Elite Swimmer Dolphin Kick Analysis

Take Home Points:

1. A qualitative, kinematic and hydrodynamic analysis of Thiago Pereira (Brazil, 2012 OG silver
   
   medalist at the 400IM) underwater kick in a flume.
2. In this trial, Thiago´s maximal knee and hip flexions were 112 and 148 degrees, respectively. The feet, knees, hip and shoulders vertical displacement were 0.49m, 040m, 0.16m and 0.21m;
3. His Strouhal number was 0.54-0.40 between 1.5 and 2.0m/s making him almost as efficient as some cetaceans (dolphins and whales).

Researchers and sport analysis must adhere to ethic guidelines. For instance, in most countries we are not allowed to disclose the identity of our subjects. Not so long ago, researchers would share what they have done only after the study being concluded. These days, we live in such fast-paced world that people are eager to share what they are up to. The temptation to post on social media pictures and videos of testing sessions and data collections is very high. In several countries, if one posts this kind of audio/video recording will be in troubles with the Institutional Review Board or Ethic Committee. If such posts involve underage participants, things will be nasty for the researchers. Can you image to be the parent or guardian of a swimmer that was invited to be part of a research, you authorized and a few days later pictures of your child in trunks or swimsuit are online and his/her identity is not protected? I guess the answer depends from which region of the world you are reading this post. In several countries a lawsuit would be filed against the researchers and the university. Other countries have more relaxed guidelines though.

It is not only the researchers that are sharing information on social media. Swimmers also do it on regular basis. It is a nice way to keep in touch with the fans. As long as they are adults, the ones posting the audio/video recordings and it is a public post, there are no ethical concerns. A few weeks ago, Thiago Pereira (Brazil) was generous enough to share a couple of videos over some testing sessions that took place in a flume in Tenerife (Spain). In one post we can watch him performing the underwater kick. Eventually this video became viral among the Brazilian and Portuguese swimming fraternity. It was shared by more than 2,400 people. Just a little bit of History: Brazil is a former Portuguese colony that is independent since 1822. Both countries share the same language (Portuguese) and have a common cultural heritage. 

We can perform a three-fold analysis of Thiago´s underwater kick: (i) qualitative analysis (as a coach would do); (ii) basic kinematics (as done on regular basis by sports analysts); (iii) hydrodynamics (carried out mainly but not exclusively by researchers).

Analysis of Thiago Pereira

• You may want to recap the main tips for a good underwater kick over here. You can also find
  
  some thoughts on the butterfly kick by Dana Vollmer (USA) and an analysis to her technique here.
• Thiago shows a nice body alignment (head´s in neutral position; arms fully extended; upper part of the body is streamlined for as much as possible)
• Wave motion (it seems to move all the way back to the feet; wave motion is not too wide so that would increase the drag force or impair the propulsion)
• Kicking (feet are in plantarflexion, i.e. tiptoes; seem to be pointing slightly inwards to increase the propulsive area; ankles are flexible enough; there is a quick change of direction from the upbeat to the downbeat and from the downbeat to the upbeat; he bent slightly the knees and hip to help the kicking; kick is short, fast and seems to have a good tempo).

 

Dolphin Kick Kinematics

• The kinematic analysis of the underwater kick involves mainly the assessment of joint angles and joint vertical displacements.  
• At least in this trial, Thiago´s maximal knee flexion was 112 degrees. Interestingly, in a sample of 19 international level swimmers Arellano (2002) reported an average angle of 113 degrees. His maximal hip flexion was 148 degrees.
•  The feet, knees, hip and shoulders vertical displacement were 0.49m, 0.40m, 0.16m, 0.21m, respectively. So, we can see that the extremities of the body have a higher vertical displacement.  Hence, wave motion moves all the way back (i.e. caudal direction). We can find in the literature a few papers reporting the same thing (e.g., Hochstein and Blickhan, 2011).

Dolphin Kick Hydrodynamics

• The questions that most of you will have right now are: “why should we perform the wave motion? What is the advantage?” and “Is it possible to measure it? To quantify it?”
• The wave motion (i.e. the vertical displacement of all major joints up and down as described earlier) will help to produce a vortex-wake. The same way fishes and cetaceans do it. When Thiago performs underwater kick the wake generated is very specific in the form of a sequence of vortices that alternate the direction of the rotation. While the feet moves one direction, creates a clockwise vortex, and then to the other, causing a counter-clockwise one (Arellano, 2002). Interestingly it was found that swimmers create this vortex in the front part of the body and then it will travel backwards (Mason et al., 1992) (Fig 1, bottom). Long story short: the vortex on the feet will produce thrust due to jet propulsion. That is why swimmers should perform the upbeat-downbeat and the downbeat-upbeat changes as fast as possible. A little bit of math: the circulation vortex increases with the angular speed and area. The induced velocity increases with the circulation vortex. Bottom line: if you are fast enough changing from up-down and down-up (i.e., short and fast kick) you increase the speed, likewise the circulation vortex and the induced velocity. Small and translating vortices are created in the end of the downbeat and no vortices at all are created in the end of the upbeat by poor swimmers (Arellano, 2002). Good swimmers created a big static vortex in the end of the downbeat and a small vortex in the end of the upbeat (Fig 1, top).

• To quantify the efficiency of the wave motion we can compute the Strouhal number (St). In fluid mechanics St is defined as a “dimensionless number describing oscillating flow mechanisms” or “the ratio between unsteady and steady motion”. For the layman I would say that it is the ratio of tip-toe speed to body speed and enables us to monitor the optimal creation of thrust associated with jet and vortices. So, the lower the St (i.e. less jet needed for a given same speed) the better.
• We can benchmark Thiago Pereira against other national and international level swimmers (Arellano 2002; von Loebbecke 2009 et al., 2009; Hochstein and Blickhan, 2011) and even against other animals such as cetacean (i.e., dolphins and whales) (Rohr and Fish, 2004). As expected, human swimmers were outperformed by their cetacean counterparts (fig 2). Overall the international level swimmers are better than the national counterparts.
• I am not completely sure of the flow speed in Thiago´s trial. I got values between 1.7 and 2.2m/s over several attempts, but most of them at 2.0m/s (mode=2.0m/s). So my guess is that the trial was performed at around 2.0m/s. Anyway, I estimated his Strouhal number for a range of speed between 1.5 and 2.0m/s. Each red dot is his trial at different hypothetical speeds. As speed increases his Strouhal number decreases. He is clearly better than the swimmers reported in the literature. What is surprising is that at higher speeds he reaches values similar to some cetaceans. He is within the range of values often reported for dolphins and whales, i.e., between 0.2 and 0.4 (Taylor et al., 2003).
• Short question for a short answer: “Why is the Strouhal number lower in cetaceans than humans?” Cetaeans have many vertebrae leading up to the end of the tail, which allow them to pass a much smoother wave. Humans have a very limited number of joints. So the increase of displacement along the length of the body is not smooth. Humans also have smaller propulsive areas and a few muscle-skeletal constrains. Elite swimmers, as Thiago are not cetaceans albeit are more flexible, have more strength and larger surface areas than poor swimmers.
• How can one improve the Strouhal number (i.e. the wave motion efficiency during underwater kicking)? To start, the body speed must be the same or higher. Anything that would you make impair the forward speed, not good at all. So make sure that the forward speed is the same or increases. After that, improve the kicking tempo (i.e. less time to perform the kick) and decrease the kicking amplitude (i.e. less vertical displacement). Follow-up question: how can one improve kicking tempo and amplitude? There are several ways, I´ll cover only two key-factors: strength and conditioning and technique. The swimmer must build-up strength power (e.g., to improve the kicking tempo) and enhance the kicking technique (e.g. decrease the joints range of motions, notably the knee flexion and therefore the kicking amplitude).

References

1. Arellano, R., Pardillo, S., & Gavilán, A. (2002). Underwater undulatory swimming: Kinematic characteristics, vortex generation and application during the start, turn and swimming strokes. In Proceedings of the XXth International Symposium on Biomechanics in Sports, Universidad de Granada
2. Hochstein, S., & Blickhan, R. (2011). Vortex re-capturing and kinematics in human underwater undulatory swimming. Human movement science, 30(5), 998-1007
3. Loebbecke, A. V., Mittal, R., Fish, F., & Mark, R. (2009). A comparison of the kinematics of the dolphin kick in humans and cetaceans. Human Movement Science, 28(1), 99-112
4. Rohr, J. J., & Fish, F. E. (2004). Strouhal numbers and optimization of swimming by odontocete cetaceans. Journal of Experimental Biology, 207(10), 1633-1642
5. Taylor, G. K., Nudds, R. L., & Thomas, A. L. (2003). Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency. Nature, 425(6959), 707-711

By Tiago M. Barbosa PhD degree recipient in Sport Sciences and faculty at the Nanyang Technological University, Singapore.