Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
Graphic Removal of Water Wave Impact in the Pool
Wall during the Flip Turn
Pereira, s.M. 1,2 , Gonçalves, P. 2 , Fernandes, r.J. 2 , Machado, l. 2 ,
roesler, h. 1 , Vilas-Boas, J.P. 2
1 University of the State of Santa Catarina, Florianópolis, Brazil
2 University of Porto, Faculty of Sport, Cifi2d, Porto, Portugal
The aim of this study was to develop a graphical removal technique of
the water wave that precedes the contact of the swimmer with the wall
during the front-crawl flip turn, as well as to characterize and quantify
it. To do this the researchers used an underwater force platform and two
digital video cameras. In a pilot approach, a swimmer performed 8 turns
without touching the wall. Following the pilot study, 17 swimmers of
both genders, included the swimmer who participated in pilot approach,
performed 154 complete flip turns. The water wave was removed graphically
through a routine programmed in MatLab that was based on the
symmetry of the wave, and the instant in time that the wave made
contact with the wall. The average values for the maximum force of the
wave were similar in both trials as well as to those previously published.
Furthermore, the removal technique provided satisfactory results.
Key words: dynamometry, swimming, flip turn, wave measurement
IntroductIon
To move a body through the aquatic environment, swimmers transmit a
volume of water that is explained by hydrodynamic drag. The volume of
water depends, amongst other factors upon the swimmer’s body volume
and the speed of travel (Barbosa & Vilas-Boas, 2005).
When the swimmer approaches the wall in the turn, some of the
water volume displaced, hits the wall before the swimmer’s feet make
contact. This anticipated force record makes it difficult to assess the real
contact force of the swimmer. It is difficult to calculate the force generated
by the wave and to remove it from the total force curve (Blanksby
et al., 1998; Lyttle, 2000; Roesler, 2002).
Blanksby et al. (1998) were the first authors to recognize the presence
of the bow water wave in the kinetic analysis of the turn in breaststroke
swimmers of different ages. These researchers tried to eliminate
the wave’s effects through signal processing. However, as the frequency
of the wave was similar to the frequency of the swimmer’s force signal,
this was not possible to totally eliminate the bow wave without losing a
considerable amount of useful data.
Lyttle and Mason (1997) attempted to explain the slope of the wave
shape during the kinetic analysis of turns in front crawl and butterfly
swimmers. The data at the instant in time that the swimmer touched the
wall (instrumented with a 3D force platform) was recorded manually. The
force records from the wave reached a maximum value of 500 N, and the
duration of the wave was not mentioned. The authors concluded that the
magnitude of this force was proportional to the size of the force platform
since the force recorded by the wave should have been equal to the pressure
generated, multiplied by the surface area of the force platform.
Roesler (2002) conducted a study using two force platforms, placed
side by side on the wall of the pool, where the swimmer’s contact zone
was only on one of the platforms. Recorded force peak values were
around 371 N with a wave duration of around 0.14 s. The second platform,
even without the swimmer’s contact, detected a load seven times
smaller than the force produced by the wave on the first platform. This
decrease was attributed to the distance that the wave travelled to reach
the second platform.
The aim of the present study was to develop a graphical removal
technique of the bow wave that precedes the contact of the swimmer
with the wall during the front-crawl flip turn, as well as characterizing
and quantifying it.
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Methods
In order to characterize and quantify the force time curve produced by
the wave before the turn, an extensometric underwater force platform
was used, developed in accordance with Roesler (1997). Dimensions
and characteristics were: 500 x 500 x 70 mm, load and sensitivity of
4000 and 2 N (respectively), natural frequency of 400 Hz, gain = 600.
The force platform allowed data acquisition of force only in the perpendicular
component to the surface of the platform. The platform mean
measurement error was calculated after calibration and was given from
the weighting values obtained from the platform and the real weight of
dead-weights previously measure in a precision scale. The mean difference
between the values acquired on the platform and the dead weights
was less than 1%. The acquired signals were converted by an analog to
digital (A / D) converter, with a 16 bit resolution (BIOPAC Systems,
Inc.) and with input voltage of ± 10 volts. The acquisition rate was of
1000 Hz, powered by a 12 volts source, which allowed the subsequent
import of the signal to a PC.
The platform was set in an upright position on the opposite wall of
the starting blocks, in a metallic support that also included a stainless
steel frame for improved comfort and safety of the swimmer. As the
platform was 0.7 m apart from the turning wall, the black lines at the
bottom of the pool were modified to fit the new setting, remaining at the
official distance to allow normal turning performances.
Data from the force platform were acquired through the program
Acqknowledge ® (BIOPAC System Inc.). Signal processing was programmed
into a MatLab base and consisted of: calibration, filtering
(Butterworth 4th order, with a cutoff frequency of 100 Hz) and DC
offset removal. Normalization was done digitally by the program sharing
files and turned over by the force of the weight of the swimmer.
Two video cameras were used to monitor the water and the swimmer
movements, particularly the time of the initial contact of the swimmer
at the force platform. A digital type HC42E Sony camera was installed
in a waterproof case (Sony SPK-HCB) and attached to the bottom of
the pool perpendicular to the swimmers direction of movement (frontal
to sagittal intermediate plan). A surveillance video camera (type B7W
submergible camera - AC 230V) was fixed at the bottom of the pool,
facing upward perpendicular to the swimming direction (frontal plan),
in order to monitor the movement of the wave during the turn.
When the swimmer started swimming towards to the turning wall,
a trigger was activated to the A/D converting system, which connected
a underwater light visible by the video cameras. This process allowed
synchronizing the video setup and the force records.
A male swimmer of 22 years old, 1.82 m in height and 84.9 kg body
weight, finalist of sprint races at the Portuguese National Championships,
performed 8 flip turns. He started from the center of the pool,
12.5 m apart from the turning wall, with maximum speed, but without
touching the platform, as shown in Figure 1.
Figure 1. Sequence of images of the wave generated by the movement of
the swimmer performing the turn without touching the force platform.