Turning Force Measurement in Swimming Using Underwater Force Platforms

Transcription

1 Turning Force Measurement in Swimming Using Underwater Force Platforms Helio ROESLER Aquatic Biomechanics Laboratory, Santa Catarina State University, F1orianopolis, Brazil. br Abstract This paper describes some tests carried out in the Aquatic Biomechanics Laboratory concerning turning force measurement using underwater force platforms and cinemetry. Two platforms fixed on the edge of the swimming pool, one beside the other, were used to measure the forces during the flip turn. Various attempts were used to minimize the water effect on the platform. The markings on the bottom of the pool were covered, so the athlete would not feel any difference caused by the thickness of the platform. nitially the turns of three male swimmers of varying technical level was measured. Then, nine turning forces of five male athletes of good technical level were measured. Finally, the performance variable and dynamic variables were measured in twelve turns of an athlete. The variables of contact time and maximum peak force proved to be good indicators to the flip turn. However the total impulse variable did not prove to be a good indicator. ntroduction n swimming, the turning force measurement and a subsequent dynamometric analysis provide quantitative data that can be used in many ways, for example in the choice of new swimmers and in the improvement of swimming technique for high-level athletes. nvestigating kinetics in freestyle flip turn push-offlyttle et al. (1999) measured kinetics, hydrodynamic and kinematic variables like push-off force, total impulse, wall exit velocity and hydrodynamic peak drag force. Using Roesler (1997), underwater force platforms the flip turn force in freestyle was measured. A platform was fixed with a support in the swimming pool and the surrounding area was adapted in a manner that the athlete achieved a natural sensation. The lack of work on this subject prompted this sequence of exploratory research. nitially the turns of three male" swimmers of different technical level were measured, in a way that the idea of the force vs time curve was obtained [Araii"jo (2000)]. Then, nine turning forces of five good technical level male athletes were measured [Martins (2001)]. After this, measures were carried out with a second force platform beside the main force platform for water effect verification. Finally, a performance variable and dynamic variables were measured in twelve turnings of an athlete (HUBERT, 2002). This work describes how such measurements were carried out. 243

2 Helio ROESLER Methods For canying out the measurements, two underwater strain-gauge force platforms were used [Roesler, (1997)] 500x500xl80mm in dimension, in orders to measure all forces and moments and with 2N of sensitivity. Platform static calibration demonstrated that linearity was less than 1 % and cross talk less than 2 %. Dynamic tests showed that the platform fundamental frequency was 60Hz and signal analysis in the frequency domain showed that the system is reliable for underwater applications. Waterproofmg was achieved through positive internal pressure. The platform was sealed and kept under pressure. Since overall response was linear, force generated by the internal pressure could be easily compensated for acting on the system baseline. Software used for data acquisition and processing was SAD32 and the signal conditioner and AD converter was CO-EXP-BRDGE and CO-DAS-16Jr respectively. For the data collection, an acquisition rate of 600 points/charmel/seconds was used. The forces Fx, Fy and Fz were defmed as tangential horizontal force, tangential vertical force and normal force respectively. The normal force is the force in the direction of movement. First, a platform was placed vertically on the wall of the swinnning pool, at the center of the third lane. Once the platform cover was placed at a distance of approximately 20 cm of the swimming pool edge, the black strips in the swimming pool bottom were covered over and new strips were made in a marmer that the athlete felt no difference. For athlete security, the platform was coated with foam rubber in the four corners and a blade of foam was fixed in its upper side. With this configuration, three swimmers of different technical levels performed 3 flip turns. Only the normal force was measured. After this acquisition the data were calibrated, a 0-15 Hz, butterworth filter was applied, the data obtained per swimmer were processed, and fmally an average and standard deviation were calculated. The data verified were the initial time, the final time, the value of the maximum force peak and the total impulse. The total impulse was obtained from the curve force vs time integration during the athlete's contact time with the platform. n a second test, the three turning force components of five good technical level male athletes were measured. The same treatment of data was used. Additionally, the data were normalized by each athlete's body weight (BW). n practically all measurements one noted that, before the force carried out by the athlete's feet, a force with magnitude of approximately 1/10 of the maximum force was verified. Hence it became necessary to test if the cause of this eventual force was the athlete or the water in front of him. So, a second force platform was placed beside the first one, according to Fig. 1, and a new phase of measurements started. An athlete performed a sequence of turnings on the main force platform and the response in the two platforms was analyzed. 244

3 Turning force measurement in swimming Fig. 1: Two sub aquatic force platforms (Roesler 1997) fixed on the swimming pool board. n the center of the lane the main platform and on the left side another platform for water effect measurement. At last, twelve turning forces of a high technical level athlete were measured. Additionally, in this case a cinemetry system was used to verify the relationship between the dynamometric variables "force platform contact time" (CT), the "maximum peak of force" (Pmax) and the "total impulse" (mp) with the time taken for the athlete to swim from and to the 7.5 m line before and after the turn. Results The flrst phase results are summarized in Fig. 2, where one can observe the force vs time curve, and the great difference between the curve obtained from a good technical level athlete and the one obtained from a low technical level athlete. The main result of this phase was the verification that the underwater force platform system has a good operation in these applications, which is slightly different from those for which the system was designed. o.s $... llme(!i) Fig. 2: Difference in swimming turning force of athletes with different training levels. 245

4 Helio ROESLER n the second phase, where five male swimmers carried out nine turns, the curves were superposed and the mean curves were taken. Also the mean plus standard deviation, the mean minus standard deviation, for the vertical and movement direction were plotted, all normalized by the body weight. An instance is presented at Fig. 3. i i z.4f-t-+-+-f=:'*":.-l.-..dr=+--+--l.'-r-f---t--+-l~+z t m-ee+n~ F:z c.c-1"' :Scl 1_ lr--.,-~cl'/--fl,:-,+-.. l\'\-,1--1-i Z\-i l,-m-e+~-rn _F +-f-_+~-±1-1--rk-+'1--l--~--~-,me nfz-1s1--.. Y/1\' t r,c-!11./-f-1-+--\-\f\ r--1-- i LG H'- ~-~ ~ ----,--r-- ~;) l 1 f o 1-4-!L v-m a~jy+1fd. r...!jl!.,.s'--~-f--l Lr ~L ' \ J v-mr-~y--j-! o.:.. =~=j--t--~~---t-;1/~ :;;~G~~;.1kcl o., ~'- j;;:/.j::'--~ ~! l ~--::\..- 1-?~~./ T -0.1-~-J, m.. :! o.s Time (s) Fig. 3: Mean curves, mean plus standard deviation and mean minus standard deviation, for the vertical and movement direction, all normalized by the body weight for a good technical level athlete. n terms of normal maximum force, one noted that the athlete who applied the least force exerted 1.6BW, and the athlete who applied the most force exerted 2.2BW on average. The contact time average stayed between 0.47 s and 0.65 s, the percentile of time in which the maximum force occurred varied between and N.s!N. One could observe that the tangential horizontal force is insignificant and the vertical tangential force varied between 1/5 to 1/10 of the normal force (in the direction of movement). The athletes were ranked according to the relation to the variables: contact time (CT), maximum force peak (Pmax), maximum peak time percentile (% Pmax) and total impulse (mp). The Table 1 presents this rank. n this picture one can observe, for instance, that athlete 2 had the least one can conclude that athlete 2 is the one that best performed the flip turning technique. Table 1: Rankin of five athletes in relation to measured variables. CT Pmax %Pmax mp S2 S2 S4 S4 ss S4 ss S2 Sl ss S2 83 SJ SJ Sl ss S4 S 83 Sl 246

5 Turning force measurement in swimming The third phase results are summarized at Fig. 4, where one eau observe that on platform l, used to perfoi:m the turning, the force reached 2.3BW. The initial load reached 0.6BW (371N) with time duration of0.143s. Platform 2, which suffered only the water impact had au initial load seven times less (0.088BW), but in a synchronised way. Once platform 1 was placed in the turning axis and platform 2 displaced 0.5m from the turning axis, and the force increase occurred on both platforms, one can conclude that the first load increase observed is caused by water effect. From this information, the contact time must be considered only after this event, causing a difference between the anterior and subsequent studies on this variable. i :: 1 ' 1. ~....'.. - ' ' l ~.. 0 L ' i. ' P atfo m1 f atf m (a llary) i ~ 11)_ "= R-V\ 1- \. 'X J! ) (\ { J c:.l _/\ 1\ e.- ~... 11me (s) Fig. 4: Comparison between the normal force performed on the platform during a turn and the force caused by the water on a platform placed beside the main platform. The fourth phase results showed that the time used for the athlete to swim from the 7.5 m line, to make the turn and to reach again the 7.5 m line varied from 7.93 s to 8.23 s (8.108±0.089s), the "CT" was ± s, the "Pmax" was ± N/N and the "mp" was ± N/Ns. Discussion Generally way, the turning force measurement was carried out successfully through underwater force platforms. The athletes didn't relate discomfort, the modified pool markings provided the same sensation of a normal swimming pool and the foam blade provided security needed for the turning. The platforms fixed on the board had shortened the swimming pool length. By 20 cm in the case of a real length measurement, a modification of the swimming pool would be necessary for the adaptation of the platforms. The comparison between Lyttle et al. and these results was difficult because in his paper he hadn't normalized the load by the body weight. The mean of propulsive force in his paper was 1.190N and the mean of body mass was 75.7kg, generating a mean of normalized propulsive force 1.57 BW. n these results, one found 1.6 BW to 2.2 BW per athlete. n terms of contact time one found ± sa result which agrees with Daniel et al. (2002) who found 0.27 to 0.48 s. Several attempts were used to minimize the water effect on the platform. Once the platform cover surface is big (0.25 m 2 ), any for~ applied by the water is recorded. The intent of placing another platform beside the main platform to diminish the water effect on the main platform measurements 247

6 Helio ROESLER was not achieved. t happened because the water effect at O.Sm distance was minor, with a delay to the main platform. For this reason, a contact time value from the second phase to the others recorded a decrease. This decrease happened due to the impact caused by the water that antecedes the swimmer and was erroneously taken into consideration in the first two phases. Using only the dynamometric analysis, one could provide the athletes' ranking. Therefore it is possible to perform many analyses with the utilization of this system (which is easily transported) without the necessity of cinemetry equipment installation. The study with the dynamometric techniques and cinemetry allowed, further of the variables measured with force platform, the measurement of a performance variable. n the twelve turning measurements a difference of 0.30 s between the best and the worst turn was verified. Taken into consideration that 0,01 s can be the difference between a gold and a silver medal, and that the athlete analyzed had high technical level and regular results, one can conclude that turning techniques training based on accurate measurements can help the athletes to achieve better performances. This study allowed verifying that the variables contact time and maximum force peak are good indicators on turning. On the other hand, the variable total impulse is not a good indicator. Maybe because when the athlete spends too much time preparing himself for a very strong turn, he ends up wasting time he could use to swim. Hence, for a good swimming turn, it is advisable for the athlete to do it with a lot of force and quickly. References Roesler H. (1997). Desenvolvimento de plataforma subaquittica para medi9oes de for9as e momentos nos tres eixos coordenados para utiliza9ao em biomecanica. Ph.D. thesis. PROMEC, UFRGS, Porto Alegre, Brasil. Araujo L.G. (2000). Arullise dinamometrica da virada do nado crawl comparando nadadores de niveis diferentes. CEFD, UDESC, Florian6polis, Brasil. Martins E.R.S. (2001). Analise Dinfunica da Virada corn Rolamento no Nado Crawl CEFD, UDESC, Florian6polis, Brasil. Hubert M. (2002). n:l:luencia de variaveis dinilmicas no tempo de execu9ao da virada corn rolamento do nado Crawl. CEFD, UDESC, Florian6polis, Brasil. Lyttle AD. et al. (1999). nvestigating Kinetics in the Freestyle Flip Turn Push-Off. Australia. Daniel K. et al. (2002). Kinematic and Dynamographic Study in Different Swimming Turns. Book of Abstracts Xth World Symposium Biomechanics and Medicine in Swimming. St. Etienne, France. 248