PRINCIPLE AND TVVO FORMS OF SWIMMING PROPULSION
Transcrição
PRINCIPLE AND TVVO FORMS OF SWIMMING PROPULSION
PRINCIPLE AND TVVO FORMS OF SWIMMING PROPULSION Falk Hildebrand, Volker Drenk, Dieter ~ l i c h e ' .Institute for Applied Training Science, Leipzig 1 Olympic Training Centre HamburglKiel, Germany Propulsion in water is primarily done by generating torque in the shoulder, hip, knee, and ankle. To do this water resistance is used as support. By the of arm movements the body is pulled through the water, by the leg movements it is pushed. Locomotion is also found without any direct backward motion of the water. KEY WORDS: swimming, propulsion, water resistance, torque I ! INTRODUCTION: Analyses of swimming technique cover classical methods of threedimensional (3D) analyses (Deschodt, Rouard, Monteil, 1996) as well as complex studies including the energy yielding process (Miyashita, 1996). The swimmers move both in the water and above it: This makes the evaluation of the effect of the body parts above the water on the remaining body more complicated. But there is a trend in swimming, caused by a higher flexibility of all parts of the body, that all body parts actively act on water resistance. Additionally, the swimmers set water as a wave in front of them in motion. Several authors have tried to define propulsion. Councilman (1968) described the Newton principle "actio est reactio" as an impulse transfer from the moving water to the body. This attitude has been revised ten years later by Schleihauf's studies showing that backward moving water creates almost no hold for support (Councilman, 1980). He recommended an S shaped path of the pulling arm. For an explanation the lift force, deduced from the Bernoulli Principle, was introduced. Schleihauf concluded that both principles (IVewton and Bernoulli) contribute to propulsion. Maglischo (1993) again trusts in Newton: "Swimmers must push water back to go forward". He considered the hydrostatic drag to be of secondary importance. But he introduced a new idea: If the hand moves linearly the boundary layer flow is disrupted to produce turbulence instead of laminar water flow. In contradiction to this approach Colwin (1992) presented his vortex theory taking the water turbulences into account. In his opinion vortexes create a compact support to prop against (Fling-Ring-Mechanism). Colwin distinguished between three kinds of vortex: starting vortex, bound vortex and shed vortex. Hollander and Berger (1997) speculated that propulsion could be regained only from the water turbulences that are caused by the swimmer's propulsion. In 1993 Prichard observed the active rolling of the hip in top swimmers. He explained a portion of the propulsion by the impulse generation caused by turning the hip is transferred to the arms. METHOD: Data on body motion are gained from video recordings above and under water. All subjects are elite swimmers. The procedure is described in the paper by Drenk, Hildebrand, Kindler and Kliche (ISBS 1999). The calculated 3D coordinates form the basis for our analyses. Especially the velocity of the centre of gravity is calculated (see lower curve in Figure 1). We try to relate the increase in velocity to propulsion measures. According to our opinion the propulsion measure can only be defined by water resistance. Below we will explain how it is possible to move without impulse transfer, e.g. without moving the water backward. In training, swimming movements are presented using animation programmes (chosen phases in FiguresI and 2). ayl aleu!ysa 01 eaJe s!yl uo uo!paJ!p pue anleh A l ~ o l jo a ~uo!lnqyls!p ayl pue w ~ ayl e jo eaJe paueaqsMoy ayl asn a M 'luauow ley1 lsnl u! 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U J I~ M ~ 6uunp J ~ z J O ) ~3 jo Aqpolan :Mollaq alqua uo A)polan jo ~ ~pua A qqad :anoqa Gu!ww!~s A~yavnqu! quawanow 6a1- c a~n6!d integral of that area as a measure for water resistance. Water resistance creates a support for hands and arms which is applied to generate torque in the shoulder ankle pulling the trunk forward. It becomes especially obvious in modern breast stroke swimming with the hands and moving backward arms only for a very short time. 2. When applying this procedure to leg movements in butterfly swimming - the subjects were swimming underwater and without using their arms - we found evidence for a second form of propulsion. Especially in skilled swimmers the resistance parameter has never been positive. Figure 1 shows that all parts of the body are moved against the flow though the swimmer moves forward (compare velocity curve of centre of gravity). Our explanation is as follows: Leg movement in butterfly swimming is characterised by an upward res. downward movement of the lower leg res. of the more or less stretched legs. Synchronous to that movement pattern we find an increase and decrease of the centre of gravity's velocity in our animation programme. Velocity vectors are almost constantly pointing (exception and then only for a short time are foot portions) in the swimming direction (Figure 1). Thus water is never moved backward, the body is exclusively faced with resistance. The resistance which originally is breaking propulsion is applied as support to generate torque in knee and hip. This makes it possible to stretch the ankles without working in accordance with the angular momentum conservation law. According to this law a movement of the centre of gravity with only such movements would be impossible in quasi airborne conditions. Consequently, the centre of gravity is moved forward by stretching the knee and hip ankle. CONCLUSION: The question for real propulsion force values is still insufficiently answered (compare even Rouard et al.). This will be possible only after having a good procedure to determine the resistance coefficients. REFERENCES Berger, M. & Hollander, P. (1997). Internet Sportsci.org-news-news0709. XI1 FINA World Congress on Swimming Medicine, Goteborg. Colwin, C.M. (1992). Swimming into the 2 1st Century. Champain, Illinois: Leisure Press. Councilman, J.E. (1968). The Science of swimming. Englewood Cliffs, New Jersey: Prentice-Hall. Councilman, J.E. (1980). Handbuch des Sportschwimmens fiir Trainer, Lehrer und Athleten. zur schwimmsportlichen Trainings- und Bewegungslehre. Bockenem Harz: Schwimmsport Fahnemann. Deschodt, V.J. ,Rouard, A.H. & Monteil, K.M. (1996). Relative displacements of the wrist, elbow and shoulder. In Biomechanics and medicine in swimming VII (S. 52-58). London: E & FN Spon. Grimsehl (1954). Lehrbuch der Physik. In W. Schallreuter (Hrsg.) 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