! Collard, L., Gourmelin, E., & Schwob, V. (2013). The fifth stroke: the effect of learning the 430 ! dolphin-kick technique on swimming speed in 22 novice swimmers. Journal of 431 ! Swimming Research, 21(1), 1–15. 432 ! ! 18! Connaboy, C., Coleman, S., Moir, G., & Sanders, R. (2010). Measures of reliability in the 433 ! kinematics of maximal undulatory underwater swimming. Medicine and Science in 434 ! Sports and Exercise, 42(4), 762–770. https://doi.org/10.1249/MSS.0b013e3181badc68 435 ! Connaboy, C., Coleman, S., & Sanders, R. H. (2009). Hydrodynamics of undulatory 436 ! underwater swimming: A review. Sports Biomechanics, 8(4), 360–380. 437 ! https://doi.org/10.1080/14763140903464321 438 ! De Ridder, R., Willems, T., Vanrenterghem, J., Robinson, M., Pataky, T., & Roosen, P. 439 ! (2013). Gait kinematics of subjects with ankle instability using a multisegmented foot 440 ! model. Medicine and Science in Sports and Exercise, 45(11), 2129–2136. 441 ! https://doi.org/10.1249/MSS.0b013e31829991a2 442 ! FINA. Federation International de Natation swimming rules (2013). http://www.fina.org. 443 ! Gavilan, A., Arellano, R., & Sanders, R. (2006). Underwater Undulatory Swimming: Study of 444 ! Frequency, Amplitude and Phase Characteristics of the “Body Wave.” Portuguese 445 ! Journal of Sport Sciences, 6(2), 35–37. Retrieved from 446 ! http://articles.sirc.ca/search.cfm?id=S- 447 ! 1051315%5Cnhttp://search.ebscohost.com/login.aspx?direct=true&db=sph&AN=SPHS- 448 ! 1051315&site=ehost-live%5Cnhttp://www.fcdef.up.pt 449 ! Guzman, R. J. (2017). The swimming drill book. Human Kinetics. 450 ! Helmy, A. (2013). The effects of combined program (land- and aquatic exercises) on gliding 451 ! underwater for young swimmers. Science, Movement & Health, 13(2), 118–123. 452 ! Higgs, Allison J. Sanders, Ross H. Pease, D. L. (2016). Kinematic assessment of human 453 ! undulatory underwater swimming. In Raúl. Arellano, E. Morales-Ortiz, A. Ruiz-Teba, S. 454 ! Taladriz, F. Cuenca-Fernández, & G. López-Contreras (Eds.), Swimming Science II (pp. 455 ! 312–317). Granada: Universidad de Granada. 456 ! https://doi.org/10.13140/RG.2.1.2675.4166 457 ! ! 19! Higgs, A. J., Pease, D. L., & Sanders, R. H. (2017). Relationships between kinematics and 458 ! undulatory underwater swimming performance. Journal of Sports Sciences, 35(10), 995– 459 ! 1003. 460 ! Lucero, B. (2015). The 100 best swimming drills. Meyer & Meyer Verlag. Meyer & Meyer. 461 ! Lyttle, A. D., Blanksby, B. A., Elliot, B. C., & Lloyd, D. G. (1998). The effect of depth and 462 ! velocity on drag during the streamlined glide. Journal of Swimming Research, 13, 15–22. 463 ! Lyttle, A. D., & Mason, B. (1997). A kinematic and kinetic analysis of the freestyle and 464 ! butterfly turns. Journal of Swimming Research, Vol.12(5p), 7–11. 465 ! Mason, B., & Cossor, J. (2000). What can we learn from competition analysis at the 1999 pan 466 ! pacific swimming championship? In XVIII Symposium on Biomechanics in Sports: 467 ! Applied Program: Application of Biomechanical Study in Swimming. (pp. 75–82). Hong 468 ! Kong. 469 ! McLeod, I. (2009). Swimming anatomy. Champaign, Illinois: Human Kinetics. 470 ! Mujika, I., Busso, T., Lacoste, L., Barale, F., Geyssant, A., & Chatard, J.-C. (1996). Modeled 471 ! responses to training and taper in competitive swimmers. Medicine and Science in Sports 472 ! and Exercise, 28(2), 251–258. 473 ! Naemi, R., Easson, W. J., & Sanders, R. H. (2010). Hydrodynamic glide efficiency in 474 ! swimming. Journal of Science and Medicine in Sport, 13(4), 444–451. 475 ! https://doi.org/10.1016/j.jsams.2009.04.009 476 ! Naemi, R., & Sanders, R. H. (2008). A “hydrokinematic” method of measuring the glide 477 ! efficiency of a human swimmer. Journal of Biomechanical Engineering, 130(6), 1–9. 478 ! https://doi.org/10.1115/1.3002764 479 ! Navarro, F., Oca, A., & Castañón, F. J. (2003). The young swimmer’s training [El 480 ! entrenamiento del nadador joven]. Madrid: Gymnos. 481 ! Novais, M., Silva, A., Mantha, V., Ramos, R., Rouboa, A., Vilas-Boas, J., … Marinho, D. 482 ! ! 20! (2012). The effect of depth on drag during the streamlined glide: A three-dimensional 483 ! CFD analysis. Journal of Human Kinetics, 33(1), 55–62. https://doi.org/10.2478/v10078- 484 ! 012-0044-2 485 ! Papic, Chris, Mccabe, C., Naemi, R., & Sanders, R. (2019). A method of quantifying torso 486 ! shape to assess its influence on resistive drag in swimming. In 37th International Society 487 ! of Biomechanics in Sport Conference (pp. 113–116). Oxford,. 488 ! Papic, Christopher, McCabe, C., Gonjo, T., & Sanders, R. (2020). Effect of torso morphology 489 ! on maximum hydrodynamic resistance in front crawl swimming. Sports Biomechanics. 490 ! https://doi.org/10.1080/14763141.2020.1773915 491 ! Pataky, T. C. (2012). One-dimensional statistical parametric mapping in Python. Computer 492 ! Methods in Biomechanics and Biomedical Engineering, 15(3), 295–301. 493 ! https://doi.org/10.1080/10255842.2010.527837 494 ! Penny, W., Friston, K., Ashburner, J., Kiebel, S., & Nichols, T. (2011). Statistical Parametric 495 ! Mapping: The Analysis of Functional Brain Images. London, United Kingdom: Elsevier. 496 ! Shimojo, H., Sengoku, Y., Miyoshi, T., Tsubakimoto, S., & Takagi, H. (2014). Effect of 497 ! imposing changes in kick frequency on kinematics during undulatory underwater 498 ! swimming at maximal effort in male swimmers. Human Movement Science, 38, 94–105. 499 ! https://doi.org/10.1016/j.humov.2014.09.001 500 ! Ungerechts, B. E. (1983). A comparison of the movements of the rear parts of dolphins and 501 ! butterfly swimmers. In A. P. Hollander, P. A. Huying, & G. De Groot (Eds.), 502 ! Biomechanics and medicine in swimming (pp. 215–221). Champaign, Illinois: Human 503 ! Kinetics. 504 ! Vennell, R., Pease, D., & Wilson, B. (2006). Wave drag on human swimmers. Journal of 505 ! Biomechanics, 39(4), 664–671. https://doi.org/10.1016/j.jbiomech.2005.01.023 506 ! Vilas-Boas, J. P., Fernandes, R. J., & Barbosa, T. M. (2011). Intra-cycle velocity variations, 507 ! ! 21! swimming economy, performance, and training in swimming. In L. Seifert, D. Chollet, 508 ! & I. Mujika (Eds.), World book of swimming: from science to performance (pp. 119– 509 ! 134). New York: Nova Science Publishers. 510 ! Willardson, J. M. (2007). Core stability training: applications to sports conditioning programs. 511 ! Journal of Strength and Conditioning Research, 21(3), 979–985. 512 ! https://doi.org/10.1080/16070658.1983.11689315 513 ! 514 ! ! 22! Table and figure captions: 515 ! 516 ! Figure 1. Training volume and units (T.U.) of the monitored 11 weeks, from the beginning of 517 ! the second macrocycle until the end of the eight-week intervention. PRE: UUS and 518 ! underwater gliding performance evaluation during the first week of intervention; POST: UUS 519 ! and underwater gliding performance at the end of the intervention. 520 ! 521 ! Figure 2. Percentage distribution of the time spent weekly on each content during the 522 ! underwater training protocol. 523 ! ! 23! 524 ! Figure 3. Linear regressions of PRE and POST between time to cover 10 m (s) and UUS 525 ! parameters. Individual value and 95% confidence lines are represented. Uavg: average 526 ! underwater velocity; Upeak: average underwater peak velocity; Umin: average underwater 527 ! minimum velocity. 528 !